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Topics - K9DJT

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1
Test Equipment / Electrical Safety
« on: July 17, 2014, 09:36:20 AM »
This is my 12th installment of “Understanding Test Equipment” and it just dawned on me I hadn’t addressed anything relating to Electrical Safety.  I am ashamed because it should have been one of the first things, and something none of us shouldn’t take lightly no matter how much experience one may have.

Did you know there is actually a correct way to connect and disconnect a piece of test equipment to a unit under test (UUT)?  Especially if you are using leads with alligator clips on them.  The correct way is to connect the low side of the test equipment, i.e., the ground lead (negative) to the UUT ground first, and then connect the high side, i.e., the positive lead to the potential (voltage).  When you disconnect from the UUT you just reverse the process.  Disconnect the positive first and then the negative.  So what difference does it make?  If the UUT is already powered up, it makes a big difference.  Let’s take a look at why.  During my travels, I had met an electrician who was working on a motor-drive system and disconnected the negative lead first and let it drop.  He created a lot of unnecessary damage in doing so because the negative was in reality “HOT”, i.e., it was at the same potential (voltage) as the positive lead of the test instrument.  The SCR’s the lead struck on the way down were not happy.  Remember the meter or scope you are using has an internal impedance (resistance), which no matter how large, has the same voltage at both ends until the negative side of that instrument is connected to the negative side of the voltage you’re measuring.  Let’s say you didn’t have an insulated boot around the negative alligator clip (dumb).  You grab it and remove it from the chassis and you are now holding onto the voltage you were measuring.  And yes, you might be unwilling holding it.

During my early years in electronics, age 12 to 20, it wasn’t uncommon to see many cartoons of some character being electrocuted and supposedly unable to let go of the wires.  It was always common to hear that we should work with one hand behind your back, which is still a very good practice, and stand on a rubber mat, which is also a good thing to do.  Well, of course I didn’t believe in this stuff.  I had already had so many shocks working on five tube radios and TV’s and was always able to pull away.  What nonsense I thought.  Well, it caught up to me while attending MATC.  My lab partner and I were setting up an experiment and all the stuff we were using was bread boarded.  The mature (older) hams will remember transformers, inductors, potentiometers, resistors and capacitors mounted on wooded boards with their leads to some type of spring connector which you pushed down and slid a wire trough.  It turns out my lab partner, who’s name I still remember and called on as a customer at GE Medical, had taken a power transformer, which had a line cord permanently attached to the primary, and plugged it in…WITH NOTHING CONNECTED TO THE SECONDARY!!!  No one would do anything like that.  Right?  Being fearless as I was, I proceeded to set up the circuit without ensuring the transformer was dead…and in doing so had a pair of fingers of one hand holding one end of the secondary while only one finger of the other hand just touching the terminal of the other end of the secondary.  400VAC hand-to-hand!  Unable to let go…unable to speak…everything going dark.  The momentary passing out is what save me as I fell off the lab stool.  I still remember the helpless feeling when trying to talk and all the other students laughing at me while thinking I was pretending and acting silly.  The above is a long story to make the point of; never trust anyone else regarding a circuit being dead or disconnected!  An example being a coworker or friend who may say, “I turned the breaker off.”  Don’t believe them…test all dead circuits yourself to ensure they are in fact dead.  How do should we test for a dead circuit?  No matter what you use, a VOM, DMM or Volt-Alert wand device, always test the instrument first on a known live circuit first.  This lets you know it is in fact working,  Then test the circuit which you believe to be dead.  Assuming it is dead by showing a zero voltage/potential, take the instrument back to the know live source again and ensure it is still working.  In this way you know the measuring device is working before, during and after the test of the dead circuit.  This is most important when using an instrument having test leads which may become intermittent.

I met many many people during my years as a sales engineer who experienced many close calls or knew someone who wasn’t as lucky as I.  The thing is, I never met a dumb electrician…all the dumb ones are dead!  Safety, relating to anything, is being smart and aware of your surroundings.  THINK about what you are doing!!!

Be safe out there!

73, Gary
K9DJT

2
Test Equipment / Ohmmeter and Insulation Testing
« on: June 06, 2014, 11:23:47 AM »
Last month I presented the idea of diagnosing electrical problems by measuring, or should I say looking for voltage drops in unlikely places, e.g., simple components such switches, connectors, lugs, terminals and wires.  One might ask why I wouldn’t recommend using the ohmmeter in the DMM (Digital Multimeter).  The main reason is that a higher energy component, whatever it might be, is not being stressed by the ohmmeter in contrast to being under a full load or operational stress.  So when would I use an ohmmeter?  I only use it for very simple, extremely low voltage and/or signal carrying connections.  The best example I can think of would be a mic cable, connector combination.  Basically from one pin or wire to the other end of the wire/connector.  When doing so, I would normally use alligator clips on the test leads so I am able to use both hands and flex the cable at both ends while looking for an intermittent open or short.  It’s a great measurement for such things.  Could I use it to measure the contact resistance of a relay?  Yup!  It would be perfect for antenna relay…you’re not going to look for a voltage there as you might on a DC circuit.  What you need to remember though, is to take the test lead resistance in consideration with the measurement.  A good set of test leads when shorted together should read anywhere from .1 to .5 ohms max.  You can mentally subtract that from your test measurement, or use the “Delta” feature I explained in a previous article, which will cause the meter to display the difference of the first measurement to the second.
OK, let’s say you want to check a transformer or choke in that old radio you just bought at Dayton before turning it on.  It isn’t that you don’t appreciate the adrenaline rush with a smoking transformer, but prefer not stinking up the whole house with the smell.  What you really want to do is measure the resistance of the insulation of the wire used in the transformer.  So, is this a place to use the ohmmeter?  Yes and No…meaning you would be able to detect a direct short to the transformer housing…maybe.  Think of what type of stress a standard ohmmeter places on that winding.  If it isn’t an old analog Simpson, it’s less than a volt and a half.  Do you think that is even close enough to detecting a failure in a questionable winding?   You’re right, it isn’t!  We need to stress this thing with some real voltage to see if the insulation has any leakage or will actually break down.  Hence, the “Insulation Tester” which is aka “Megger”.  Basically, an Insulation Tester is an ohmmeter which uses a very large voltage, i.e., up to 5,000 VDC, but a low current which is limited to about 2 mA.  It has selectable  voltage ranges and will detect leakage between windings and/or the housing.  The display reading is typically in the meg ohm range.  You normally will test a winding at twice its operating voltage, e.g., 480 VAC would be tested at 1000 VDC and a good winding would be 1000 ohms per volt.  In this case, 1000V X 1000 ohms = 1 meg ohm or greater.  Any value less than 1 meg would be considered a questionable device.  I just presented the very basic test.  We can make it more complicated in a future article!

See you on the radio!

73, Gary
K9DJT


3
Swag (Hats/Jackets) / New Items Available!!!
« on: May 12, 2014, 02:20:00 PM »
A nice wind-shirt, dark blue with yellow lettering, will cost about $36.  (We could also get them in yellow with blue lettering if we want.)  There is no minimum order which means we can order one at a time.

A decent polo-shirt, blue or yellow as above, will cost about $26.  Again, no minimum order.

T-shirts will cost about $12, any color, but because of the silk-screening there is a minimum order of twelve.

I am planning to order a wind-shirt and polo for myself which I plan to picture on the ORC website.

We're ready to accept you order!!!  Don't forget to also provide me your size...

73, Gary
K9DJT

4
Test Equipment / Digital Multimeter (DMM) - Voltage Drops
« on: May 12, 2014, 02:13:30 PM »
There are a lot of different ways to use a DMM (Digital Multimeter) to diagnose problems.  One of the more common usages is to find a missing voltage while tracing a circuit.  Most technicians will do the tracing by placing the negative (black) lead from the DMM to a chassis, if you’re measuring DCV, or to one side of an ACV power source, and then moving the positive (red) probe from point to point back from the load toward the source. The assumption is a known power being applied to an appliance but the appliance (load) is not functioning, i.e., dead.  What I would like to do is present a little different method of finding an open circuit, or a highly resistive one, by measuring the voltage drop “across” simple components such switches, connectors, lugs, terminals and wires.
What I mean is to test simple devices which “appear” to be OK when looking at it.  The car battery terminal in the picture is a perfect example.  There is no visible corrosion, it’s not lose, but it isn’t necessarily a good connection.  Especially under load, i.e., when attempting to start the vehicle and drawing a large current.  I helped my neighbor one day, who is a very good mechanic but was unable to figure out why the starter solenoid would chatter on his brothers car when he turned the key.  The battery was relatively new, the dome light and radio worked, all the connections looked good.  He was thinking it was the starter or solenoid when I asked if he measured for a voltage drop across the various connections.  He said, “no”.  So I made several  measurements similar to the one shown in the photo as he turned the key to start.  Guess what…there was a voltage of 0.4 Volts DC!  Look at the picture and ask yourself why would there be ANY voltage between those two points???  It should be ZERO!  If you leave the black probe where it is (chassis-frame) and put the red probe on the positive terminal of the battery, chances are you would think the voltage drop was due to a poor battery which wasn’t supporting the starter current .  Now the scenario even gets better.  Let’s say your conclusion is a bad battery, and you replace it.  You remove both battery connectors…clean both of them…clean the battery terminals on the new battery, and reconnect.  Wow…you fixed it.  It starts!  The downside is that it wasn’t the battery…IT WAS A DIRTY CONNECTOR, which happened to become clean when replacing the battery.  I’ve seen the same thing at the terminal of a starter.  Probe to center post and probe on the lug showed a voltage!  Why?  Again, it was a dirty and a highly resistive connection.  This happened on my sons jeep.  The garage he took it to wanted to replace the starter. I told him what I had found and the mechanics response was that he never seen anything like that…replacing the starter always fixes it.  Well, I am sure it does.  Again, remove the starter, clean the lugs and reconnect.  Yup…that fixes it.  But again, poor connection, not a bad starter.  Returning to the photo again, if both probes are placed on the center posts of the battery you wouldn’t have see the voltage drop at all.

OK.  The message I am trying to get across is to make voltage measurements across devices which you would not expect to be a problem.  Center post to lug, lug to lug of a wire, ground post of a device to ground,  the female part of a connector to the male part of the connector and even across a closed switched.  These are all places where there should be absolutely NO VOLTAGE INDICATED!  If there is ANY voltage indicated, it means the connection is highly resistive or it is open.  Could you find a bad connection using an ohmmeter?  Yes, but there are times when a fault will only appear under load…when a current is being drawn.

5
Test Equipment / Digital Multimeter (DMM) - Part 7
« on: May 12, 2014, 02:09:45 PM »
Last month we looked at making AC current measurements using a Current Clamp in the form of a “Current Transformer” which extends the capability of a DMM (Digital Multimeter) to measure up into thousands of AC amps depending on the specifications of the clamp used.  This month we’ll take a look at the yellow clamp at the right in the picture.  It also measures AC current, but in addition it will measure a DC current.  It is not a transformer type of device as the grey one on the left, but rather a “Hall-Effect” device.  Instead of a transformer, an active component is used in this type of clamp and  therefore requires a battery to power it.  The “Hall-Effect” was discovered by Edwin Hall in 1879 while working on his doctoral thesis in Physics.  He learned there was a voltage difference across an electrical conductor which was transverse to an electric current in the conductor and a magnetic field which is perpendicular to a current.  At the left is an illustration  of the makings of a clamp showing the “Hall-Effect” sensor.  Because we are looking at a voltage generated in reference to the current, we will be plugging this clamp, red plug into the millivolt jack on the DMM rather than the current jack.  I would expect you to know the black plug goes into the negative jack.  (The pictured clamp will help you plug it in correctly)  We then need to interpolate the mV reading into a current measurement.  This particular clamp will generate 1 mV per Amp.  Therefore, if the display shows 150 mV, you will have 150 Amps of current flow.  Pretty easy!  This is one reason the Fluke brand has a separate mV switch position.  I will discuss the other reasons in future articles.  So what do you do if you have a DMM without a mV switch position?  Well, you are still able to use the clamp as long as your DMM has mV resolution, i.e., .001 display capability.  Now for the same measurement, your display will not be a direct read out, but rather show .150 volts which as we know is 150 mV’s, or 150 Amps when we interpolate.  The need of plugging the clamp into a voltage jack, instead of a current jack, is not the only thing you need to be aware of.  The clamp, which uses a 9 volt battery needs to be turned on and/or off.  It also makes sense that you want to ensure the battery is good.  The other thing is that when you originally connect the clamp to the DMM, but before it is placed on a wire to be measured, the clamp requires a DC “Zero” adjustment.  Quite easy to do with the knob on the clamp.  You just look for a Zero value on the display and then proceed to place the clamp on the wire you want to measure after adjusted.

73, Gary
K9DJT

6
Test Equipment / Digital Multimeter (DMM) - Part 6
« on: May 12, 2014, 02:03:00 PM »
Last month we looked at making current measurements (A for amps) in circuits which were 10 amps or less.  What if you want to measure larger currents up into the hundreds or even thousands of amps?  Is that really possible?  Yup…by using an accessory referred to as a Current Clamp.  The concept originally came from a company called Amprobe who manufactured a clamp-on instrument used just to measure current hence the name.  Those type of meters (clamps) are still manufactured with the addition of a lower level voltmeter incorporated.  We’ll discuss those at a later date.

A current clamp accessory is typically available in two flavors.  One being a transformer device, pictured on the left, used to measure AC current, and the second is a Hall-effect device used to measure AC or DC current which we’ll discuss next month.

The current transformer, depending on its rating, will convert 1A to 1mA.  The connection to the DMM (Digital Multimeter) is the same as if you inserted the meter in series with your load.  The black lead is placed in the negative-jack and the red is connected to the mA-jack.  You’ll need to turn the rotary knob to AC mA, and in addition interpolate the reading in your mind from mA to Amps.  The mA reading is direct, i.e., there is no decimal point.  You might see a display of 25 mA, not .025 Amps, which makes it pretty easy to see your circuit has a current flow of 25 Amps.  This is one of the reasons there is a mA range on the meter.  The floor lamp on the left is drawing 1.83 amps, but you will notice the range is mAAC.  What do you do if you don’t have a mA range?  You may still use the clamp in the current mode of your meter as long as you remember 1 Amp = .001 Amp.  Resolution is usually lost and you won’t have the “direct” read out on the display.  Remember the measurement you are making is the current flow which is occurring in one leg of a circuit.  One of the biggest mistakes I used to see was someone who wanted to know how much current an appliance was drawing by placing the clamp around the line cord.  Guess what…the draw was zero!  That’s because the clamp was around both wires, and the opposite flowing current cancelled the other.  You need to have a means of capturing the current in just one of the wires as shown using a current clamp adapter (pay attention to the rating of the device) for line cords.  The one shown in the picture is using a x1 opening while the other opening is a x10 which may be of help with measuring an extremely low current.  The other neat benefit to using a current clamp with a DMM is that you’re able to make use of the Min-Max feature of the instrument.  That means you are able to capture the inrush current of a motor or appliance at start up.

73, Gary
K9DJT

7
Test Equipment / Digital Multimeter (DMM) - Part 5
« on: May 12, 2014, 01:58:08 PM »
At this point we’ve discussed voltage and resistance measurement using a DMM (Digital Multimeter) and even some convenience features such as Min-Max, Touch-Hold and diode check.  So it’s about time we take a look at the third piece of Ohms law, current, using A as the symbol on the rotary switch to indicate Ampere measurement.  As there is AC and DC voltage selection, there is AC current and DC current selection.  Some instruments might also provide a better resolution by supplying  a milliampere or microampere selection.  Typically, the maximum current which can be measured directly is 10 amps but that is dependent on the model and brand.  Higher currents reaching thousands of amps can be measured using accessories such as current clamps or current shunts.

Current measurements are primarily used to diagnose performance issues with a piece of equipment or process, e.g., the apparatus you are servicing is working, but not working well.  Maybe it’s overheating, running too slow or too fast.  The point being is that it is working and you have a very good idea of what the current should be.  If you do not have a clue of what to expect, do not attempt a measurement.  (This rule holds true to any electrical measurement; if you don’t know what value to expect, why bother?)  If the equipment you are servicing is dead, non-functional, K9DJTyou should make voltage measurements, or resistance measurements once power is removed and caps  discharged.  Yes, you could make a current measurement and maybe discover no current being drawn, but if the problem is a short, the current will be sky high and hopefully you’re using a meter which is fused.  If not, it could blow up or at minimum fry the insides.  Naturally, the severity is dependent on the power you’re working with.  The biggest mistake people make with a DMM setup to measure current, is instead of placing the meter in series with the load, they place the probes across the load or source.  Again, depending on the power you are working with, this situation becomes an extreme safely issue.  The DMM or VOM (Volt-Ohm-Meter) has a very low input impedance in the current mode and thus it’s like placing a short across the potential.  So remember, when measuring current with a DMM/VOM directly…meaning no accessory is being used…place the meter IN SERIES WITH THE LOAD, NOT across it.  This means you need to break the circuit under test in order to attach the test leads.  In addition, you need to place the RED test lead into the appropriate current jack on the DMM relating to the value of current you intend to measure.  In many cases there will be a 10 Amp jack and a 400 mA jack.

73, Gary
K9DJT


8
Test Equipment / Dgital Multimeter (DMM) - Part 4
« on: January 20, 2014, 07:49:30 AM »
Let’s take a look at something I believe is pretty simple, but yet have been asked many times of what two different symbols on a DMM (Digital Multimeter) rotary switch are used for.  The first one is a small cone shape having multiple curved lines, and the second is a schematic symbol for a diode.

The cone shape with multiple curve lines is representing a speaker, and is used to measure continuity.  Most of us have done continuity measurements using an ohmmeter, which is fine.  But what if you just want to know if something is making contact or not, without having to look at the display of the DMM?  Let’s say you’re looking at a pair of contacts on a relay and it is taking both your hands and eyes to active the relay in some manner.  Well, by using the “Continuity Check” you can do just that.  You connect your test leads across the contacts you want to check; activate the relay, and LISTEN for a steady tone.  When you release the relay, the tone should disappear.  You may check this function of the DMM just by touching the probes together, or if you’re in the need of a Morse code practice oscillator, here it is.  Just connect the probes across the key terminals and you’re ready to go!!!  But wait, what if you are also interested in the integrity of the relay contacts?  What if they’re highly resistive?  At this point you do need to look at the display which will indicate a resistance as high as 600 ohms.  With perfect contacts the DMM should indicate 0 ohms as in the picture at the above right.  If the contacts are in poor shape they might indicate a resistance like the picture to the left.  In both cases, take note to the position of the rotary switch and the symbol on the left side of the DMM display.  It’s the little speaker symbol, and not the ohm meter.


OK…So why is there a “Diode” selection on the rotary switch?  In the past when we would test a diode with a conventional VOM, we would use the ohmmeter in its lowest range and place the probes across the suspect diode, first in one direction, and then the other.  With the negative on the cathode there would be a needle deflection indicating forward bias of the diode and therefore conduction.  Reversing the probes should indicate no conduction, i.e., a good diode.  If there was conduction in both directions, the diode is shorted.  That still holds true using a DMM except for one thing.  In many cases, the DMM, because of the processor it uses, doesn’t provide enough voltage to forward bias a diode while in Ohms.  That is why there is a separate “Diode Check” on most DMM’s.  The neat thing though is that it just doesn’t indicate a conduction when forward biased, but it will display the amount of voltage it takes to do it.  Take note to the picture on the left showing the voltage and a little diode icon to the left of it.

Can’t remember if it is the long or short lead on an LED which is the cathode?  Why not check it with the DMM?  Just connect the test leads across the LED and see if it lights up.  If not, reverse the leads and take note to the negative lead and the length of the LED lead it is connected to.  (It will not be full brightness and therefore you must be sure you are looking at it straight on and not off to the side as in the picture.)  Again, the DMM will display the voltage it takes to forward bias and turn the LED on.  In this case it is 1.63 volts which is enough to turn it on but not to full brightness which is usually around 2.2 volts.

Next month we’ll discuss the various current measurement capabilities and options available with a DMM.

73, Gary
K9DJT

9
Test Equipment / Digital Multimeter (DMM) - Part 3
« on: December 04, 2013, 01:58:58 PM »
This month I would like to explain two convenience type features available on Digital Multimeters (DMM).  One is the “Min-Max” and the other is “Auto-Hold”.  Although most brand DMM’s have buttons labeled as such, they do not all function in the same manor.  As in the past, I will be referencing the FLUKE brand with my examples.

The “Min-Max” button, when depressed while making a measurement will do as it implies, and that is retain the maximum and minimum values while the probes/leads are connected to your source or device you are measuring.  In addition, it will also compute an Average value between the minimum and maximum.  For example, if you would like to see how well your regulated 12VDC power supply is actually functioning, you would first turn the main switch of the DMM to measure DC Voltage, connect the test leads across the output terminals of your active supply and then press the “Min-Max” button on the DMM.  At this point, you may press the “Min-Max” button, and each time you do, it will first display the Maximum, the next press will display the Minimum and the third press will display the Average measurement.  (Holding the button  down will turn off the Min-Max function.)  Because you haven’t really drawn any current yet, the values should really be close to each other.  Now, without changing anything with your test setup, draw some current, e.g., transmit for a couple of seconds, assuming you are also connected to a transceiver.  You most likely will see a slight difference in the values when you toggle trough the Min-Max again, but nothing great.  If the regulator is poor or not functioning properly, you will notice the Minimum value much lower than the Maximum value.  Instead of monitoring voltage, you might want to measure current instead.  Just make sure the meter can handle the current in question.

“Min-Max” will work within all the measurement functions of the DMM, e.g., Voltage, Current, Resistance, temperature and frequency.  It is important though to remember the proper sequence in which you connect the DMM and turn on the “Min-Max”.  Always connect the DMM and start measuring your parameter prior to depressing the “Min-Max” button.  If you connect the DMM to the circuit with the “Min-Max” already turned on, your “Min” value will equal Zero!  That is because that is what the DMM was seeing prior to connection…zero!  Connected in such a manor will display the Max and compute an Average between the Max and Zero.  The same thing will happen if you disconnect the test leads prior to toggling through the “Min-Max” values.  As soon as you disconnect, the new minimum value is zero!

Use your imagination with this feature.  I’ve monitored my line voltage over a period of time and even made remote continuity measurements by using the ohmmeter on a pair of wires, going to the far end, momentarily shorting the wires and returning to the meter to see if my new minimum resistance went to zero or not.  It also works great when working on a trailer wiring harness.  Connect the DMM, go and depress the brake pedal and then return to the DMM.  Did you have a voltage on the correct lead?  See, no need to bother the wife or kids to step on the brakes!!!

The other feature is the “Hold” and “Auto-Hold” button.  While making a measurement, you might want to retain the reading on the DMM to record somewhere…especially if your short-term memory is starting to fail you.  All you need to do, while the DMM is connected, is depress the “Hold” button. Now it is stored on the display so you’re able to disconnect the DMM and carry it over to your PC or desk to record it.  To bring the display to normal, just press the hold button again.  Now you should be asking, “what’s the difference between “Hold” and “Auto-Hold?”  Have you ever worked around lethal voltages or currents?  Like those inside a breaker-panel?  How about an HV power supply for you amplifier?  These are places where you need to be extra careful of what you are doing.  One slip of a probe can damage components or worse yet be a “widow-maker”.  This is where “Auto-Hold” or “Auto-Touch” saves the day.  Set up the DMM for the proper parameter, in this case let’s say AC voltage.  Press the “Hold” button twice, and AutoHold or AutoTouch should show up on the display.  Now carefully go into your breaker panel while watching both hands, having no need to look at the DMM, and listen for a beep.  Once heard, remove your probes and look at the DMM.  The measured value was retained on the display!!!  Now, without changing anything on the DMM, go back in and make a measurement at a different point, wait for the beep, remove the probes, and again look at the display of the DMM.  The new value is now displayed!!!  So as you can see, each time you touch a different test point, the DMM will HOLD the last measurement without ever having to reach over and do anything with the DMM or even look at it while making the measurement.  It’s Automatic!   To turn the function off, just press the Hold button one.

Next month I would like to discuss the use of the “Continuity” and “Diode.” functions.

73, Gary
K9DJT

10
Test Equipment / Digital Multimeter (DMM) - Part 2
« on: December 02, 2013, 10:50:40 AM »
Last month I touched on how most digital multimeters (DMM) will “Auto-range”, meaning they will choose the correct range for the most accurate reading for the value you’re measuring.  So why would you want to manually range the instrument?  Well, actually there are two reasons.  The first would be to increase the response time; meaning it will provide you a value faster.  The second would be to better utilize the ”Analog Bar Graph” as a peaking or nulling tool.  What happens when “Auto-ranging” is engaged, the DMM needs to take a moment for it to think.  It needs to look at the value you are measuring and make a decision on what range it should select for you.  We are not talking about a real long time.  It usually takes only a second or two depending on the brand but that might be too slow to capture an intermittent or let’s say a momentary switch closure.  I once met a customer who carried a DMM and an old analog meter with him.  I asked him why he still bothered to carry the analog with him?  He explained he serviced a piece of equipment were one of the tests was to momentarily see 120 VAC on the fly.  It presented itself for maybe a second or two and the DMM was trying to figure out what it was going to display…and by the time it was done, it was gone…in others words, he couldn’t tell if it had been there or not.  He said, “with the analog meter, I can see the needle swing up and back down.”  I asked him how the DMM reacted when he manually ranged it?  He said, “what do you mean, manually range it?”  I showed him the manual range button on the DMM, and chose the 600 volt range.  He then conducted the same test, and the DMM numbers not only came up immediately, but the “Analog Bar Graph” display went up and back down just as fast as the old analog meter.  In some cases, it might even be faster.  Guess what…he only carries one meter with him now.

As I mentioned at the beginning, the bar graph can be used for peaking and nulling. You are able to make adjustments using the bar graph just as you would an analog meter.  The thing to remember though, is to manually range the DMM first.  Depending on the brand, the bar graph should respond within all functions of the DMM, i.e., voltage, current and resistance.  Just for fun, measure the resistance of a potentiometer sometime and watch how the bar graph changes as the shaft is turned.

Now with all this being said about a DMM “Analog Bar Graph”, and being a senior citizen myself, there is still something to be said for the “Feel of watching a needle swing.”  Just as all of us have test probes, and even hand tools, which have a certain feel to them, we will all have a preference or feel for a certain type of meter for a particular measurement or adjustment.
 
Next month I would like to discuss the application/use of “Min-Max” and “Auto-Touch.”

73, Gary
K9DJT

11
Test Equipment / Digital Multimeter (DMM) Part 1
« on: October 21, 2013, 08:49:12 PM »
Now that we have reviewed some history on the VOM and VTVM, I think it’s time we got into some of the current technology used today, i.e., the Digital Multimeter (DMM).  Not only is a DMM easier and safer to use, it also provides a whole host of what I call “Convenience” features.  First, when I say safer to use, I mean it is less likely you will damage it or literally blow it up due to a misapplication.  With that said, anyone working within a high voltage, high current situation needs to take necessary precautions…most importantly, THINK!  Be aware of what you are measuring, and if you do not know what you’re measuring, why are you doing it?  So what makes a DMM safer and easy to use?

Using the Fluke brand as an example, you simply turn a rotary knob to the function you are interested in measuring, e.g., AC-DC voltage, AC-DC current, resistance, diode check, frequency or capacitance.  (With some models, there are even more parameters which can be measured.)  If you like, you can choose a range like the older instruments, but the neat thing is that you don’t need to.  The meter will “Auto-range” and select an appropriate range for you.  If you are making a DC voltage measurement, you want to watch the polarity of your test leads (+/-) but again, you don’t have to. The meter will display either a negative or positive voltage in reference to the way your leads are connected.  If you had been using a VOM or VTVM you might have damaged the meter movement because it was pegged in the wrong direction.  Resistance measurements do not require a manual range selection either.  You turn the meter to ohms, connect the leads and the DMM will choose the correct range for the best resolution.  For those of you who have used a VOM/VTVM in the past, how many times have you left the meter in the ohms position, picked up the probes and made a voltage measurement???  Did you enjoy the aroma of the resistors cooking, or was a major arc?  With a Fluke DMM, as long as the voltage you apply in the ohms mode does not exceed the highest rated voltage of the meter, nothing happens.  No smoke or arc!!!  The only thing you will notice is an “OL” (Out of Limit) displayed on the meter.  How cool is that?  This holds true for whatever function you left the meter in.  The current mode operates just a little differently.  You need to physically move the positive test lead (red) to the appropriate jack, i.e., milliamps (mA) or the 10 Amp when making a current measurement.  Both jacks are fused for safety.  In this case you turn the rotary knob to either mA or A (Amps) and choose either AC-DC.  If you know for fact the current will be less than 10 Amps, but not exactly sure of how much less, it would make sense to start by placing the probe in the 10A jack, and if it is way less, .4 Amp (400 ma), you can move the probe to the 400 mA jack for more resolution.  Again, there is no need to select a range.  It will automatically select the best range and it will also provide the polarity (+/-) in reference to the way you connect the leads.

Next month I will explain why you might want to manually range the meter and will get started with some of the convenience features.

73, Gary
K9DJT

12
Wanted / WTB: THRUST BEARING and/or TOP TOWER PLATE
« on: September 11, 2013, 11:16:23 AM »
I would be interested in talking with anyone who might have a THRUST BEARING and/or TOP TOWER
PLATE for a ROHN 25G they might want to sell.  Please contact me at 262.707.4279 or k9djt@sbcglobal.net

Hope to hear from someone.

Thanks and 73, Gary
K9DJT

13
Test Equipment / VOM, VTVM and DMM - Part 2
« on: September 11, 2013, 11:09:29 AM »
Last month I touched base on the VOM (Volt-Ohm-Meter) and ohms-per-volt.  As you should remember, the VOM is a useful measurement tool on a low resistant circuit but questionable on a high resistant circuit because of the “loading effect” which can result in an erroneous reading.  So how can we confidently make a voltage measurement in a high resistant circuit?  Originally it was accomplished by using a VTVM (Vacuum Tube Voltmeter), and most recently a DMM (Digital Multimeter).The VTVM was first introduced in 1942 by David Packard, Model 400A, and was manufactured until 1958.  Many other companies followed with their own designs and became the choice of instruments by engineers and technicians through the 60’s.  Basically the meter operates by using a tube amplifier to generate the current required to deflect the meter pointer, hence the name Vacuum Tube Voltmeter.  By using this technique, an input resistance or impedance (the resistance applied in parallel with the circuit under test) of up to 20 megohms can be achieved.  The key benefit is that the 20 megs is independent of the range selected which is complete opposite of a VOM.  Now by using ohms law, if you were to place 10 to 20 meg ohms of impedance across a high resistant circuit, you will conclude it will have a negligible effect, and therefore provide a accurate measurement.  A value of 10 meg ohms for input impedance is now the accepted standard.

The above is the upside which the technical community felt outweighs the negative side of requiring an AC line cord for the power supply, the inability to measure current and the need to deal with a little larger test probe.  In the years to come, the tube amplifier was replaced using a FET solid state device which brought back the portability, but still the need for a larger probe and no current measurement.  But wait, couldn’t we measure a current if we measured the “voltage-drop” across a 1 ohm resistor of proper wattage placed in series with our circuit?  You bet we can.  With that being said, it is not always practical but still a solution in many cases.

There is not enough space to write about all the features and benefits of purchasing a DMM this month, but I will say it has the best of both the VOM and VTVM in one package, i.e., it typically has a 10 meg input impedance, it is portable (no line cord), makes use of a variety of slim probes and  in most cases can measure a current up to 10 amps.

73, Gary
K9DJT

14
Wanted / WTB: 7/8" Helical Connectors
« on: August 20, 2013, 09:38:27 AM »
I just installed a couple of 7/8” hardline and in need of two female “N” connectors.  Would be interested in talking with anyone who might have some laying around.

Hope to hear from someone...

73, Gary
K9DJT

15
Test Equipment / VOM, VTVM and DMM Differences - Part 1
« on: August 12, 2013, 09:15:44 AM »
With our fall swapfest soon approaching, we all have an opportunity to maybe find a deal on a multimeter.  But do we know the difference between the offerings?  How does a VOM (Volt-Ohm-Meter) differ from a VTVM (Vacuum Tube Voltmeter), or either to a DMM (Digital Multimeter)?  Do they all perform the same functions?

One of the earliest multimeters manufactured was the Simpson-260 which was introduced in the 1930’s and has since evolved into a series of them.   As a matter of fact, they are still being produced and sell for about $299 new.  You will find many old-timers out there who still believe there is nothing better.  It measures AC and DC voltages up to 1000 volts and DC currents up to 10 amps.  In addition, it will measure resistance from zero to 20 Meg ohms.  Accuracy is reasonable and is specified as a percent of full scale reading.  When looking at the spec’s of a traditional VOM such as the Simpson, you will notice it is rated as a certain “Ohms-per-Volt,” e.g., 20,000 ohms per volt DC, and 5000 ohms per volt AC.  This relates to the input impedance (resistance) of the meter when making a voltage measurement and the impact it has on the measurement itself.  It is also considered an indication of the sensitivity of the meter, i.e., the higher the ohms-per-volt, the greater the sensitivity.

Looking at the above VOM example (Single click 1st pix below), you can see that there is a different resistance used for each voltage range.  The actual lead-to-lead resistance is different for each voltage range you might use in making a measurement.  Now consider the impact of that resistance you are placing in the circuit under test.  What is the chance of it altering the circuit enough that your measurement really isn’t correct?  It most definitely will in low current electronic type circuits.  It is referred to as the Loading Effect.

Looking at the circuit example below(Single click 2nd pix below), you can see what happens when you use a low impedance VOM on a high resistance circuit.  You do actually alter it unknowingly BY applying a parallel resistance path in the circuit and obtaining the equivalent circuit.  This is the reason you want a high impedance meter to do electronic diagnostics.

We will discuss high impedance meters such as a VTVM and DMM next month.

73, Gary
K9DJT

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