I'm finding I get different results (vastly different in some cases) when measuring the total resistance of a circuit with a) a DMM and b) an old analog meter with a physical needle. And this doesn't only happen at high impedance points, either. What could account for this?
I've got four DMMs and two analogs. The DMMs agree with the other DMMs and the analogs agree with each other. But the different types don't agree with each other!
Are they calibrated?
NT
Do your DMMs and analogues agree with each other when using fixed resistors out of circuit?
Yes they do. Sorry, should have mentioned that in the OP.
If you are measuring in circuit resistance of solid state circuits, then you often will.
The analog meter has enough voltage/current to turn on the junctions of diodes and transistors. The DMM will not have enough to turn them on.
If you have a diode out of the circut and use an analog meter you will often see a small resistance in one direction and if you reverse the leads a high resistance. The DMM will usually show a high resistance in both directions unless you use the diode setting if the dmm has one.
Yes, and have reversed the leads as Ralph suggests...
Mike.
Damnit, Ralph! You beat me to it. I was going to say that. ;-) I have a couple of analogue meters too. They test for resistance at 15V which is more than enough to turn on those semiconductors, but also more than enough to destroy a lot of chips that can't tolerate much more than
5V. Horses for courses/different strokes and all that.
Try with different probe leads and resistors with different termination metals, maybe small dissimilar metals producing voltages that are interpreted differently by the different metering systems
It's okay now I believe Ralph has nailed the problem entirely in his post. (thanks, Ralph).
If you're doing in circuit resistance measurements, you're probably measuring the resistance of a non-linear device such as a transistor or diode. These will show different resistances at different applied voltages. Umm... this assumes that you've unplugged the circuit that you're testing and have discharged any BFC's (big fat caps).
Disconnect whatever you're measuring. Take one of the DVM's that has the highest input resistance, set it to VOLTS, and measure the VOLTAGE across the leads of the other meters. You'll find quite a bit of variation. My guess(tm) is that the meter with the highest voltage, will read the lowest resistance.
If you have an ESR (equivalent series resistance) meter, you can do in circuit low resistance measurements without worrying much about the effects of semiconductors. That's because the voltages involved are so small, that the semiconductor doesn't even being to conduct, and is therefore essentially out of the circuit.
There's really no way to "fix" the problem of measuring in circuit resistances. If I want to accurately measure a resistor that's in a circuit, I usually have lift one lead, and measure only that resistor.
DMMs
The analog meters (VOM) require more current in order to obtain a resistance reading. More current means more applied voltage across the leads, which means that the semiconductors in your test circuit are well into conduction. Try measuring a resistor and diode in parallel and you'll see the problem in action.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
I have heard this several times about analog meters destroying chips, because the chips cant tolerate over 5v. At the same time, I have never seen any analog meter using more than two batteries. That's 3 volts. I have several analog meters and they all use two AA batteries. Except for the one mini meter which only has only one AA battery.
So we have this theory about these analog meters ruining chips for exceeding 5v, but none of the meters can provide more than 3volts. This makes me think that this theory is based on advertisers trying to sell digital meters, or just an old wives tale with no backing. (Unless there are some analog meters which use 4 batteries [6volts], or use a 9v battery). If meters do exist, which are powered by more than 3v, I have never seen them.
Ok, I guess you have meters that I've never seen. I have to wonder why they need such high voltage to measure resistance. However, I would not use those meters on modern circuits if I owned them.
My analog meters are all older Radio Shack meters, which I have owned for many years. All (except the mini), have two AA batteries.
I also have a few VTVM's. I am not sure what they output, so I dont use them on any solid state devices. But they are well suited for tube gear, and can handle the high voltages in tube gear, which a lot of the battery operated portable VOM's cant handle.
The Simpson 260 VOM used a single D cells and a 15V battery (Eveready
417) for the Rx10K range. Later models switched to a 9V battery and then to 4ea AA batteries. The Triplett 630 used a single D cell and a 30V (Eveready 413).-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
You've never seen any AVO/Megger meters?? Like this for example:
They've been exported all over the world for decades!
They typically use one 15V battery (about the size of a AA cell) and a single 1.5V D cell.
On Sunday, July 16, 2017 at 4:50:45 PM UTC-4, snipped-for-privacy@tubes.com wrote: But they are well suited for tube gear,
Maybe the Harbor Freight free-with-any-purchase DMM can't handle higher vol tage, but I've never seen any DMM that can't handle tube gear voltages. If you're talking about the old plate voltages of the horiz output or HV rect ifier tube in televisions, no meter without an accessory HV probe will read those without some damage.
Unless you're doing peak and null, you shouldn't be using your analogue met ers in my opinion.
The Simpson 260 series used a 1.5 V D cell and several voltages for the highest resistance range. Very old ones used a 15 volt battery I think, some used several AA batteries and the two I have use a standard 9 volt battery. Tripplit made a similar VOM. I have not looked inside one of those, but would bet it had 2 batteries similar to the Simpson.
The caution before ICs came out was not to use the lowest current range for solid state devices. That 1.5 volt battery could supply enough current to destroy some simiconductors. Not voltage, but current.
The caution was way before the DMMs and ICs came out.
Radio Shack tells all. Anyone that says old school should have heard of and maybe used the Simpson or Tripplit meter. Don't recall the number of the Triplett as I only used one in school over 40 years ago.
The RS meters may have a meter of around 30 uA and the others have 50 uA meters. It takes more voltage to operate them in the higher resistance ranges. I don't know what the RS meters have for full scale of the resistance ranges, but it may not be as much.
Even the Free HF meter I have says it will do 1000 VDC and 700 VAC. Not too much common tube equipment has voltages over that, unless much higher and special HV probes are needed. I think my old Heathkit VTVM may do 1500 Volts. Have not had it out in years except to give it a check a year or two ago to see if it still works.
You haven't seen any of these? These were probably the most common VOM's available. At one time, I had about 5 of them. They've been replaced by more modern meters, but I still keep one of them on the shelf, just in case.
Most (not all) VOM's did NOT have amplifiers with gain. Therefore, resistance measurements needed to be using the basic meter sensitivity. The meter sensitivity and battery voltage put a limit on the highest resistor value that could be accurately measured.
The meter face usually had the meter sensitivity. In this example: It says "20,000 ohms per volt" (on DC scales) which is the same as:
You could probably read 1/10th of full scale accurately. So, what's the largest resistance that you could read at 1/10th of full scale, using a 9V battery? R = E / I = 9V / 5*10-6A = 45M Good enough to measure common resistors of the 1960's. However, if you tried it with a 1.5V battery, you would get: R = E / I = 1.5 / 5*10-6A = 7.5M That's too low, because there were plenty of resistors up to 22M in older tube sets, that such a meter could not measure.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Many old meters used 9v or 15v batteries as well as a 1.5v.
To enable the highest R range to work
they're fine, just don't use the top R range on anyhing delicate.
voltmeters don't output anything. They should be fine measuring solid state.
NT
The Simpson did have a 20 meg mark on the scale. It is almost worthless at that resistance. About all that can be told is that the reistor is not totally open. Around 2 to 5 meg ohms is abut the best anyone can tell close to the resistance. The resistance scale is similar to a log scale so as the resistance value goes up and below about 1/4 scale the values start getting very close together.
I have not looked into the 260 working in many years. I know of the 20 K per volt and how it works on the DC ranges, but not sure where it comes into play on the ohms scale for this meter.
The meter movement is just under 50 uA and has a pot that I think goes across it so the first step in calibrating one is to set it for 50 uA full scale.
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