Measuring Resistors in Circuit

Measuring Resistors in Circuit

Not quite, but,


I having trouble measuring a resistance in my project while the project is turned on.

I isolated one side of the resistance from the live circuit, but my multimeter still doesn't measure it properly.

I though I could measure a resistor on a circuit if one end of it was disconnected, even if the rest of the circuit was powered up, or can't I?

This really stuffs up my project :evil:
Going to bed now. Hope I left enough info for you guys.
Thanks.

Screech said:

Measuring Resistors in Circuit

Not quite, but,


I having trouble measuring a resistance in my project while the project is turned on.

I isolated one side of the resistance from the live circuit, but my multimeter still doesn't measure it properly.

I though I could measure a resistor on a circuit if one end of it was disconnected, even if the rest of the circuit was powered up, or can't I?

This really stuffs up my project :evil:
Going to bed now. Hope I left enough info for you guys.
Thanks.

Click to expand...

Yes, as long as one end is completely disconnected it 'should' have no effect. This assumes the rest of the circuit isn't injecting some kind of intereference?, mains hum etc.

But digital meters are really pretty poor with resistance measurements, I've always found in-circuit work far better with an old analogue meter.

I agree with Nigel, in theory it should be possible.

But a good digital multimeter will inject so little current into the resistor that any stray voltages, even voltages picked up on the leads to the meter will make the meter readings meaningless.

What puzzles me is, why do you want to measure the resistance of a half disconnected component while the rest of the circuit is powered?

There are ways and means of doing this, an old analogue meter is a good start, but tell us what you are trying to achieve.

JimB

do remeber that tyour body has resistance i think my multimeter used to read me as 1 meg so on values up to like 100 K i didn't worry but u should make sure that u r not touching both leads of the component and that as stated previously noise is a problem and ur body will sure carry that too.

You have to be quite careful when measuring components in-circuit with one leg desoldered. It's all too easy to accidentally re-make the connection when you stick your multimeter probe onto the component.

Usually I try to measure components in this manner but, if I feel I'm getting a strange reading, I then take the component completely out of circuit altogether so I can be absolutely sure about what I'm testing.

A lot of the people I work with would agree with Nigel regarding the analouge meters but personally I really can't stand them. I grew up on the digital meters so it's probably just a case of what you're used to. I think it's fair to say that you need a very good digital meter to measure resistance well, especially when you start talking about measuring very small resistances.

Brian

I was almost a prowd owned of one of those analog multimeres, but,
seems i found out the problem.

Not an expert, but I believe I found a fault in my circuit caused high frequency oscilations, creating voltages that were zapping the multimeter.

I removed a few suss components and things are looking promising again.

Let me share some of my experience of using analoge meter in troubleshooting.

When I was young I worked in the summer vacation as a repair technician in a company building transitor radio on assembly lines.

Not only none of technicians use a digital voltmeter but all of them use a particular brand of multimeter, the SANWA 360-Y, which I later been told by senior technicians is the meter in use by all repair technicians across the radio assembly industry.

Why?

1. It has a large Rx1 scale with sub-division of resistance value below 1 ohm. Very good for finding out short circuits of intermediate freq. transformer(IF). One never like to desolder them just to test that they are ok.

2. The range selector is recessed. By loosening the range selector, one can use the probe to change between volt and resistance ranges, which we need to do so continously during trouble shooting. Changing from voltage to resistance measurement be done in less than a second without needing to let go of the probe.

3. Its Rx10000 range using 22.5V battery let the leakage current of capacitors show up easily if they do leak.

I've just never been able to get into analouge meters. I appreciate that they have different properties to digital meters, and that some of those properties can be used advantageously, but I really can't use them. I've always used a digital meter and so I just hate the analouge ones. I think half the trouble is that I've never really been shown how to use one properly so they're a bit of an unknown to me. I can't trust the readings they give me because I don't fully understand them.

Regarding the high frequency interference thing, yes you can get peculiar readings on your meter if you stick your probes near a powerful high frequency point in the circuit. It's like if you get your scope probe and hold it near the line stage of a television - you can pick up the oscillations without even touching a component.
This kind of problem is worse when you start using capacitance meters. They same to be even more suseptical to inaccuracies caused by this kind of interference.

Brian

ThermalRunaway said:

I've just never been able to get into analouge meters. I appreciate that they have different properties to digital meters, and that some of those properties can be used advantageously, but I really can't use them. I've always used a digital meter and so I just hate the analouge ones. I think half the trouble is that I've never really been shown how to use one properly so they're a bit of an unknown to me. I can't trust the readings they give me because I don't fully understand them.

Click to expand...

One of the BIG advantages is with adjusting for a peak or dip, this is much more difficult to do with a digital meter (and some even have a bar graph display to make it easier).

At work I use a combined digital/analogue meter, the best of both worlds!, although being an electronic meter it still has some of the problems of digital meters.

A lot of the people I work with would agree with Nigel regarding the analouge meters but personally I really can't stand them. I grew up on the digital meters so it's probably just a case of what you're used to.

Click to expand...

Very true. Although I appreciate the storage and calculation capabilities of a DSO, I'll stick with my trusty analog scopes.

And a lot of us that like the capabilities of analog meters you will find do not deplore the use of digital meters. We just know, appreciate and use each to their advantage. Most of us who can use the analog meters with ease grew up with them because the digital instruments were not yet invented. We also grew up using slide rules because electronic calculators were still not in the marketplace.

My suggestion is to get a decent analog meter and learn to use it. It really isn't as bad as you may think. Just as with an analog scope, certain ranges are easier to use than others. Typically any "5" ranges are easiest to read and "1" ranges are second easiest. As with the scopes, it's the "2" ranges that are a bit more difficult. But once you learn to use one, you'll appreciate their specialized capabilities.

An older 3% analog meter with a 5000V range can read a 1000V level in most circuitry more accurately because it loads the circuit less than a 10M ohm 0.05% digital meter!

However, those VOMs with the 22.5V battery (or 30V battery in some cases) in the ohmmeter sections were known to blow a lot of solid state junctions in their day.

Dean

you can determine the approximate resistance based on the colour of the bands on the resistor itself, or get a high quality ohmmeter and measure the resistance of the resistor when it is not in a circuit.

It's a no brainer - just turn the power off...

Dean Huster said:

However, those VOMs with the 22.5V battery (or 30V battery in some cases) in the ohmmeter sections were known to blow a lot of solid state junctions in their day.

Click to expand...

I've never heard any suggestion of that?, and the Avo 8 series were used almost exclusively for many decades. The current is far too low to blow any transistor junctions.

What about the voltage ? isn't 30 volts high for some transistors ? particularily the Vbe

Thunderchild said:

What about the voltage ? isn't 30 volts high for some transistors ? particularily the Vbe

Click to expand...

Yes, it is. So the junction breaks down. Remove voltage, nothing changed.

I suggest you try to test a transistor using 30V in series with a 10M resistor and try to see whether you can damage it or not.

Its a different matter with FETs and MOSFET gate.

what are you getting at about the 10 meg r in series ?

Thunderchild said:

what are you getting at about the 10 meg r in series ?

Click to expand...

I would expect you don't realise that. The current that the multimeter sent out when in Rx10000 range is a few microamperes, just like a high voltage supply in series with a 10M resistor.

Nigel, it was mostly the germanium point-contact devices that were taking it in the shorts back in the 1960s and early 1970s when there were still a lot of VOMs out there with higher voltage batteries in the upper-resistance ranges. It was the voltage that got a lot of them in the reverse bias mode since a lot of those earlier parts had really low breakdown voltages compared to todays parts. And some of the base-emitter junctions couldn't handle the higher current of the Rx1 ranges when forward-biased, so it was a two-edged sword on those poor little parts. Most still used a single 1.5 volt battery for the Rx1 range or the two or three lowest ranges if they had a lot of resistance ranges. The Simpson 260 has used 15V and 6V and is now using 9V. The U.S. military AN/PSM-4 series used 30V batteries with the 1.5V cell on the higher ranges. My Knight-Kit VOM uses 6V and the 1.5V cell.

Dean

Dean Huster said:

Nigel, it was mostly the germanium point-contact devices that were taking it in the shorts back in the 1960s and early 1970s when there were still a lot of VOMs out there with higher voltage batteries in the upper-resistance ranges. It was the voltage that got a lot of them in the reverse bias mode since a lot of those earlier parts had really low breakdown voltages compared to todays parts. And some of the base-emitter junctions couldn't handle the higher current of the Rx1 ranges when forward-biased, so it was a two-edged sword on those poor little parts. Most still used a single 1.5 volt battery for the Rx1 range or the two or three lowest ranges if they had a lot of resistance ranges. The Simpson 260 has used 15V and 6V and is now using 9V. The U.S. military AN/PSM-4 series used 30V batteries with the 1.5V cell on the higher ranges. My Knight-Kit VOM uses 6V and the 1.5V cell.

Click to expand...

Avo meters only used the high voltage battery on the high resistance ranges, so the current was only very small (presumably a maximum of 50uA?). All the lower ranges (including the high current x1 range) used a 1.5V battery.

I've repaired plenty of germanium based gear, I started work servicing in 1971, so pretty well everything was germanium (or valve!).