Analog meters output more current to measure resistance at low currents? Vacuum Tube

[danny davis]

I was having trouble measuring resistance on a low current board because I was using a Fluke 87 DVM.

My Manager said I need to use a Analog Meter or a Vacuum Tube multiple meter to measure resistance because it outputs a high current than a DVM

When do you use an analog meter because it outputs a higher current than a DVM?

Does a Vacuum Tube output a higher current when measuring resistance better? but for what kind of circuits?

 

[chuckey]

Without doubt analogue meters do push out a lot of current on their low ohms range, the AVO 8 wacks out 100 mA on its low ohms range with zero ohms in the circuit. I do not see why the current should alter the accuracy of the measurement. Bad leads and their plugs and sockets and probes are the most likely suspect areas.
Frank

 

[danny davis]

Why does analog meters output more current for their low ohms range?

Digital meters only output 1mA?

What advantage does output 100mA do for testing? testing what?

 

[BradtheRad]

My analog VOM puts out 60mA at 1.5V, on the ohmmeter x1 range.

I think it could ruin small-signal components (such as germanium diodes and transistors).

When is the analog type needed? I suppose when testing components that (a) have a threshold voltage (such as silicon diode junctions, which are in a range to be turned on by 1.5V from my ohmmeter), and (b) will present low ohms to my ohmmeter.

I wonder if, by attaching my analog meter to a board, it could energize a coil somewhere, then when I detach a lead, the coil would generate a high voltage spike? Could be bad for neighboring components.

 

[chemelec]

If you really want to measure Low Resistances, Build yourself a "LOW-OHM" Meter.
Like These ones on my Website.

http://chemelec.com/Projects/Low-Ohm/Low-Ohm.htm

 

[danny davis]

I think it could ruin small-signal components (such as germanium diodes and transistors).

So a Digital DVM can't do this? why not?

Measuring the resistance of a germanium diodes and transistors?

When is the analog type needed? I suppose when testing components that (a) have a threshold voltage (such as silicon diode junctions, which are in a range to be turned on by 1.5V from my ohmmeter), and (b) will present low ohms to my ohmmeter.

So digital DVM can't do this? why can't it?

Are you measuring the resistance of the silicon diode junctions?

 

[chemelec]

Diodes and Transistors are Not "Resistive devices".
Most Diodes typically create a 0.6 Voltage drop. (Some Higher and some Lower.)
But this drop Depends on the Type of diode and Current Flow.

Transistors also create Voltage Drops, but this is dependent on Bias Current applied to the Transistor.

Many Digital Multi-Meters have a DIODE Test feature.
It Measures this Voltage Drop.

 

[chuckey]

A typical analogue meter, when the leads are shorted pushes out, say 100mA, which comes from an internal 1.5V battery, so the internal resistance must be 1.5/.1 = 15 Ω. Now if the external resistance is 15 Ω, the current will be halved (50 mA), so the meter reads 50% of its full scale. If you put the probes across a diode, the current will be (1.5 -.8) / 15 + Dr (20 Ω?) = .7/35 = 20 mA and the meter will read 20% of FSD. I don't think 20 mA would damage many diodes, only micro wave mixer diodes?
Brads meter, 1.5/.06 = 25 ohms internal resistance, on diode, (1.5 - .8)/ 25 + 20 = 15.5 diode current.
Frank

 

[danny davis]

So when or what do u test using an analog meter for because of its output current is higher? Sine an analog meter outputs more current what can I test that a digital DVM can't bbecause of the meters output current?

 

[chemelec]

Simply PUT:

Most Ohm Meters are Designed to just Test just DC Resistance values.

Such as VTVM's, DMM's and VOM's.
And they are usually Only Accurate down to ONE Ohm.

For Resistance Measurements, "Lower than ONE Ohm", you need a LOW OHM Meter, like in the Link I Posted in #5.
It uses 4 Wires, to Avoid Errors from Contact Resistance of the Probes.

A "Sine" Ohm Meter measures AC Resistance, which is Impedance at the Frequency of Testing.
In a "Pure" Resistive circuit, this is the Same value as the DC Resistance test.
But will be a Different Resistance reading if ANY "Capacitance or Inductance is present"

 

[danny davis]

I know but I want to Focus on how an analog meter outputs a High Test Current

Since it outputs a higher Test current, what are some things I can test with an analog meter that I can't test with a Fluke DVM meter 87?

 

[FvM]

Why does analog meters output more current for their low ohms range?

I'm under the impression that the question hasn't be yet clearly answered.

The answer is in the different circuit used by analog meters for resistance measurement. The circuit has been already sketched by chuckey. It's a battery, a current limiting resistor, the meter in current measuring mode and the resistor under test. Digital meters (or analog meters with built-in amplifier) are in contrast using a current source, voltage measuring configuration. The series circuit is used for two reasons, simplicity and automatic disconnection of battery with open terminals. An obvious disadvantage is the nonlinear scale.

The series circuit involves that for sufficient resolution in low resistance ranges, the current limiting resistor must be low and the maximum measurement current respectively high. So you can say that the relative high current is more a necessity than a feature.

The question if higher current is possibly advantageous for some measurement problems should be separated from the "why". In my view there are very few cases. I don't remember that I took out an analog meter from the depths of it's shelf during the last 30 years or so...

 

[danny davis]

So an analog meter is better to measure resistance in circuit test because of parallel components?

I was getting weird readings with my DVM fluke meter, My manager said use the Simpson 260 because the output test current is higher he said , I think because there is components in circuit that are parallel component , so when measuring resistance in circuit a DVM meter is to low of test current?

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The Simpson 260 can put out over 100ma with the leads shorted on the Rx1 scale. That's considerably higher than a DVM which puts out 1ma. Case closed.

Why would they want an Analog meter to output 100mA for testing what components? any reason?

Is this test current at 100mA better for testing resistance that have parallel resistance?

What is it used for? it's ment for something to do certain test on components that you can't do on DVM meter for sure

The 260 definitely has a much better resolution for Ohms. It has 10 grads between 0 and 2 ohms, which means each small grad is 0.2 Ohms. So this thing can tell the difference between 0.2 Ohms and 0.4 Ohms

So the Simpson 260 can measure Milliohms? isn't 0.2 ohms and 0.4 ohms , milliohms?

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Measuring in circuit resistance that has parallel resistance can cause to much loading on a DVM meter that has an output current of 1mA , that's why I was getting weird or NO readings at all. The Simpson 260 can measure resistance with a lot of Load resistance in circuit?

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Well no, the 260 can not measure milliohms. It can only measure hundreds of milliohms.
0.1 ohm is 100 milliohms, 0.2 ohms is 200 milliohms, 0.3 ohms is 300 milliohms, etc.

Thanks, Ya my DVM fluke 87 can't measure 100 milliohms or 300milliohms, so the Simpson 260 is better

A lot of parallel resistors do not "load" a DVM too much. A DVM can take a short circuit which is the ultimate low ohms load. The problem, again, is that there may not be enough voltage to get a good reading.

But isn't that loading the DVM meter if there isn't enough voltage to get a good reading?

Yes true that was happening to me at work, when I was measuring In circuit resistance, my DVM meter was measuring ZERO ohms or just not picking up

If the resistance is very low because of many parallel resistors (as in a load bank), then the only way to accurately test it is with a test current designed specifically for that particular set of parallel resistors

Yes True, When I was measuring the resistors with my DVM fluke meter 87 , I was measuring ZERO ohms or it wasn't pickup up or random voltages up and down weird scramble

So these circuit boards at my work much have parallel resistance banks or its measuring low resistance or it needs a higher current to go through the traces because its loading the DVM meter to not get a good reading. The output test current is being sucked up by other components or traces in the board

 

[danny davis]

My Manager said that when he tests Semiconductors an Analog meter tests them better because it forward biases and reverse biases them at a high current

My Manager said that my Digital DVM fluke 87 meter won't measure the semiconductors because of the forward biasing and reverse biasing to know if they are working or not

I'm not sure if he is measuring Resistance of the semiconductors
and I'm not sure if he is talking about testing power transistors

I think he is using the RX1 scale on the analog meter to test semiconductors , but i guess it forward biases and reverse biases tests are better than an DVM meter

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Because when I test a diode or transistor, I use the DVM diode checker it will measure .7 volts if the diode or transistor is good

But I also measure the resistance forward bias and reverse bias and compare it will a GOOD BOARD compared with the board i'm troubleshooting

I'm guessing an Analog Meter, has a higher TEST voltage or current output when doing this

 

[chemelec]

Unless your measuring Very high voltage Diodes, Your Fluke wll test them quite well.
The Test is NOT about CURRENT.
It IS about Forward Voltage Drop and No Reverse current flow.

Your Fluke puts out about 2.7 Volts on the diode Test.
This is more than enough for most diodes.

Diodes like 1N4001 will be about .6 to .7 volts.
Schottky Diodes will be Lower, About .3 or .4 volts.

Diodes do NOT have Resistance.
They will only show some resistance reading on the meter, because the Output voltage of the meter is Greater than the Breakover Voltage of the Diode.
But the reading Means almost Nothing.

And WHY do you keep Posting this question on More than One Discussion Forum?

 

[danny davis]

And WHY do you keep Posting this question on More than One Discussion Forum?

Because there is other answers I get from other Techs that come on different forms

Your Fluke puts out about 2.7 Volts on the diode Test.
This is more than enough for most diodes.

Diodes like 1N4001 will be about .6 to .7 volts.
Schottky Diodes will be Lower, About .3 or .4 volts.

What about for power transistors? or High voltage diodes?

wouldn't a Analog meter be better for this? and what setting do you use on the analog meter since it doesn't have a diode check mode?

 

[BradtheRad]

Are there any electrolytics on the boards?

An analog ohmmeter is good for doing a quick and simple test on a capacitor.

Attach the leads. The meter swings quickly to the right (if the capacitor is uncharged). Then as the capacitor charges, the meter drifts back to the left (high ohms).

Action is slow if the capacitor has a high farad value and your ohmmeter puts out tiny current. Hence an analog ohmmeter on the x1 range will charge large electrolytics quickly, so you don't have to wait forever to find out whether that low ohm reading is a short circuit.

 

[danny davis]

An analog ohmmeter is good for doing a quick and simple test on a capacitor.

You put the meter to RX1? and you just look for if the capacitor is charging? or what are you looking for , testing the cap for what?

 

[chemelec]

When I Said: "High Voltage Diodes", I mean Really High Voltage.
I have some SHV-24 Diodes used in TV Flybacks. They are rated at 24KV and 2 mA and the Foreward Voltage drop is 75 Volts.
So the 2.7 volts from the meter is FAR TOO LOW to conduct through these diodes.

Yes the Diode test is OK for ALL Transistors. Both Power and Signal Types.
But NOT for Fets or Mosfets.

As to Testing Caps, All it shows is the Capacitor has Capacitance and a general indication as to how much capacitance.
Large Caps (1000uf), will Respond SLOW.
Small Cap (.47uF), will Respond Quite Quickly.
It May also give an indication of Leakage, But only at this VERY LOW VOLTAGE.
It does NOT really tell you if the capacitor is really Good at its Actual Rated Voltage.

 

[danny davis]

Can't a digital meter show capacitor leakage? Or only analog meters?

at my work they do test diodes using an analog meter set a RX1, Why is that?