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

[Billy Mayo]

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

 

[Billy Mayo]

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

 

[throbscottle]

So, tvtech, how much current does your Yew push out? I ask because I'd like to build a reliable junction tester and could obviously learn from your experience here!

 

[MrAl]

So, what's the consensus here as to what is considered "normal" for a good quality dmm? 1mA? 2? 3? or more? And for measuring low resistances what is considered "normal", and at what point would heating of the DUT be considered excessive? I'm seeing figures of 100mA and 130mA in this thread but that looks really high.

Hello there,

Most digital meters put out a 1ma test current and that's about all there is too it. It may be a little higher but it wont be 100ma like the Simpson 260.

The Simpson 260 is the main analog meter we're talking about, and because that has an 11.5 ohm resistor connected to the 1.5v battery, if the leads are shorted together (as in measuring a very low resistance like 0.1 ohms) then the current is simply 1.5/11.5 which comes out to about 130ma. That seems high but that is what it is for THAT meter. My Simpson 160 only puts out 50ma in that same test configuration.

So it really depends on the meter. I have a very expensive meter that only puts out about 1ma (maybe up to 10ma) but it measures down to lower resistance like 0.01 ohms. So it just depends on the meter what current is being used. You have to check the model and schematic or try to find the specs for that meter.

 

[MrAl]

Yes it has an 11.5 Ohm Resistor, But it also uses a 2,000 Ohm Meter.
So the Resistance is Actually 2,011.5 Ohms.

If it were Really just 11.5 Ohms, the 50ua Meter would be Burned up, VERY QUICKLY.

Hello again,

Sorry but you are still not getting this. The 11.5 ohm resistor is in parallel with the meter circuit, and the meter circuit when adjusted will total about 30k ohms resistance, which is very high. That 30k is in parallel with the 11.5 ohm, so when the leads are shorted (or very low ohm test like 0.1 ohms) the current will be 130ma through the 11.5 ohm resistor and 50ua through the meter. But the meter circuit and 11.5 ohm resistor are in parallel and that is in series with the 1.5v battery and the meter leads, so when the leads are shorted the current through the leads is that 130ma PLUS the 50ua. So the meter never "burns out" because it gets it's required current while the leads still get a much much higher current.

Now if we connect another 11.5 ohm resistor in series with the leads, the current drops to half that, about 65ma, and that means we only see 25ua through the meter. That gives us the half scale reading which is around 12 ohms.

So you see the meter leads get 130ma through them while the meter only receives 50ua, so all is well with the meter movement and it does not burn out just because the leads have a higher current.

This is a very common scenario too. A higher level current is measured with a much much lower current meter like a 50ua meter. In fact, 50ua was a very popular choice for many current levels using the appropriate shunt resistance (like 11.5 ohms or even less for higher current measurements).

If we wanted to measure 10 amps for example, we would chose a shunt resistor that would match the application like maybe 0.15 ohms. Now with 10 amps flowing through 0.15 ohms we see a voltage drop of 1.5 volts. With 28k in series with the 50ua 2k meter movement and that in parallel with the 0.15 ohm resistor, we'd get 1.5v/30k which equals 50ua again. So we manage to measure 10 amps with a 50ua meter movement. And that technique is nothing new but has been used for many many years.
How do you think it would be possible to measure high current with a 50ua meter movement in that same meter even. Same thing there, using a shunt resistance to shunt most of the current while measuring just a small sample in a parallel circuit.

 

[chemelec]

OK, I See the Parallel Circuit now.

But I would think that this Higher current on Low Resistances (Less than 1 Ohm) would cause Greater Errors in Contact Resistance.

A 4 Wire System is MUCH Better.

 

[ChrisP58]

Just lumping all multimeters into the two categories of analog or digital is making a gross oversimplification. According to the specs on my bench digital meter, an Agilent 34401A, it will display down to 0.1 milliOhms and uses 1mA to do so. Of course, you have to be using the 4 wire connections to get a reliable number there.

However, when I am measuring anything that low, like the winding resistance of a switch mode transformer, I prefer to use a higher current. Then I set my bench supply to be a 1 amp current source and apply that to the unknown resistance an use my bench meter to measure the voltage across that resistance. But it is important to do this test quickly, or the resistance will change due the the excitation current.

I think that the knowledge that we need to learn from this thread, is that it is important to properly choose the test equipment to match the testing to be done. What is right for one circuit may be dead wrong for another. A low excitation current from your meter may be important if you need to reduce self heating in one circuit, but won't adequately bias a transistor junction in another.

 

[Reloadron]

I think Analog meters can measure AC voltages at Higher frequency's than a Digital volt meter

I think my fluke 87 DVM meter can only measure AC voltages at maybe 1khz max.

I think you should read the Fluke 87 and the Simpson 260 manuals with a focus on specifications.

Most analog meters like the Simpson 260 for example are Average Responding RMS Indicating when measuring AC. That means the meter responds to the Average value of an AC signal being measured. That means unless the AC Waveform is a nice clean pure sine wave the meter will not correctly respond to it let alone indicate it. Next, even with a nice pure sine wave the Simpson 260 is reliable to around 10 KHz depending on AC Range selected. Additionally the Simpson 260 AC Sensitivity is: 5,000 ohms per volt. The Fluke 87 is a True RMS Responding meter. It responds to the True RMS value of a measured signal. Therefore the signal measured need not be a pure sine wave. The uncertainty is +/- 1% up to 1 KHz and between 1 KHz and 20 KHz it is about a 2% meter. However, if you look at the error specifications for the Simpson 260 the error is specified as % full scale while the Fluke 87 is specified as % RDG.

Ron

 

[tvtech]

It's at stages like this I bail out of Technical threads..

I say: An Analogue Meter is better at testing forward biasing of Semiconductor junctions. An Analogue Meter will normally reveal a failing Junction.

And yes, obviously on X1 on the Analogue. Quote me if you want. I will have maybe taught people something.

That's all.

tvtech

 

[Reloadron]

It's at stages like this I bail out of Technical threads..

I say: An Analogue Meter is better at testing forward biasing of Semiconductor junctions. An Analogue Meter will normally reveal a failing Junction.
And yes, obviously on X1 on the Analogue. Quote me if you want.

That's all.

tvtech

Agree with that.

I have a few analog meters I still use as well as a few digital meters, all in the hand held flavors. I figure it this way, when it comes to test or measure. You choose the instrument that is best suited for the application. Knowing how to choose comes with experience.

Ron

 

[tvtech]

Agree with that.

I have a few analog meters I still use as well as a few digital meters, all in the hand held flavors. I figure it this way, when it comes to test or measure. You choose the instrument that is best suited for the application. Knowing how to choose comes with experience.

Ron

Thanks Ron

The only reason why I post here is try and help people stop making the mistakes I made as a tv tech. I have been around the block many times. I have found easier and better ways to fault find. That is why I am the way I am.

So, maybe people should just listen???

All the best,
tvtech

 

[Billy Mayo]

My flocked 87 can't tell when an AC voltage is in phase or 180 degrees out of phase, it doesn't show a + or - for the polarity of a AC voltage

which meters do?

 

[cowboybob]

My flocked 87 can't tell when an AC voltage is in phase or 180 degrees out of phase, it doesn't show a + or - for the polarity of a AC voltage

which meters do?

( Do you mean a "Fluke" 87?)

DMMs can not detect phase.

AC does not have a static "+" or "-" condition. Thus DMMs can not "show" an AC polarity.

 

[Billy Mayo]

my test fixture at work can output an AC voltage in either polarity. i can have a negative ac voltage at -5volts

 

[Billy Mayo]

The AC voltage is Inverted by 180 degrees , you can see it on the oscilloscope but on my Fluke 87 meter it doesn't let me know if my AC voltage is out of phase or in phase

Which DVM meters do this ?

 

[ChrisP58]

The AC voltage is Inverted by 180 degrees , you can see it on the oscilloscope but on my Fluke 87 meter it doesn't let me know if my AC voltage is out of phase or in phase

Which DVM meters do this ?

Out of phase with respect to what?

I am not aware of any standard DVM that has any kind of phase indication. A scope is the right tool for that kind of work.

But again, phase is a timing relationship between TWO signals of the same frequency. So any phase measurement device has to be looking at TWO inputs.

 

[Billy Mayo]

NO, not phase

It's an 180 polarity switch on an AC output voltage , you can flip the polarity

So out of phase would start on the negative cycle not the positive cycle , that would be in phase AC

At my work , they do check the resistance of the junctions of diodes to test if the junctions are not leaky or has leakage

The diode checker my manager said, doesn't tell the diodes leakage or if the junction has conductance or a partial short

An Analog meter will show these things right?

 

[cowboybob]

NO, not phase

It's an 180 polarity switch on an AC output voltage , you can flip the polarity

So out of phase would start on the negative cycle not the positive cycle , that would be in phase AC...

All this means is that the sine wave output of your test fixture starts the output on the negative going portion of the sine wave. After the initial 1/2 wave, the signal is indistiguishable from any normal sine (or other shape) AC signal. Your DVM (DMM, whatever) recifies the incoming AC (thus eliminating the negative going portion) and is designed such that the RMS value is what is then displayed.

Now if your sinewave was, for instance, slow enough, (say, 1 to 10hz) your meter (analog or digital and IF set to DC) would display a polarity . Try it. But once you select AC as your input for the meter, it will never display a polarity.

And, as ChrisP58 points out, phase issues only relate to two (or more) AC signals and their relationship to each other as to their starting (or instantaneous) measurement points.

Do not confuse what an o-scope displays and what a DMM is capable of displaying.

...At my work , they do check the resistance of the junctions of diodes to test if the junctions are not leaky or has leakage
The diode checker my manager said, doesn't tell the diodes leakage or if the junction has conductance or a partial short
An Analog meter will show these things right?

First, understand that an analog or digital meters, essentially, give you indentical values when testing electrical characteristics of a circuit or component (albeit with differing accuracy).

With regard to leakage or partial short anamolies of a diode (one in the same), without a more sophisticated test bed than a simple resistance test, thay cannot be detected with a VOM. Often, though, some devices have a transistor/diode checker feature that can display the more subtle characteristics of a simple solid state device.

 

[Billy Mayo]

I can't see the AC waveform 180 because i need to use the ext. trigger input , but what do i use as a reference for the oscilloscope in order to see the AC waveform out of phase? the ac waveform is to fast to see the polarity switch from 0 to 180 , so i need to use the ext reference input on the oscilloscope, but what do i use as a reference?

 

[Nigel Goodwin]

I can't see the AC waveform 180 because i need to use the ext. trigger input , but what do i use as a reference for the oscilloscope in order to see the AC waveform out of phase? the ac waveform is to fast to see the polarity switch from 0 to 180 , so i need to use the ext reference input on the oscilloscope, but what do i use as a reference?

Use whatever it is you're wanting to check the phase against - as you've been told repeatedly, phase is purely relative, so you MUST have something to compare it to if you have a need to know the phase.