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

[chemelec]

I REALLY DOUBT that the Simpson Meter puts out 100 mA of Current. Especially Considering it has a 50uA Meter,
The Schematic of it Does Not appear to allow that either.

As I PREVIOUSLY SAID:
On the Analogue Meter I Have, Measuring on the Lowest Resistance Range, (0 to 10 Ohms) the Maximum Current is just 3.1 mA.
THIS IS ALSO A 50uA METER, And it Also has 10 Divisions between 0 and 2 Ohms.

If you are just trying to check Traces for BREAKS, ANY Digial Meter will do that.

If you are actually trying to test RESISTORS on the PCB, You MUST Disconnect ONE END of the Resistor, to get a CORRECT READING.
Or you can be measuring Other Series and Parallel resistors on the PCB.

 

[MrAl]

I REALLY DOUBT that the Simpson Meter puts out 100 mA of Current. Especially Considering it has a 50uA Meter,
The Schematic of it Does Not appear to allow that either.

With regard to your own meter:
THIS IS ALSO A 50uA METER, And it Also has 10 Divisions between 0 and 2 Ohms.

Hello there,

I didnt realize there was a law passed by Congress that says that we have to have resistance meters that put out only a certain level of current and no more than that.
Just kidding of course, but there are a couple things wrong with your statements above.

First, the meter movement has nothing to do with the output current during a resistance test. A 50ua meter movement is quite common or at least was in the past, and can be used with any current from 50ua up to ten thousand amps or more, although i think few of us have used it up that high. It is the basic movement used in lots and lots of analog meters of all kinds. The current range is adjusted via a shunt resistance, and that was a very typical thing to do before maybe around 1980.
So it really doesnt matter if your meter has a different output current even if other things about it are similar, and im sure we could find other meters that put out different currents than that too. The Simpson 160 puts out 50ma with very low resistance tests as it has 1.5v and 30 Ohms on the Rx1 scale, and it also uses a 50ua meter movement. We could certainly build one from a 50ua movement that does any current we care to measure.

Second, the schematic clearly shows a 11.5 Ohm resistor in series with the battery and the battery is 1.5 volts. Did i read this right or are my eyes too tired right now? Check for yourself on page 17 i think.
Anyway, a 11.5 Ohm resistor and a 1.5 volt battery mean that if the leads of the Ohm meter are shorted together (very low Ohms under test) the current flow will be 1.5/11.5 which comes out to around 130ma.
130ma is surely greater than 100ma which i quoted previously.
Now if different models put out a different current that's not my concern unless someone comes up with a different schematic and he measures his short circuit current and it comes out to be less, and then we would just have to update our notes. But going by the only schematic we have available right now, it puts out 130ma. If you dont agree then simply state your reason and i'd be happy to look into this further.

 

[Nigel Goodwin]

I REALLY DOUBT that the Simpson Meter puts out 100 mA of Current. Especially Considering it has a 50uA Meter,
The Schematic of it Does Not appear to allow that either.

Never heard of Simpson?, but the old Avo meters (Avo 8 etc.) used to have two batteries - a special 15V one for the high ohms ranges, and a D cell for the normal and low ohms ranges. A large D cell was required because on the lowest ohms range it pushed substantial current through the load.

 

[Billy Mayo]

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?

 

[chemelec]

Nigel, Shown in #15, The Meter they are discussing.

old Simpson 260 Analog VOM manual.

According to the Schematic, this one uses Four, 1.5 Volt Batteries.

 

[Billy Mayo]

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?

 

[MrAl]

Hi,

The higher the test current, the higher the measured voltage across the resistor and thus the better the measurement, all other things equal. For low value resistors a higher test current is better because we get a higher voltage to measure.

100ma would be better than 1ma for testing resistors in parallel given that they are low value resistors themselves. The only way to test without taking them apart though is to calculate the resistance going by the color code or package stamping, and then test with the meter to see if the combined resistance is correct.

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.
You may be able to use a digital meter with the 260 to get more accurate readings however, by knowing the current that the meter is putting out at the time. But that would require a second meter to measure the current.

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. 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 (the load bank).

What i would suggest that you try is to use a simulator program to simulate a circuit with several parallel resistors and a small test current, adding more and more parallel resistors and see what happens to the voltage. Then try to figure out how you would measure the resistance in the real world by measuring the voltage and by using a meter. Compare the two methods to see what the difference is. Also try different test current levels.

 

[Billy Mayo]

Thanks Mr. Al for your time and info on this, this helps me a lot at work know why and how to use this meter

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

 

[MrAl]

Hi,

You can call it loading if you want i guess, but i dont think that is a good description of what is happening. The DVM simply can not measure resistance low enough to measure those values you seem to want to be able to measure accurately.
Most DVM's will read values like 1.2 ohms, 1.3 ohms, etc., but only as low as 0.1 ohms. So if you have a 0.050 ohm resistor you will find the reading to be questionable because it may read 0.1 ohms or 0.0 ohms, or bounce between 0 and 0.1 ohms. I did not look up your meter to see what i can do though.

But for the best results you have to make your own test current and measure the voltage and do the math. that's the only way. You should do it such that you get at least another 1/2 digit more than what you really want to measure. So to measure 0.5 ohms you should have a meter or test setup that can read with resolution of 0.05 ohms at the very least. For best results though another full digit would be good, so for that same 0.5 ohms you would want a meter or test setup that can read with a resolution of 0.01 ohms.
This works for many applications except those that have to be extremely accurate, and then you have to go for another digit (0.005 ohms resolution to measure 0.5 ohms).
You can determine the required resolution by the percentage it will be off if there is an error equal to the resolution. So for measuring 0.5 ohms with a resolution of 0.05 ohms, the error could be 10 percent, which is sometimes too large. With a resolution of 0.01 ohms, the error could be 2 percent, which is much better. With a resolution of 0.005 ohms the error is down to 1 percent which is even better yet.
This assumes that the basic measurements are accurate too.

 

[Billy Mayo]

You can call it loading if you want i guess, but i dont think that is a good description of what is happening.

Since it's not called Loading, What would you call it?

 

[MrAl]

Hello,

Well, since the DVM can take a short without really overloading i would call it just inappropriate test equipment. Just because it cant read down that low does not mean it is overloaded. It just does not have the capability of reading below a certain value that's all.

When a meter tries to read a voltage that is too high for it we usually say is over ranged. But when it can not measure a voltage that is too low we simply say it does not have the sensitivity to measure the required voltage.

 

[Billy Mayo]

True I have heard those seems also

So the DVM meter is not sensitivity to measure the required voltage

I get it now

Thanks Mr. Al

 

[chemelec]

Second, the schematic clearly shows a 11.5 Ohm resistor in series with the battery and the battery is 1.5 volts. Did i read this right or are my eyes too tired right now? Check for yourself on page 17 i think.
Anyway, a 11.5 Ohm resistor and a 1.5 volt battery mean that if the leads of the Ohm meter are shorted together (very low Ohms under test) the current flow will be 1.5/11.5 which comes out to around 130ma.
130ma is surely greater than 100ma which i quoted previously.

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.

 

[chemelec]

True I have heard those seems also

So the DVM meter is not sensitivity to measure the required voltage

I get it now

Thanks Mr. Al

It also is True you need a Higher Current to measure Lower Resistances, as this increases the voltage being measured
But for really Low resistances, You also need a 4 Wire Ohm Meter.

With Just a Normal 2 Wire Ohm Meter, the Contact Resistance between your probes on the circuit being Tested, Can Cause a LARGE ERROR in the resistance Reading.

 

[tvtech]

Hi

As many here that have followed my posts regarding Analogue vs Digital for fault finding on Semiconductor Junctions....
An Analogue is the master. An Analogue "loads" the forward biased junction. Specifically good at that on the x1 range on Ohms...

I don't argue with people anymore or discuss it. My Fluke 75 I use for new Projects and accurate Voltage and Current measurements. Never on my repair bench.

My Yew though, stays permanently on my repair bench. That is the only home it has ever known. And has helped me through thousands of TV repairs with much success where the Digital guys miss failing junctions.

That's all

tvtech

 

[throbscottle]

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.

 

[tvtech]

Hi

As many here that have followed my posts regarding Analogue vs Digital for fault finding on Semiconductor Junctions....
An Analogue is the master. An Analogue "loads" the forward biased junction. Specifically good at that on the x1 range on Ohms...

I don't argue with people anymore or discuss it. My Fluke 75 I use for new Projects and accurate Voltage and Current measurements. Never on my repair bench.

My Yew though, stays permanently on my repair bench. That is the only home it has ever known. And has helped me through thousands of TV repairs with much success where the Digital guys miss failing junctions.

That's all

tvtech

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.

Hi throb

You missed my point. Digital no matter what quality is not able to do the raw work that an Analogue can do with detecting forward biasing problems. Today again, a set came in with Frame Collapse....my genius Technician (has been shown here before) with his state of the art Fluke...started changing the Frame IC...

I asked him "Have you checked the 1n4007 feeding the IC....He says yes. I say take it out and give it to me. I want to see what my Yew says".

Fail. Please don't change the Frame IC. Please put it back. Tighten it and make it like it was.

This is what makes me tired. It is like hitting a wall and using my head as a hammer.

I never post crap tech advise...only what works for me.....and then people think they know better....

Why test me?

tvtech never requests advise here. Never asks someone to bail him out. Once I can remember Alex trying to help. We failed. Set was thrown away.

Only Nigel will understand. Maybe.

That's why I totally loose it sometimes....

Regards,
tvtech

 

[large_ghostman]

if your going to be measuring stuff that low get a 5 1/2 digit bench meter that can do 4 wire,

sorry did it again!! this is GM11 i didnt log the lad off! i will log in under my own lol

 

[picbits]

Hey - Big GM ! Just use the switch users function when you've finished (if you're using a Windows machine)

Windows and the "L" key will boot you straight into the lock screen so you don't forget.

 

[Billy Mayo]

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.