This morning I was using two DVMs to measure voltage and current in a circuit . The current readings were impossible so I checked the 9 volt battery which was down to 6 volts. I replaced it and assumed everything was good again. But not so. I finally put all four meters I have (one an old analog) in series and connected a fully charged LiPO cell and a 450 ohm resistor. Three of the meters read 8.4 MA but the cranky one read 30.something!

So I have four questions--is this correctable? Is it common? Is there a known probably cause? And how can it be right on voltage and wrong on current unless the current measuring resistor is suddenly wrong?

 

Those of us how learned electronics using analog meters learned that meters can affect the circuit being measured. Make sure this isn't happening in your case.

It would be helpful if you gave information on the meters; the shunt resistance in particular and the range being used. Also whether the "wrong" reading was given by the analog meter.

 

Those of us how learned electronics using analog meters learned that meters can affect the circuit being measured. Make sure this isn't happening in your case.

It would be helpful if you gave information on the meters; the shunt resistance in particular and the range being used. Also whether the "wrong" reading was given by the analog meter.

Three meters, including the culprit, are digital. One is analog. They are in series. It does not matter what the shunt resistance is--the same current flows through them all. The analog meter read about 8ma (not too easy to be precise with those meters.) Two of the three digital meters read 8.4 ma. Given my voltage and resistance that seems dead on. The other read over 30 ma.

And I bet I grew up with analog meters before you did.

 

It does not matter what the shunt resistance is--the same current flows through them all.

Each meter will have some resistance because they aren't ideal. Having four current meters in series could give a different answer than one.

What purpose would having multiple current meters in series have?

 

Each meter will have some resistance because they aren't ideal. Having four current meters in series could give a different answer than one.

What purpose would having multiple current meters in series have?

Dennis, I don't mean to be cross, but it has been a bad day with a lot of time wasted due to a faulty meter. Now any combination of resisters place in series and connected to a single current sourced will ALL have the same current through them. It doesn't matter if each meter is different.

My original post explains why I put them in series--I wanted to find out if my meter really was giving the wrong readings or if, by some misteak I had used the meter incorrectly or my circuit was not what I though it was or that some spirit had magically made more current flow than Mr Ohm predicted. I found the bum meter. I'm asking the local wisdom what might have damaged the meter and if such damage is common.

Boy--it has been a bad day--an ed left of off "placed"; an ed incorrectly added to :sourced" and a misspelled mistake. Tools are put away and I'm settling down with a glass of red,

 

First you have to feel lucky, if 2 meters would have read 8.4mA and the other 2 read 30 something then the typical whitehaired man who believes in democracy could have had a bigger dilemma on his hands.
If once in the past you left the current meter on for a long time then the shunt resistor inside got hot and changed value. You can check that by looking inside.
The shunt resistors are below ratings because you normally measure current for a short time.

 

First you have to feel lucky, if 2 meters would have read 8.4mA and the other 2 read 30 something then the typical whitehaired man who believes in democracy could have had a bigger dilemma on his hands.
If once in the past you left the current meter on for a long time then the shunt resistor inside got hot and changed value. You can check that by looking inside.
The shunt resistors are below ratings because you normally measure current for a short time.

Could be--I'll look tomorrow. Thanks. And yes, 2 at 8.4 and 2 at 30 something would have called for more than a glass of red.

 

Most DVM's measure the voltage across a fixed resistor (the resistor varies by range).

Many also have fused low ranges (e.g. < 2A) and unfused 10 A ranges that's supposed to be used for a short time. The wacko one could have a blown fuse.

 

Most digital DVM's need 20mV for full scale reading. For 20mA range the resistor is 1 ohm. Yours will be 4 ohm. Look for a resistor that looks charred.
Don't forget tonight to leave some in the bottle for the celebration.

 

Measuring the shunt resistance of the bad and good digital meters when set to the same current range may tell you something.

 

Most DVM's measure the voltage across a fixed resistor (the resistor varies by range).

Many also have fused low ranges (e.g. < 2A) and unfused 10 A ranges that's supposed to be used for a short time. The wacko one could have a blown fuse.

If any of them had a blown fuse (on that range), then no current would have flowed in the circuit at all and all should have read zero.

 

This morning I was using two DVMs to measure voltage and current in a circuit . The current readings were impossible so I checked the 9 volt battery which was down to 6 volts. I replaced it and assumed everything was good again. But not so. I finally put all four meters I have (one an old analog) in series and connected a fully charged LiPO cell and a 450 ohm resistor. Three of the meters read 8.4 MA but the cranky one read 30.something!

So I have four questions--is this correctable? Is it common? Is there a known probably cause? And how can it be right on voltage and wrong on current unless the current measuring resistor is suddenly wrong?

Where did you get a 450 ohm resistor? That's not a standard resistor value in any of the preferred series.

But, assuming that's the value and that your 8.4 mA is correct (note that 8.4 MA is 8.4 megamps, which is probably not what you meant), then I'm assuming your LiPo cell is 3.6 V? Without looking it up, that sounds familiar.

That would be consistent with all of the meters having current-sense resistors below about 4 ohms (1% of the resistor used), which is reasonable, particularly for the DMMs. The meter reading 30 mA (or about 3 to 4 times too much) would appear to still have a fairly low resistance, but something has happened to mess things up. Try using a different range and see if you get something that is more consistent with the 8.4 mA or the 30 mA. If the former, then the current sense resistor is probably damaged. If the latter, then the circuitry that reads the voltage across the current sense resistor is either damaged or out of calibration. Whether or not it is repairable depends on the specifics of the meter.

 

3.6v is the nominal voltage of a lipo. A fully charged lipo is 4.6 to 4.7v (or multiples thereof).