I have two separate problems with my 5 digit panel meters so I am posting two threads.

I bought two nice 5 digit 0-100 microamp meters. Some weeks ago the meter did not display the 5th digit—just went out—and the reading that did show was off by somewhere around 2 but not exactly 2.

I tried really hard to go back in my mind to find what I had done and the only thing I could think of is that I might have brushed the +5volt power lead (these are 4 wire meters) with 8 volts.

So I put the other meter in the circuit and treated it with great care, never to overload it. To be sure that I could not burn out the power I added a 5 volt regulator and insulated all the points that provided power to the meter

This morning the second meter displayed the same symptom. Now, I know this isn't magic; I know I have done something stupid (twice). I do know that this meter did NOT get 8 volts where it should get 5. But I don't have a clue as to what.

Any advice?

 

Hello,

Can you show the meters you bought? What's the model?

 

Any advice?

Maybe after you provide meter model numbers and how they were connected.

 

I apologize for the careless wording of my question; I was so angry with myself for burning out two meters that I did not take time to think about what I really want to know.

Let me try again. Is anyone familiar enough with dual slope integration panel meters to know the typical failure modes? Is it common for abuse to cause only one digit to go blank? In my case, both times the least significant digits went out--typical or just a fluke? What kind of abuse causes this--serious over voltage or over current? Or a power supply over 5 volts?

There is no sense in my showing the circuit or the panel meter number. The circuit is wired so a slip of a meter probe might cause over current. The meter is generic Chinese. If you care to read my other post you'll see what I did to solve my problem with the new meters.

Thanks

 

There is not enough information for an answer to your question without a schematic and the data sheet on the meter modules. My suspicion is that the power supply to the meter is NOT isolated from the input connections.

Les.

 

I am going to take a wild guess -your panel meters have a common materials or factory defect so they both suffered the same failure.

Don't be angry with yourself -be frustrated with the meter supplier, it will be more constructive.

 

Both worked correctly for weeks. I have poor small muscle control so my probes and screwdrivers twitch. The circuit isn't going to help anyone. I'm certain it was not a set of faulty meters. I'm not out much--about $12 each.
I've gotten over it but---

I found that each one, with a proper shunt, makes a decent 4 digit 0-.9999 uamp meter. I adjust the shunt by trial and error while the meter is in series with my DMM on the 2mA setting. Not precise, but as precise as my DMM. Obviously, I could get a 0 to 9.999 mA meter or a 0 to 99.99 maa mA meter.

The range is odd since the first digit in the unblown meter was 1 at the most but now it is 9.

 

you really need to post info on what you have there - what meter, how is it connected, etc.
note, you may have damaged it even while experimentally determining shunt value.

 

No, I don't need to show the circuit; I accept the fact that I burned the meters out. The meters are inexpensive generic, Chinese meters; even if they were defective I would have no real recourse.

All my talk of shunts comes after I burned the meters out; as they came they read exactly in the range I need.

I made shunts for the blown meters merely to see if I could salvage them for some future use. I made shunts for the mis-labeled meters to see if I could retrofit them to be of use in the first circuit. In both cases I was successful.

 

It almost sounds like you've blown the ADC / display chip. Something to keep in mind is that cheap meters (DVMs too) don't have a lot of input protection. If your circuit contains relay or other coils without a catch diode, the back EMF is plenty to fry a low cost meter. I had a client go through 3 'looks-like-Fluke' DMMs measuring a 48VDC contactor. The ~700V spike on release was more than enough to fry the 450V rated protection. Just grasping at straws without more info but something to think about.

 

JohnInTX thanks. My circuit doesn't but it is good to know for the future.

 

the ' .. with great care, never to overload it ..' does it mean that you allocated additional circuit components to prevent accidental reverse polarity? or that you are certain the polarity was not accidentally reversed?
Mechanical shocks, vibration, dust can also inadvertently damage circuits.
Once I met a guy who killed his own CPU, an early Intel Pentium, or so he claimed, because he held a 60W light bulb really close to the CPU, because he had to read the specs printed on the chip .. and left the light right there, one inch on the CPU, for enough time to damage the chip.
So, are those 100mA meters back to work?

 

John, I just meant that I was extra cautious the second time around. The way they are set up I could not have reversed power either to power the meter or on the meter's input. And I know I didn't beat them up. But, as I said earlier, my fingers tremble; my small muscle control is poor; I don't see detail as well as I once did. Somehow I must have used a screwdriver or meter probe to apply too much voltage.

The 100 ma meters are in the storage box awaiting duty sometime in the future.

I'm pretty sure I'm on track to forcing those mis-labeled meters into service. I can't find much on google about 75 millavolt meters used with shunts for very large ammeters, but I am pretty sure that's what I got. I vaguely remember in my youth reading about a 75 millivolt standard measurement.