Testing a Panel Meter
HOW TO TEST A PANEL METER 
 for the newcomer, casual tourist, or experienced tech.

**disclaimer - until this note vanishes, this is still a work in progress.  Use at your own risk**

Testing, verifying, or measuring a panel meter is a topic that often comes up when trying to test, repair, or restore a vintage electronic instrument. The following pages will attempt to address this simply and directly, in order to cover the majority of meter movements that one is likely to come across as a casual tourist. At the END of the paper is the math behind the testing (ohm’s law applied) for the curious (in other words, what you are really doing). For those that want to jump right to testing and read later, go ahead, but heed the WARNING there or risk sacrificing your meter to the god of smoke.

The tests below will safely tell you what the sensitivity in uA or mA the meter under test is, within the accuracy of your own DVM. The precise voltages and resistances referred to for these tests is relatively unimportant … your DVM is the precision part of this test circuit.

A BIT OF THEORY - how they work”– skip this if you just want to get to the testing phase.

First of all, it is important to realize that most, if not all, of the panel meters you will come across in these types of instruments, receivers, transmitters etc, are CURRENT measuring devices. It makes no difference what the scale says, or in what units it is calibrated. At its most basic, the meter movement is simply measuring current. The equipment that uses this panel meter is modifying whatever is being measured inside the equipment or workbench, into some value of current that this panel meter can display.

The basic sensitivity of a panel meter is therefore expressed in current, whether it is microamps (uA), milliamps (mA), or amps (A). For example, if you have a meter movement that is manufactured to be, say, a 50mA meter (50mA sensitivity), then it will deflect to full scale if a current source (the instrument it is in) provides it with 50mA, no matter what units the scale is in. (volts, ohms, degrees F, watts) …. whatever. It’s measuring current no matter what the scale on the meter movement says.

If you KNOW the sensitivity of a certain meter you want to test, (let’s say 50mA), you can simply provide it with exactly that, and if it deflects to full scale, it is at least somewhat working and not DOA. If you want to go one step further, you’d want to make sure it is also linear, which means that for every mA change in source current, the meter deflects exactly 1mA further upscale. More comments on this and other refinements after the testing section, which is why you tuned in and I feel I may be in danger of losing your interest.

ONWARD TO TESTING. This section will be followed up with some refinements and further comments for those interested.

There are a few simple, and basic, ways to test a panel meter. WARNING: DO NOT USE AN OHMMETER ACROSS THE METER TERMINALS to either see if it is “open” or what its resistance is. Unless you happen to have used a modern DVM that sources very low current, you may have just re-invented the legendary “Flashbulb Tester” …. You’ll be able to say. “yup, that one was good” before you flash it into oblivion.

So, here is how to do it. You will be supplying your meter under test with a variable current, while simultaneously observing its full scale deflection, and verifying the value with your DVM hooked in series.  You can do this with either of the 2 following setups.  In using either of these test setups, you will be connecting all of the components in series. (CAD drawing to be included eventually). If you start the test (SLOWLY!! please), and the meter under test starts to deflect backwards, simply reverse the leads to it. Ditto with your DVM.

Step 1 for either setup is to use the mechanical zero screw on the front of your meter, to set the needle over the zero (usually left hand) mark on the dial scale.

METHOD 1. Requires a fixed DC supply (a 1.5V battery will work), a 5,000 ohm potentiometer, a 100 ohm fixed resistor, a DVM, and a few clip leads.

The fixed resistor is there to protect the meter in case you accidentally connect your 5K potentiometer backwards (zero ohms).  The 100 ohm resistor will limit the current to your meter under test to a (hopefully) safe level.  If the meter happens to peg right, disconnect immediately and see what you connected wrong.  Try not to make this error, or you may awaken the smoke diety.

1. Set the DVM to read milliamps. Pick the scale with your “best guess” as to what the meter under test is … you can change your DVM* scale if it does not read anything because you set it too high. I’d start with perhaps a 200mA scale if it has one.

2. Set the 5K potentiometer to MAXIMUM, in this case, 5K ohms. Don’t hook it up backwards (0 ohms) or you risk applying enough current to the meter under test to smoke it.

3. Complete the circuit: Hook the battery, your DVM, the potentiometer, the fixed resistor, and the meter under test in series.

4. Very slowly, rotate the potentiometer towards lower resistance, and note that the meter under test has started to deflect upwards. Your DVM should also be indicating a similar reading, but probably not precisely the same reading.

5. When you reach full scale deflection on your meter under test, stop rotating the potentiometer. That’s all the meter can handle. Your DVM should now be reading 50mA, if your meter movement is 50mA … etc. **


METHOD 2. Requires a variable DC power supply, 0-15 volts will suffice.  A 100 ohm fixed resistor, a DVM, and a few clip leads. It wouldn’t hurt if this DC supply also has adjustable current limiting built in, but it’s not required if you are careful.

1. Set the DVM to read milliamps. Pick the scale with your “best guess” as to what the meter under test is … you can change your DVM* scale if it does not read anything because you set it too high. I’d start with perhaps a 200mA scale if it has one.

2. Set the variable DC supply to zero.  If it has a variable current adjustment, set that for just slightly over what you believe is the maximum rating of the meter under test.  You can increase this while testing, if the meter stops deflecting during test due to the current limit kicking in

3. Complete the circuit: Hook the power supply, your DVM, the fixed resistor, and the meter under test in series.

4. Very slowly, increase the DC voltage, and note that the meter under test has started to deflect upwards. Your DVM should also be indicating a similar reading, but probably not precisely the same reading.

5. When you reach full scale deflection on your meter under test, stop increasing the DC voltage. That’s all the meter can handle. Your DVM should now be reading 50mA, if your meter movement is 50mA … etc.

Notes:
(*) you can use a VOM instead of a DVM, but the DVM is probably going to be far more accurate especially for more sensitive meter movements under test.

(**) If, upon rotating the potentiometer all the way to zero, or the power supply to maximum 15V, and your meter under test still does not deflect to full scale, you can reduce the 100 ohm resistor to 50 ohms and try again. This would indicate that your battery is weak, the meter under test is open or sticking, or it’s sensitivity is less than your setup can supply, and you’ll need more voltage.  


NEXT SECTIONS COMING SHORTLY:
Short explanation of Ohms Law
How it applies to this test (mathematical explanation of what you are testing)
What if any the resistance of the meter movement itself contributes to this equation
Notes on meter movements that might have internal shunts or rectifiers

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