Re: Radio Shack Ammeter

Mark Mathu spake thus:

Whatever. I say "good riddance" to that chain, which I've always hated. Too bad it leaves so many places without any retail electronics store. (I'm lucky to live in an area that still has some *real* old-line electronics places. Just to give one example, I found an exact replacement drive belt there for my 25-year-old cassette deck there last week.)

Wierd Stuff!!

The ampmeter movements are usually center zero movements so that they can read either plus or minus currents. Your method works fine for a unidirectional ampmeter. For the shunt, you want to use some length of 20 gauge wire if you're going to be measuring a few amps. It is better to use a longer length of wire than a smaller wire of shorter length as this will help to dissapate the heat when a short occurs. For a 1 amp resistance, I'd not go under 28 gauge wire. The technique is to put a length of wire across the terminals of the meter movement and run enough current through the meter to move the needle 1/2 way (use a 10 ohm resistor or larger to reduce the current from your power pack - 1 ohm at 1 volt passes 1 amp of current 10 ohms at 1 volt will pass 1 tenth of an amp and so forth) and measure the voltage and/or current with a multimeter - current measuring being more accurate than measuring the voltage as the resistor may not be exactly 10 ohms but may be 10% more or less than that. Adjust the length of wire so that the meter reads what you want it to and things will be fine. Note that you want to set the meter so that you can't pass more than 200% of the max. reading when the power supply is directly shorted or you will be wanting to put a fuse in the line so that the meter won't be burned up. Anything more than 2x the max. current tends to fry the really fine wires in the movement of the meter.

-- Yeppie, Bush is such an idiot that He usually outwits everybody else. How dumb!

You might want to check with places like Mouser, Digikey and other electronic parts stores that are online. Radio Shack generally buys parts from other makers and puts them in their packaging and thus, the meter design is usually available from other sources, often at lower prices than RS. Then again, you bought the first meter at a garage sale so you probably paid very little for it.

-- Yeppie, Bush is such an idiot that He usually outwits everybody else. How dumb!

I got four of them for $1.00, still in the plastic packaging and they included a 15k ohm resistor to put in series with the voltmeter across the power pack lines. I think I am going to try to convert two of them to ammeters using your and David's information - just to try it. Thank you.

Really? We had this guy in our club who had a cab control layout. But instead of using "store bought" toggles, he used a 2x4" with a bunch of nails in it, each with a bare wire wrapped around it connected to the track. He then used a pair of alligator clips from his power supply to clip on the proper nail for the block he wanted to power. It worked...but I wouldn't say it's "cooler". More like "primitive" or "cheap", but to each their own.

Paul A. Cutler III

I believe your trying to make an Amp meter out of a volt meter. General Motors did this in the late 60's by balancing the resistance of the battery feed wire to the alternator to act as the shunt that would be used in an ampere meter.

Just rambling with the thoughts of a retired auto electrician.

Here is an other crude method of controlling twin coil switch machines.

I called this the magic wand system. The two outer wires from each machine coil went to a hole for the corresponding route on the control panel. A round head brass screw was installed into the hole to act as a terminal and also effect a contact with that wire. The third wire was run through a gutted out ball point pen to act as a wand (handle).A little bit of bare wire now replaced the ball area of the old pen. All you had to do was touch the wand to a screw on the panel and you completed the circuit to the switch machine.

Hi Phil, If you look up to the top of this thread, with Dave's help, that is exactly what I was attempting. The thing worked out great. I have much better scaling now than I did with my old ammeters and all my voltmeters and ammeters now match.

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This circuit was made using the meters that you have.