Calibrating salvaged milliampmeters

[snuffy]

I have struggled with trying to calibrate and reuse old milliampmeters for years.  Calibrating them for a known current is tricky.  Paralleling up resisters has never worked for me.

I have finally figured out a way to do this very simply.  Starting with a 1 milliamp meter (full scale).  I attached a piece of brass model railroad track (because that's what I had) with eye terminal on it to one bolt of the meter.  A screw cable- end that will fit over it provides the calibration slide.  

Attach a small wire from the screw cable-end back to the other post on the meter.

Get your polarity right.  

Give it a known current.  5 amps in the picture.  Slide the cable-end on the brass until you get the reading you want tighten it down and your done.  

Make sure you solder all your joints to reduce resistance.  It only takes a few tenths of an ohm to screw you up.  

[TomW]

#10 copper wire works pretty well, too.

About 1 millivolt drop across one foot per amp of draw thru the wire. I figure good for 30 amps continuous and 50 intermittently.

Try it sometime.

Cheers.

TomW

[ghurd]

Pretty Slick.

I wonder about the resistance of the test leads and clips.

G-

[snuffy]

It could be done the same way with a little more care to get more accurace but I'm pretty sure it's close enough for these tired eyes to see on an analog meter.  Sure is a simple way to use up some of these old meters though.

[coldspot]

SWEET !!!

Just the type of post I needed. I had just gotten a couple of AMP meters from old dead auto battery chargers, (one goes only left to right, up. And the other is centered and goes either way).

 I'd played with them a little using a 12 VDC plug in type auto heater, (H.F.'s new rubber coated one, pretty nice little unit), and a 1 watt PV panel and a 5 watt one into very small SLA's and that isn't a good playground but did show that they do work and the effort I put into cleaning and replacing the gage covers was worth it.

Thanks!

[richhagen]

Now, in case you didn't see it, about 3 and a half years ago or so, BTHumble posted a visual basic application he wrote to reprint the back of the meter to a scale of your choosing.

http://www.fieldlines.com/story/2004/2/3/191649/8833

I have used it a few times with success.  Rich

 

[Opera House]

Having the meter connected to a current terminal is bad practice. Shunts should always be four terminal. At best a bad connection leads to errors, worst it will kill a meter.  Solder a separate connection for the meter.  Instead of the slide connection, solder a wire a wire a a distance longer than you expect to need. Then use a 100-500 ohm pot in series with the meter to calibrate.

[FrankG]

I just completed a similar project using a 16" length of AWG#12 that I coiled to reduce it's size...

http://www.theworkshop.ca/energy/dirt_e/4/meter%20tub.JPG

The coils idea worked well for me... I used a pair of clip wires to the meter and just moved them along the coil until I matched the DMM at 10Amp (1/4 scale of the 40 Amp full scale). Once calibrated I marked the spots & soldered the meter in place.

http://www.theworkshop.ca/energy/dirt_e/4/Bat%20&%20Motor.JPG

The meters are mounted in a couple of plastic tubs I fished out of our recycle bin...

The original face was scanned and modified with Corel Draw...

http://www.theworkshop.ca/energy/dirt_e/3/a_meter_face.jpg

[snuffy]

Great looking meter FrankG.  I love the new face you made.