custom circuit boards

[ctartist236]

I need some help. I need to find a circuit design for a digital ampmeter. Does anyone know where I can find one? Better yet, does anyone have one? We are looking to test up to 200 amps. I need all the help I can get.

[bigqueue]

Someone noted that you can measure the voltage drop across a measure of wire. Yes, you would have to probably make that wire long to get a high enough resistance to measure.....but it is possible.

Now, why not just add a very small resistor in series....because it does not seem to be allowed. The rules say that the high impendence inputs of a circut may be attached to some number of points described, but it clearly indictates that this is OK because it does onto affect the circuit appreciatbly.

Placing a series resistor violates all the rules regarding how one MUST wire all the motors/spikes and other controllers......so it would be against the rules.

Still, the job is not as hard as it seems.....and with an op-amp or two (depeending on whether you need to build a true DIFFERENTIAL AMPLIFIER or if you can get a good common ground reference point).

There are of course other ways to do such a measurement using the magnetic radiation off the wire as current flows. I think this might be the better, more noise-free way to skin this cat.


-Quentin

[ErikJusten]

As far as using fields to measure DC current, those sensors aren't cheap, and I'm pretty sure they aren't in Digi-Key's catalog (Newark carries some, but they are a little pricey (~$75, if memory serves me correctly), and aren't allowed anyway). Now AC is a whole different story - those are easy to find. But.......... that's not really point........


Erik
(who has spent a lot of time thinking about this exact question)

[bigqueue]

I'm not sure that these work as well as you mite like because some of them require differential power supplies, but take a look at this:
http://www.digikey.com/scripts/us/dk...9&Cat=23265747

They seem to range from $19.00 to $50.00...which fits in the budget.

[Greg Ross]

Originally posted by bigqueue
I'm not sure that these work as well as you mite like because some of them require differential power supplies, but take a look at this:
http://www.digikey.com/scripts/us/dk...9&Cat=23265747

They seem to range from $19.00 to $50.00...which fits in the budget.

Hey Q, Just exactly what were you trying to point us to? When I tried the URL you gave, DigiKey.com told me "your dataset has expired."

[bigqueue]

Oops....sorry.

Well, go to Digikey and look for the following Digikey part number:
398-1011-ND

This is a 100 Amp LEM Inc. Transducer. These seem to come from 6A to 150A max.....some run off 5v, and others +/- 15v.

I hope this helps.....

-Quentin