# Ammeter wiring?

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#### ThSpeakerDude88

I want to put an ammeter in my HV power supply, but am completly new to the concept of ammeters.

my power supply is adjustable from 150vdc-450vdc, and will have a maximum draw of 150-200 ma.

I have a Vellman 500ma panel meter that I got for like \$5. Can I use this in a HV circuit? Does the shunt resistor have to be very high and if so would it cause any problems with heat dissipation?

#### nhuwar

Make sure the meter is on the negative line of the high voltage supply not on the positive side.

You can use the vellamen gauge and the type of resistors shouldn't really matter.

Just find the current draw full scale for the meter and find the corresponding resistor value.

Are you going to use a .1 ohm shunt or a 1 ohm shunt, this will decide what the size of the multiplier resistor is.

Hope this helps,

Nick

#### ThSpeakerDude88

the meter's resistance is .5 ohm if that helps any.. are there any calculators on the web that show how to connect a panel meter and/or calculate shunt resistance? I havent been able to find any...

#### nhuwar

If the meters resistance is .5 ohms then it probably has a built in shunt.

You can test is with a power supply and a resistor and the meter put in the circuit and see what you get.

But I'm guessing it has a built in shunt. Most of the amp meters I have working with do.

Nick

#### ThSpeakerDude88

so... I can just stick it in series on the positive rail with the output?

#### nhuwar

No the negative rail !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

If you put it on the positive rail it could arc to ground. You always put an amp meter on the side of the supply that is grounded.

Nick

#### ThSpeakerDude88

I'm confused? If it is in series with the plate supply, how can it arc to ground?

I basically want to put it in line with the B+ output ... it would go between the B+ output and the + lead of the output transformer to measure plate current. I would put it in the cathodes but this wouldn't do me any good as I want to be able to monitor my entire power supply's current draw to keep it within the transformers ratings, and to see if there are any problems in the circuit causing excessive current draw.

#### kuroguy

put it on the ground side. that way if something inside it shorts to the case it is at gropund potential instead of at B+.

#### ThSpeakerDude88

Case is made of oak...and any panels are made of PVC. Yeah I know, maybe a fire hazard but I have a main kill switch nearby and a fire extenguisher handy at all times. Its very well built and shielded inside as well.

I guess I could put it in the negative rail...will the .5 ohm resistance mess anything up with the ground potential? I wouldn't think so but just asking...

#### korneluk

Here is an example from Yamamoto:

Go here for more details...
http://www.jacmusic.com/Yamamoto/html/meters/index.html

Enjoy,

-- josé k.

#### ThSpeakerDude88

thanks for the reply, but those show the meter in the cathode. This is great information on measuring bias for when I built my power amp prototyping box, but right now what I want is a master current meter for the entire B+ line so I can see what draw my amps are pulling when operating so that I can have the right size transformers made for my guitar amps.

#### ThSpeakerDude88

ok, I think I can see clearly now!

the rain has...ehh nevermind.

does the polarity still run from the "positive" side of the circuit to the negative side? (bridge negative output?)

sorry im a bit tired tonight..

#### nhuwar

The polirity will be reversed from what it would be in the positive rail. But it shouldn't matter for the meter though. Digital meter auto correct for that I think. But if not you just reverese the connections on the back of the meter.

Nick

#### ThSpeakerDude88

Thanks guys! Any notes on calibrating my meter? I thought of using a light bulb and testing the curent draw with a multimeter, and then just calibrating the gague set screw until it reads the same... right?

#### nhuwar

I would use a resistor of know resistance instead of a light bulb. This way you can calculate the current via ohms law. The just hook it up and put the desired voltage on it and calibrate you ammeter.

Hope this helps,

Nick

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