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Question about a panel meter.

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Voltwad

Fri Aug 28 2009, 06:58PM

Registered Member #1829 Joined: Sun Nov 30 2008, 01:06AM
Location: Raleigh N.C.
Posts: 74

Hey all, I've been working on my washer launcher. To monitor the voltage I purchased a 3 kV analog panel meter on ebay. It's made by Burlington Instrument Co. It says full scale = 1ma. Now, when I hooked it to my cap bank and started charging, it pegged immediately. I thought F.S. = 1ma meant that at 3 kV the meter would draw 1ma. But now I wonder if it means I need to make a circuit that will generate 1ma at 3kV for the meter to work at full scale or if it's just broken. (I hope that makes some sense.) Googling Burlington Instrument Co. turns up nothing useful. Also, on a side note, has anyone else noticed a significant decline in the quality of Googles search returns lately? I'm just getting more and more disappointed in it.

aonomus

Fri Aug 28 2009, 07:01PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Measure the resistance across the meter terminals. At full deflection, 1mA of current will flow across it. Now with V = IR, you can calculate the true voltage at full deflection. Once you have the actual full deflection voltage, you can figure out what resistive divider you need (don't forget to take into account the resistance of the meter itself if you are using high value resistors).

IamSmooth

Fri Aug 28 2009, 07:34PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

You want 1ma of current to flow through it when the voltage is 3000v.

3000v/0.001a = 3,000,000 ohms. You need a 3Mohm resistor in series with your meter. When you have 3000v, only 1ma with flow through and the meter will read 3kv.

Voltwad

Fri Aug 28 2009, 08:57PM

Registered Member #1829 Joined: Sun Nov 30 2008, 01:06AM
Location: Raleigh N.C.
Posts: 74

Alright, Thanks Smooth and Aonomus. 3 meg. is almost perfect. I was worried there for a bit. So I could use this meter for any range I wanted or, in fact, even to measure current to 1 ma. The only thing that makes it a 3 kV meter is the panel behind the needle. I also bought a +/- 100 uAmp meter as well for testing a Van DeGraaf generator. I shouldn't need any external circuitry for this to read -100 to +100 uA should I?

Myke

Fri Aug 28 2009, 09:45PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

No, you don't need anything extra. If you want to measure high voltages, you would want a pretty low current flowing through the meter so that it doesn't load the circuit too much. I would use something like a 10-50uA meter with its respective resistor.

Voltwad

Fri Aug 28 2009, 09:54PM

Registered Member #1829 Joined: Sun Nov 30 2008, 01:06AM
Location: Raleigh N.C.
Posts: 74

Oh, yeah. I'm actually going to use the uA meter to adjust the design. For vdg's you want to maximize the current. I read somewhere that you just put a uA meter between the top and ground and mess with it until you get the most current possible.

MinorityCarrier

Fri Aug 28 2009, 10:23PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

Your 3 meg resistor will be disipating ~3 watts, so select your resistor(s) wattage to handle the load.

If you are not already, be aware that sensitive meter movements are susceptable to static charge on the face plate, deflecting the needle, giving you erroneous readings.

klugesmith

Fri Aug 28 2009, 10:34PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

MinorityCarrier wrote ...

Your 3 meg resistor will be dissipating ~3 watts, so select your resistor(s) wattage to handle the load.

Good catch.
At rated power dissipation, most resistors are happily hot enough to burn fingers.
And regardless of the power dissipation, you should respect the maximum voltage rating of the resistor(s), which is 300V to 500V for common 1/4 and 1/2 watt R's.
A series string of small resistors is traditional and practical.

Voltwad

Fri Aug 28 2009, 10:48PM

Registered Member #1829 Joined: Sun Nov 30 2008, 01:06AM
Location: Raleigh N.C.
Posts: 74

Yeah, I've got three 1M 1/4W resistors on it now and I've no problems charging the bank up to 2kV.

Bored Chemist

Sat Aug 29 2009, 08:55AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

If you have 3 resistors dissipating 3 watts between them they are dissipating about 1W each. If they are only rated for 1/4W then you are seriously over-running them. They will not last long and you may find they are giving the wrong resistance.

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