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Old 2nd April 2008, 11:05 PM   #1
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Default Ammeter wiring?

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?
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Old 2nd April 2008, 11:13 PM   #2
nhuwar is offline nhuwar  United States
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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
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Old 2nd April 2008, 11:43 PM   #3
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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...
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Old 2nd April 2008, 11:48 PM   #4
nhuwar is offline nhuwar  United States
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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.



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Old 3rd April 2008, 12:10 AM   #5
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so... I can just stick it in series on the positive rail with the output?
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Old 3rd April 2008, 01:07 AM   #6
nhuwar is offline nhuwar  United States
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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.








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Old 3rd April 2008, 01:38 AM   #7
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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.
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Old 3rd April 2008, 01:58 AM   #8
kuroguy is offline kuroguy  United States
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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+.
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Old 3rd April 2008, 02:03 AM   #9
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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...
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Old 3rd April 2008, 02:04 AM   #10
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Here is an example from Yamamoto:

Click the image to open in full size.

Click the image to open in full size.

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

Enjoy,

-- josť k.
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