Adam:
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What calculation can I use to determine the correct voltage setting for a particular size part?
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You can't really. For any desired current, the voltage will vary, depending on the bath temperature, spacing between part(s) and anode, and size of part(s). Unless you carefully control these parameters every time you plate, the voltage will be different. That's why constant current is the way to go.
Having said that, you should still be able to get the current you desire:
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The rectifier allows for the adjustment of voltage, but not current, although it has both a voltage and current meter. The voltage meter registers fine, allowing me to dial in a specific voltage, but the current meter always reads <1amp, regardless of the voltage setting or the size of the part being plated. Should I be seeing a reading on the current meter? Am I missing something here. The current meter seems totally useless to me.
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I'm not familiar with this type of rectifier, but would assume that both the voltmeter & amp meter should indicate the actual operational values.
If you calculate the current you want, you should be able to adjust the dial until the amp meter reads your desired current. The volt meter will simply tell you what the final voltage is, but it's value is irrelevnat.
How large are the parts you're plating, and what current density are you shooting for? It's possible that your surface area is too small to get an accurate reading on the amp meter.
Is this amp meter single-scale, 0-25 amp? Or can it be switched to read a smaller amperage full-scale?
It is possible that the amp meter is bad. You can test that theory by hooking up a low-value resistor, and dialing the voltage up. As the voltage increases, so will the amperage. If the amp meter doesn't properly read, then it's bad.
eg. take a 1-ohm resistor: at 1 volt it should draw 1 amp, at 5 volts it should draw 5 amps, etc.
Be sure to use a resistor capable of the power you put into it.
eg. at 5 volts & 5 amps it needs to be rated 25 watts or more.
Sean