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Originally Posted by smithb98
Am I supposed to put a resistor in line with it?
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No, but sometimes it may be necessary to compensate for limitations of the power supply.
Keep in mind, that "power supplies" are designed to operate electronics circuits on the test bench, and electronics circuits typically have much higher load resistance than an electrolyte cell.
One significant difference between expensive, high-quality power supplies, and the low cost supplies (such as Mastech) is their ability to handle low-resistance loads. Good power supplies will handle a dead-short circuit, others will not, but will have protection circuitry to kick in when it thinks it has a short circuit. I think this is what you're experiencing.
Using your adjustment procedure, as you turn the current down, the voltage will also drop (see Ohms laws below). When voltage reaches the design threshold at which the power supply thinks it is shorted out, the protection circuit kicks in, and all readings drop out.
Adding an in-line resistor keeps the load high enough that the power supply never thinks it's shorted out.
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Is there a way to figure out what kind of resistor I need?
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Any low value resistance w/adequate power rating will do. Even a small 6 or 12v auto bulb will work, but then you don't know what the resistance is, so you can't tell if your meters are reading correctly or not. That's where a resistor of
known value is useful.
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A mathematical equation or any resource that has rules of thumb?
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Ohms laws are what you're dealing with, very simple math
- Ohms voltage law: V = I x R (volts = amps x resistance). Knowing any 2 of the three terms will let you calculate the third. This is why I recommend a 1-ohm resistor: you can eliminate it from the math, so V always = I.
- Ohms power law: P = V x I (power(watts) = volts x amps). eg. if you expect to run 5 amps at 5 volts, the resistor (or any other component) has to be able to handle 25 watts, otherwise it will burn out.
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Also, is it as easy as picking one up from Radio Shack ... ?
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It's that simple, if they have something suitable in-stock. Resistors of more than 2 watts are typically called "power" resistors. Just be sure to get one that will handle the
maximum power you expect to use. I doubt you'll ever see more than 3 volts while plating, so a 30 watter would handle the full 10 amps. A bit bigger would be better though, as it won't get so hot (a 30 watt resistor dissipating a full 30 watts will get hot enough to give you an near instant 3rd degree burn!)
OTOH, you probably won't need one for anything above 6 amps, as you've already found out!
Make sense?
Sean