# Determining Rheostat size to get

#### JimmyJamz

Joined May 28, 2020
2
Hi all - I just found this forum today after doing quite a bit of internet digging to see if I can figure out the puzzle I have on my plate. I'm currently looking at electro plating and rather than going out and buying a power supply, I figured I can reuse some parts laying around. Unfortunately my electrical knowledge has diminished over the years as I've been primarily focused on software engineering but I think I have the majority of the puzzle figured out.

All in all, I am essentially trying to determine what size rheostat I can get to allow for variable voltage control between 1-3 volts.

Setup this far:
9T56Y2876 Transformer - Wired for 12V - KVA .150
Rectifier to produce DC voltage

So based on my limited knowledge this far, the transformer will produce 150 watts at 12 volts at an amperage rating of 12.5. It seems that there's wattage ratings for rheostats which seems to indicate the amount of power that can be dissipated so if this is correct, I would think I would want a 150w to account for the incoming 150w (perhaps 200w for some extra buffer). The second question to the puzzle is how many ohms should I be targeting to meet my needs?

I was looking at something along the lines of this: https://www.amazon.com/Fielect-Wire...ostat&qid=1590684709&s=industrial&sr=1-5&th=1

However I'm not confident enough in my research and understanding to give it a go. Any input/information is greatly appreciated!

#### Papabravo

Joined Feb 24, 2006
15,560
I'm not entirely certain they make them for your intended purpose. This would be beyond ridiculous in terms of most of the power being tuned into heat. I think there are more than a few alternate solutions that will be more flexible for your purposes. For the electroplating process are you trying to control the current or the voltage or both?

#### JimmyJamz

Joined May 28, 2020
2
I'm not entirely certain they make them for your intended purpose. This would be beyond ridiculous in terms of most of the power being tuned into heat. I think there are more than a few alternate solutions that will be more flexible for your purposes. For the electroplating process are you trying to control the current or the voltage or both?
Thank you for the response. It is a little bit of both..... essentially my power requirements is that the voltage needs to be between 1.5-3v as this will provide a cleaner finish and the current needs to be at minimum 25ma per square inch. My application that I am plating is motorcycle/car bolts/nuts and therefore the actual surface area is not all that much. Granted, varying sizes, I would assume 50ma-200ma would be sufficient for the majority of my need

#### Papabravo

Joined Feb 24, 2006
15,560
In the rheostat you might be dissipating 12 Amps of current to get down to 1-2 volts and that current level. That looks like megaoverkill to me, but hey you're free to waste resources until there are no more.

#### ElectricSpidey

Joined Dec 2, 2017
1,538
Why would you use a 150 watt supply when your process only requires .6 watts?

Get a 5 volt supply and use a emitter follower transistor circuit to control output voltage.

#### Papabravo

Joined Feb 24, 2006
15,560
Why would you use a 150 watt supply when your process only requires .6 watts?

Get a 5 volt supply and use a emitter follower transistor circuit to control output voltage.
A wall wart would be perfect for that purpose.

#### ElectricSpidey

Joined Dec 2, 2017
1,538
And reguardless of whatever supply you decide to use the emitter follower circuit would be far superior to a rheostat due to the variable loads.

No guesswork deciding values for the pot, and just about any NPN power transistor will do.

it's a very simple circuit. (braindead simple)