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#185346 - 03/11/09 08:04 PM How can I limit the current of a power supply?  
LoneGunman  Offline
Member
Joined: Oct 2005
Posts: 141
Florida
I'm getting into some small scale electroplating. The place I purchase my supplies from suggest a constant current, constant voltage, adjustable voltage and current power supply. These things are expensive, I could get away with a 25 amp for everything except for hard chrome, roughly it takes one amp for every square inch of surface area you want to plate.

Hard chrome is almost triple this, I would need a 200 or 300 amp power supply to attempt hard chrome, I cannot afford that.

How can current be limited? In other words, say I have 100 amps of 12 volt battery power available and I want to only allow 25 amps to flow through my cathode and anode, any idea how I would do this?

I was thinking about regulating the voltage using resistors, let's say I need 18 volts and I have two 12 volt batteries wired in series and I need 100 amps, are resisters available that will handle that amount of current? If so, are they very expensive?



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#185357 - 03/12/09 12:11 AM Re: How can I limit the current of a power supply? [Re: LoneGunman]  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,103
Estero,Fl,usa
I would think about a dryer element.
Dropping 6 v at 100a is 600 watts and .06 ohms. You could get there with parallel sections of a dryer element but I bet there is some voltage drop in the plating tank. You are not going to get 100a out of a battery long anyway. Are you sure you really need that much current? The problem with resistive current regulation is you waste a lot of energy in heat.


Greg Fretwell

#185358 - 03/12/09 12:43 AM Re: How can I limit the current of a power supply? [Re: LoneGunman]  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 785
Chicago, Il.
Yes and yes but nobody does it that way. You can get just about any value you want with series/parallel combinations of common power resistors. They would be very expensive and cumbersome. Most high current supplies like you describe would be controlled by a variac on the primary of a large step down transformer or phase control of an SCR/Diode bridge on the secondary. A high current linear supply might have 8 or more 2N3055 NPN transistors (15A,115W) with emitter resistors, in parallel. Huge heat sinks and fans are often used. One or more driver transistors are needed for the relatively low gain outputs. The most common IC for this sort of supply is the LM723. It provides for both voltage regulation and foldback current limiting and has been around for decades.

Is there a maximum amount of ripple that your application can tolerate?
Joe


#185372 - 03/12/09 03:17 PM Re: How can I limit the current of a power supply? [Re: JoeTestingEngr]  
LarryC  Offline
Member
Joined: Jul 2004
Posts: 783
Winchester, NH, US
Another source of high power constant current power supplies are WELDERS. Specifically welders that are set up for Stick and TIG welding. Not MIG (wire feeders). As far as regulation, no guarantee of any precision. Be aware that normal open circuit voltage is typically 80 volts. Under load, the voltage drops down to around 15 to 30 volts. TIG based machines have the added advantage that they usually have an input for external current control, AKA the foot pedal.

A poor mans dropping resistor includes such wonderous items as garage door springs, electric elevator cast iron resistor banks, and water rheostats.

Larry C


#185373 - 03/12/09 03:24 PM Re: How can I limit the current of a power supply? [Re: LarryC]  
LarryC  Offline
Member
Joined: Jul 2004
Posts: 783
Winchester, NH, US
Another low tech current limiting device is a tungstem light bulb. I believe a standard car headlight is good to 4 or 5 amps. Place as many as needed in paralell to get up to the current needed.

Larry C


#185375 - 03/12/09 04:25 PM Re: How can I limit the current of a power supply? [Re: LarryC]  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,103
Estero,Fl,usa
I have this thing that I am not sure what to do with. It was out of a big radio transmitter shack they tore down. If you can use it you can have it.

http://gfretwell.com/electrical/Ohmite.jpg


Greg Fretwell

#185386 - 03/12/09 09:59 PM Re: How can I limit the current of a power supply? [Re: gfretwell]  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 785
Chicago, Il.
That variac would be a perfect front end if it was rated for 120VAC instead of 40VAC.
Joe


#185581 - 03/19/09 02:13 PM Re: How can I limit the current of a power supply? [Re: LoneGunman]  
SteveFehr  Offline
Member
Joined: Mar 2005
Posts: 1,208
Chesapeake, VA
You could always build your own power supply, but honestly, you're probably just as well off buying one from your supply house.

[Linked Image]


#185861 - 04/02/09 12:07 AM Re: How can I limit the current of a power supply? [Re: SteveFehr]  
LoneGunman  Offline
Member
Joined: Oct 2005
Posts: 141
Florida
Thanks for all the replies, I'm still researching my options.



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