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  #1  
Old 09-28-2008, 07:34 PM
Greg23 Greg23 is offline
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Default DC 12 volt 100 amp power supply

I'd like to find schematics to build a variable power supply.
I'm starting with a 120v 10 amp AC outlet.
I'd like to build a constant 12vdc with a variable knob
for 1A to 100A. I don't know if this possible but maybe
someone out there can help or steer me in the right direction....
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Old 09-28-2008, 08:58 PM
scubasteve_911 scubasteve_911 is offline
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You cannot have a constant voltage and specify a current at the same time. You must mean that you want an adjustable over-current protection knob.

The schematic really isn't the difficult part of the project, it will be the implementation. Finding a 1.2KW transformer for this is going to be difficult. You are going to need some extremely beefy rectifiers and a lot of bulk capacitance.

Do you really need 100A? That is a massive amount of current, you are going to need some very thick lines for that!

Steve
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Old 09-28-2008, 09:54 PM
Greg23 Greg23 is offline
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Thanks for your reply Steve,

Your answer proves my obvious ignorance here.
I don't know a whole lot about electronics.
Are you saying that amp & volts will increase and
decrease in proportional measures regardless of circuitry ?

I don't necessarily need 100 amps but I'd like to be able
to adjust from 0-12 volts and 0-75 amps or so.

It looks like a large transformer will be a part of this ?
Would it be easier for an novice like myself to purchase
something already built ?
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Old 09-28-2008, 10:13 PM
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Quote:
Originally Posted by scubasteve_911 View Post
The schematic really isn't the difficult part of the project, it will be the implementation. Finding a 1.2KW transformer for this is going to be difficult. You are going to need some extremely beefy rectifiers and a lot of bulk capacitance.
I concur with scubasteve, but you will need a 2.16KW transformer (multiply 1.2KW by 1.8), fact that only worsens the problem.
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Old 09-28-2008, 10:15 PM
scubasteve_911 scubasteve_911 is offline
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No problem Greg,

I was saying that you cannot control both current and voltage at the same time.

A voltage supply will give a constant voltage, despite load changes. The current varies to the load to keep the necessary voltage across it.

A current supply will do the opposite. It wants to maintain a constant current to the load, so it will vary its voltage to try to make it happen.

From these explanations, you know that to have both is impossible.

If you need an adjustable regulator, that might be a challenge. I do not know of any commercially available IC that will perform this task, they are usually limited to about 5-10A. You would need to design something along the lines of the following link, except your reference voltage will be adjustable.

http://tangentsoft.net/elec/opamp-linreg.html

The 0-75A must be a current-limit / shutoff input. So, if you sense more than x amount of amperes, then you can disable the output of the supply. It is also possible to switch between a voltage-source and a current-source, but this is more complicated.

If you are a beginner in electronics, I don't think it is feasible to build something of this magnitude. If you are the dedicated and have lots of time to spare to learn, build, and potentially have to repeat the design process, then perhaps you should attempt this.

I have a lot of design work to do on my own, so I cannot be specific with my answers.

Goodluck!,

Steve

Last edited by scubasteve_911; 09-28-2008 at 11:45 PM.
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Old 09-28-2008, 10:18 PM
scubasteve_911 scubasteve_911 is offline
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cumesoftware is right about the 1.8 multiplier, I guess I have been working with switching regulators too long, where I am thinking that I can approximate 100% efficient calculations :P

Linear regulators are notoriously inefficient.

Steve
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Old 09-28-2008, 10:35 PM
Greg23 Greg23 is offline
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Thank You Steve!

Your input has helped alot.

Thanks for taking the time to reply.

Greg
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Old 09-28-2008, 11:47 PM
scubasteve_911 scubasteve_911 is offline
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You're very welcome, sorry if I discouraged you. If you attempt making this, please keep us updated and feel free to ask for guidance.

Steve
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Old 09-28-2008, 11:55 PM
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Lambda used to make supplies that would put out voltages and currents like that. They weighed at around 60 pounds. They used a preregulator section with an SCR full wave regulator so the filtered voltage was about 2 volts above the desired output. That way, the pass transistors could run in saturation and minimize ohmic losses. I would hate to try to duplicate all that ironmongery.
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Old 09-29-2008, 08:52 AM
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100Amps (or so).

Well, to put this into perspective, houses in the UK normally have a 100 amp main fuse.

At 12 volts you can get 100+ amps from automotive cold start machines, although not for sustained periods.

You can also achieve this level of current from welding trnasformers.

Do you have you basic supply sorted?
I suggest you do this before worrying about control options.

It is also pertinent to enquire what you are going to do with this supply?
If you have split loads you may find it adequate to run with several smaller 12 volt supplies fed from a common source.

This would also be more reliable and reduce wiring needs.
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