High current regulator circuit needed

[Blind Bruce]

Hi folks, I have a 50 year old battery eliminator made by Mallory. I used it to repair auto radios in the 50's. It has a rating of 10 amps at 30 volts. The voltage is adjusted by a variac type of transformer. It has no regulation at all but is filtered by two very large (and probably dry) electrolytic caps. It has four separate copper oxide rectifiers that I plan to replace with a 30 A bridge rectifier. I will replace the caps as well.
Now then, what type of regulator can I use and what type of surge protection is required?
Bruce in the Peg

 

[crutschow]

If the voltage is controlled by a variac, then it's not easy to add a regulator that will follow the variac voltage. Typically for that the variac has an attached pot that controls the regulator voltage to keep it's output slightly below the variac rectifier output voltage.

Not sure you need surge protection, unless you live in an area with poor power line regulation or have a lot of lightening strikes.

 

[KeepItSimpleStupid]

I've got one of those or similar anyway. Replacement of the recifier may give you some more voltage to work with.

 

[#12]

I actually saw a Variac with a pot on its rear end...about 35 years ago. It was in a power supply by Universal Electronics, Santa Ana California, and it worked well.

 

[Blind Bruce]

I had not thought of that. Of course the variac would be set up to a value somewhat above the regulators input requirement. I could leave it there and control the output with a pot within the regulator. So, how about a circuit for that situation?
BB

 

[crutschow]

You could use a linear regulator such as an LM317 with booster transistors to carry the 10A, but you would need a huge heatsink if you want high current at a low voltage output with the variac set to 30V (up to 300W dissipation in the transistors). That's why it's desirable to adjust the variac voltage to be just above whatever headroom the regulator needs at the desired output voltage setting.

 

[MikeMl]

I have a couple of homebrew supplies that use a Variac to control the unregulated DC ahead of a high-current IC/Transistor regulator. To minimize power dissipation in the regulator, the trick is to set the unreg voltage just high enough to keep the regulator from dropping out. It helps to use a regulator circuit with a low drop-out. I provide metering to show both the regulated and unregulated voltage. In another supply, I use a simple comparator to drive a LED; if the LED flashes, the regulator is going into dropout.

Come to think of it, I have a commercial microprocessor-controlled DC supply that uses several relays and a tapped transformer to minimize dissipation upstream of the regulator. I have seen multi-KW rack-mounted supplies that use a servo-motor to set the Variac.

This is certainly not a new idea.

 

[crutschow]

Some variations on Mike ML's designs:

One would be to connect a voltmeter from the unregulated to regulated output and adjust the unregulated voltage to keep this voltage near the minimum dropout of the regulator.

Another would be to use two leds connected between the regulated and unregulated supplies with one indicating adequate voltage across the regulator, and the other indicating it is higher than needed.