I have a trasformer that is rated at either 28v or 30v at 8A (big heavy thing it is), and I need to make a power supply giving out 24V DC, reasonably stable, at anywhere between 0-8A (load is not going to be constant).

I know I need to recifiey the AC output and smooth it with a capactitor (which size capacitor I need, I do not know), but how would I reduce the voltage?, as the current is not a constant, the volatge dropped across a resister would not be constant, so I don't know how to do this?

Thanks

Adam

 

In the 1994 (and many other) edition of the Handbook for Radio Amatuers, published by the ARRL, is a 28 v, high current power supply project that uses an LM723 regulator that controls 5 - 2N3055 power transistors. It would seem that the voltage could be adjusted to 24 volts. It is rated at 10 amps continuous.

The author notes that the minimum supply voltage to the regulator circuitry must remain above 31 v (for 28 vdc output) suggesting that for 24 vdc you'd need 27 volts min. If you'd plan to use this or similar circuitry it would be wise to determine the actual ac voltage you can get under load then see where the bottom of the ripple voltage would be (given the filter capacitors you use) to see that it is above 27 volts.

Note that this is a linear regulating system and that there might be switching power supply arrangements that might work better with the transformer you have.

The author makes no mention of ripple or regulation capability but it's likely to be pretty good. IM me if I can help you get a copy of the article.

__________________
stevez

 

U can use a Filter Capacitor of considerable size.I think 2200micro farad is enough at the output of a bridge rectifier whose output will be 1.41xVac - (2X0.7) = 40v approx. Now make a voltage divider circuit. use 1kohm and 3kohm resistors.Feed 40v to it.Take 30v output from 3kohm resistor.This is your input to regulator ic 74H24, which will give u fix24v dc supply and a max of 5A current.Your Circuit should not draw more than 5A current with which u r going to use this power supply.

i hope u understand.
did u. :wink:

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A transformer which puts out 28V at 8 amps will have a higher output at lower loads, so you could be 35V or so at low loads.

You're going to need a regulator output and heatsink which handles at least 50W, in additiona smoothing cap for 8 amps is pretty large.

Frankly, I'd recommend going to a surplus catalog and finding a regulated 24V switching supply. There are plenty of them for a fraction of the cost of building this reg, although it may be a trick to find one up to 8 amps. If you don't actually need that high of a current from a single supply, you'll be better off. Far smaller, cooler, and more reliable.

Back to the old question- just what are you trying to do? What do you want to power? Are you sure you even need filtered DC or regulation?