New Power Supply

[BobK]

Well, that rule of thumb is flawed even for 60Hz. The equation for a capacitor charging / discharging is:

I = C dV/dT

So setting I to 1 A and C at 1000uF we find that dV/dT is 1000V / sec.

With a full bridge rectifier, the capacitor is going to have to supply the current for about 1/4 of the cycle or 1/4 * 1/60 = 4ms. This means the ripple in the voltage is 4V!

Simulation with LTSPICE shows that this is just about right for a 10V supply drawing 1 Amp with a 1000uF cap.

But, to answer your question. The time needed for the capacitor to supply current is inversely linear in the frequency, so at 20000 Hz you would need a capacitor 60/20000 = 0.003 as large as you would at 60 Hz.




Bob

 

[alfa88]

It looks like I under estimated the transformer. I got a 4A transformer since I was building a 4A power supply but at a 4A load the Voltage drops miserably (from 15 to 10). I'm wondering now if I should over rate it by, say, 125%?

 

[alfa88]

The new supply is just about finished. I waiting on a couple panel meters, a couple caps and new regulators. In addition I have to tweak some resistors in the current limiting section. The transformer has enough headway where if I want a couple more Amps I can add a little more circuitry. The initial front panel layout was just plain stupid so a new one is awaiting the new meters.

 

[KrisBlueNZ]

Looks nice alfa!

One tiny point. I would put a small resistor, say 1K, between the wiper of each current limit adjustment pot and the base of the transistor. Without it, at the minimum current setting, you have the base-emitter junction of a small transistor connected straight across the current shunt and a sudden short or heavy load could easily damage it.

I assume you're putting the current limiting transistors and the regulators on the heatsinks? I only see two TO3 packages in the photo.

I admire your woodworking skills!

 

[alfa88]

The MJE3055s are on the inside of the rear heatsink, the NTE184s are heatsinked on a pc board. I heatsinked the bridges as well. I'll add that 1k resistor. Thanks for the suggestion. Not bad for just hand tools hand drill and a Dremel, eh?

 

[KrisBlueNZ]

Not bad for just hand tools hand drill and a Dremel, eh?

Yep! It's worth doing a nice job for something you'll use often.

 

[mrcurrent]

Good reply by jackorocko...!!!