Variable Voltage Supply

Variable Voltage supply I am building. Three outputs and a digital readout.

I learned that the linear regulators will be extremely inefficient, especially when doing something like stepping 20V down to 5v. I should use a switched regulator (similar to a Class D amp). The lm2679 seems good. MC34063AP seems better.

I ordered some LM2575T, use the schematic on the top of page 24: http://www.ti.com/lit/ds/symlink/lm1575.pdf

• Calls for a 2000uF cap, but those were expensive so I got 1000uF and can use two in parallel

https://hackaday.io/project/4154-bench-power-supply

My Own Design

Just trying to understand the electronics...

I'd like to drop from 110AC down to as low as 3.3V DC using a switching supply.

This can be done in three stages. The first stage is just a rectifier and a ripple cap.

The second and third stage are tied together and pot sets both of them. The second stage drops it 90% of the way with a switching supply. And the third stage drops it the rest of the way with an LM317 regulator.

The second stage is the tricky bit, in particular, the square wave generator. 555 are good to about 500kHz. It's hard to pin down an exact value for a Astable Multivibrator, but it should be dependent on the transistors, and they should be pretty fast (even the cheap ones). I'm really liking the Astable Multivibrator option. Take two pots, one needs a center tie; the first leads into the center tie of the second. Adjusting the first changes the total resistance, changing the frequency. Changing the second moves resistance between R2 and R3, changing the duty cycle.

The lower the switching frequency, the more efficient, so it would be nice to be able to adjust the frequency independently of the voltage. It may also be worth looking at 2 switching stages, one at a very low speed (~1kHz) and the other at a much faster speed (~1MHz).

I was playing around with the Lush Simulator and there is a voltage regulator under Op Amps. I hooked that up to a full wave rectifier with ripple cap and the output was very smooth. And, when I took out the cap, it was still smooth. The one big change I made is that there was a voltage divider feeding the negative input on the op amp, I increase both resistors a lot.

Pre-Built

Live and learn - there are 'buck' modules on eBay for about $2.50 a piece (or less in bulk) that can step down 4-40Vdc to 1.3-37Vdc.