Much cheaper 12v power supply?

[BrianG]

I was sitting at my workbench looking at my $75 30A 12v power supply when I starting thinking about how to make something far cheaper and simpler.

So far, the only two 12v power supply options are switching and linear types.

Linear types are fairly simple, but are large, heavy, and inefficient. This is due to two reasons; 1) the low AC wall frequency 50Hz or 60Hz requires physically larger transformer (expensive) and filter capacitors, and 2) any excess voltage above the 12v output is shed as heat.

Switching power supplies are generally smaller, lighter, and are far more efficient for two reasons; 1) they operate at MUCH higher frequencies (typ 15kHz+) which means MUCH smaller/lighter/cheaper transformer and capacitors can be used, and 2) they regulate the output voltage by PWM. However, they are quite complex, expensive (due to complexity) and are harder to repair if something goes wrong.

Soooo, what about using something built around a light dimmer? A light dimmer is just a triac (and a few other support components) set up to chop off the AC signal by phase rather than a certain voltage. Since a sine-wave has a rise and fall ramp, we could set the triac to cut off on part of the sine-wave slope. I figure outputting this to a bridge rectifier (to provide 120Hz instead of 60Hz) and then to a bank of fairly cheap high uF, low V (~25-35v) caps would take the complexity out of a switching design and the bulk/weight out of a linear design, while remaining inexpensive and light. Then, a simple linear regulator can be used to fine-tune the actual output - or even provide feedback to the triac trigger. And to protect against full 120v in case of triac failure, a simple crowbar-style protection circuitry could be used to shunt this high voltage to ground, thereby tripping a fuse/circuit breaker.

Just looking for some feedback here from some more electronically-experienced members here before I go out and start buying parts.

[Serum]

I am not very electronically experienced, but here is my concern;

in order to make it safe, you need a 100%seperation from the 110V.. a crowbar style protection circuit doesn't take care of that.


and what about the efficiency of the triac at these low voltages? you are talking about ac 8.5V output to create 12V dc.

[BrianG]

I don't think total mains isolation is needed, but that's my opinion. The protection circuit to ensure full 120v doesn't get to the output should be fairly simple - the crowbar thing was just a quick-n-dirty solution for now.

Triacs are simply gated diodes that are placed back to back so that they pass AC. The only losses are from the small voltage drop. Even at 0.7v and say, 15A, that's still only 5% efficiency loss. But, there would be a little more losses since I'd probably have the rail set to around 15v or 16v, and then go to a linear regulator to provide the steady 14.5v DC. I'll probably pick up a beefy dimmer and play with it a little to see what I can come up with...