Low voltage AC power source design

[calzap]

For a long while, I’ve wanted a low voltage AC power source for my shop bench.  I have a variac, but it’s too easy for someone to go overvoltage with it.  Its output is not isolated from mains, and it has no voltmeter or ammeter.  So, we haul out a transformer with the right output voltage, wire a mains cord and plug to the input, use alligator leads on the output and go.  Not convenient or particularly safe.  I came up with a design with several fixed voltages selected with a rotary switch, but a completely variable voltage is more flexible.  I thought about getting another variac and mechanically limiting how far the knob could be turned.  But discarded this idea because I acquired a new Powerstat variable transformer with 40 VAC input and 0-40 VAC output.  Using a 120 VAC to 40 VAC input transformer for it will give AC isolation as well.  The preliminary design, which is simple, is in the pic below.

In addition to the power switch, there is an output switch.  I think it’s a good idea for bench PSUs to have an output switch.  It can avoid frying something inadvertently when the PSU is switched on.  I used a 3-pole output switch with one pole used to control power to the ammeter.  The ammeter needs 9 VDC to operate, including the light for its display.  This will provide an obvious visual indication of the switch position.

Comments welcomed.

Mike

Added note:  Displayed voltage would be a little more accurate if the voltmeter was connected to the “output” end of the shunt resistor.

[enut11]

Interesting but perhaps a little too complex? Another way would be to simply connect a line transformer primary to the output of your variac.
In my case, I have a 300v variac so I connected a 240v-30v transformer to the variac output.
Over the full output range of the variac, 0-300v, gives me fine 0-30v control and an isolated output.
Also, I used analog meters as they are much faster and accurate enough for what I need.
enut11

[coppercone2]

yeah this is useful I wanted something like this for my AC electrolytic cleaning experiments. I used a variac but it was always on my mind that it could possibly make a bigger voltage, so I only used the method for some very restricted tests.


I was just gonna make a box that it connects to with a transformer for 48VAC max output


putting it in 1 box means more reliable connections since its going to be like wired instead of plugged. But it will also be a heavy box


Ultimately combining them in 1 box leads to a more reliable product, so long you engineer it right for heat and make sure the chassis can take 2 transformers. If you have some flimsy crap, the weight of moving it around could potentially loosen something if its flexing from the weight...


so its a bit challanging to make this project, the chassis is expensive.. if you want the full safety benefit from a real world standpoint (theoretically the concept is fine)


also, there is inrush. My 20A variac likes to trip stuff during turn on. big doughnut.