AC Source Completed

[calzap]

I’ve wanted a variable, low voltage AC source for a while and have dragged out single voltage transformers for some testing.  And I have a variac, but it’s not isolated and it’s all too easy to dial the voltage too high.  So, when I got a 0-40 VAC, 7 A variable transformer, I designed and built an AC source for my bench.  The schematic is below and the BOM is attached.

The case I wanted most was US$200, so I settled for a junction box at 1/5 the cost.  I could have made it a bit more compact, but the footprint would have been about the same, and I like having a big box to work in.  As expected, most of the effort went into metalwork.  The backplane is a 3/16 inch (5 mm) sheet of PVC.  Only a couple of surprises with the components.  The Triad 120V in, 40V out transformer has green output leads … not the best color choice.  The Powerstat variable transformer came with a plastic plate to insulate it from any metal mount (like the box) and also has a small hole into which a peg on the transformer fits to keep the transformer from turning.  Unfortunately, two of the mounting holes of the plate are so close to the transformer that any screws put in the holes touch the transformer coil.  So, I used nylon screws and nuts for the plate.  Once the metalwork was done, the rest went together smoothly.  The only soldering was two pairs 22 AWG wires inserted into the 9 VDC screw terminals of the CUI power supply.  Pics of the inside and front panel are below.  The leads are connected to a nominal 50 ohm, 100 W resistor in the front panel pic.

I tested it with a group of nominal 30-ohm, 100-W resistors that were hooked up successively in parallel.  The results are in the last pic.  From no load to max load, the voltage dropped about 2 V when set at max voltage.  The internal meters did not quite agree with external DMMs but are close enough for routine work.  If I need more precise V or A readings, I’d use an external DMM in any case.

Mike

[BILLPOD]

Good Morning Calzap,   Nice job.   I built a similar variable AC supply, but I used a bigger
Variac, (10 amp), and I put a cooling fan on one end and an exhaust vent on the other.  My only mistake was not fusing the output, like you did.   I've been meaning to add one, and it
has been on my 'To-Do' list, and we all know how those lists work.  Thank you for sharing your project.

[calzap]

Good Morning Calzap,   Nice job.   I built a similar variable AC supply, but I used a bigger
Variac, (10 amp), and I put a cooling fan on one end and an exhaust vent on the other.  My only mistake was not fusing the output, like you did.   I've been meaning to add one, and it
has been on my 'To-Do' list, and we all know how those lists work.  Thank you for sharing your project.

Thanks.  I should have included a pic of the backside.  There are vent holes (6 mm) at the top and bottom of the backside.  A few can be barely seen on the inside view that was posted.

Mike

[SuperMario]

Hi, nice work on isolated Variac.
I completed one myself couple weeks back. I did dual version and added Arduino to control relays with hope one day I will implement remote control . The chassis is form old Altec amplifier, isolation transformer is a beast, paid $25 for it and variacs I had laying around.

[coppercone2]

I made one of these too, I can just recommend that you check the carbon on the variac. Mine was basically worn out so I did alot of custom work to fit a defective part, its been a long time and I still did not fix it, making it a expensive paper weight. Always check the variac brush before you start this project...

[inse]

Very nice unit, the only point of criticism is the adjustment knob 😉

[calzap]

The shaft on the variable transformer was very short with a slot in the end.  I’m guessing it was designed for an application where it wouldn’t need adjusting very often, and then with a screwdriver.  It was new BTW.  So, I sliced the end from an aluminum shaft coupler very close to the set screws which allowed attachment to the shaft as a knob.  But the coupler was quite long, so I cut it again so it wouldn’t stick out too far on the front panel.  Could have done a better job of polishing away the saw marks.  Maybe I’ll do it someday.   I used the number plate that came with variable transformer just because it was there … never look at it when using the supply.

Mike