# Variac AC/DC Power Supply.

Discussion in 'The Projects Forum' started by PatrickSmith, May 28, 2015.

1. ### PatrickSmith Thread Starter New Member

May 28, 2015
3
0
I need to create a variable output power supply that will give both AC and DC voltages up to 10Amp, to be used like a bench power supply for testing.
I have an idea of what i need but would like some help with the DC output.

What I have is:
240v 50Hz AC supply to the Variac through an isolation transformer.

The variac operates from 0 to 240v

Full-Wave bridge rectifier ( up to 400v) to convert to DC voltage.

I then need to smooth the output of the rectifier with capacitors, how would I calculate what capacitor I would require considering a possible maximum of 10Amp load and 240v AC input, but usually any load would be less than that, so 240v x 1.41 = 273vDC so to be safe a potential maximum of 300v DC.

Then after the capacitors a bleed resistor to discharge the capacitors as a safety precaution.

Can anyone suggest what capacitors and bleed resistor I require or if this design would work?

Jul 18, 2013
13,182
3,646
A capacitor size depends on what % ripple you are looking for, the % ripple is going to change with load, for simple general purpose anything over 1500μf should be OK.
Max.

wmodavis likes this.
3. ### PatrickSmith Thread Starter New Member

May 28, 2015
3
0
If 10% of the supply voltage is acceptable for ripple, would a 1500μf + capacitor be ok for potentially up to 10a load?

Jul 18, 2013
13,182
3,646
The 10% value will change according to the final DC level out.
2.5v subsequent ripple at 250v is insignificant but 2.5 at 5v!!
Max.

5. ### PatrickSmith Thread Starter New Member

May 28, 2015
3
0
What would you suggest is the best solution?

6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,071

Very idealized, but it shows the ripple vs capacitance at 240Vrms input and a load current of 10A. The progression from bottom to top is 400uF, 800uF, 1600uF, 3200uF, and 6400uF.

Watch rectifier current rating, and ESR and ripple current rating of the capacitor.

File size:
42.2 KB
Views:
42