Question on the 0-30 VDC PSU...

[Guest]

Hello folks! Great site

I'm new to the electronics world and I was looking to build this power supply. I have the opportunity to get a 38VCT@150VA Hammond transformer at a really good price. From my calculation this will give me about 2.95A into the power supply circuit. Will this be enough and compatible with the current (i.e. updated) parts list/project?

[Kevin Weddle]

I am not sure you can get the current requirement from 150VA. The transformer is rated for the current. The current is RMS. So the DC current you will be able to get after rectification for a 3Arms transformer will less than 3A. Maybe around 2ADC. The peak current for a transformer can be very high. You can surmise from the 150VA that you can have a DC power that is below 150W. The reactive power can't be calculated for however.

[audioguru]

Hi Bitsdoo,
Welcome to our forum.
A 38VAC transformer has a voltage that is much too high for our corrected project. With no load, it will probably produce 40VAC, which has a peak voltage of 56.5V. The rectified and filtered DC supply voltage will be about from 50V to 55V!
Many parts will need re-calculation and replacement or they will breakdown, including the output transistors which are rated for 60V, but only at a small current.
You can't reduce the supply voltage with resistors, but maybe a 12V/50W heatsinked zener diode will do, if anybody makes such a monster. You could also reduce the DC voltage with a 39V small zener diode and a power darlington transistor heatsinked for 50W.

BTW, your current calculation forgot to include the VAs that are wasted by heating the rectifiers. So don't subtract the rectifiers' voltage drop but instead just use the actual peak voltage (53.7V) of the 38VAC.