Power supply duel_5v/15v from 230:15v 1amp transformer

[sajsaj220]

Along with subject and ckt attached,
why getting 19.5v(more than tranformer sec rating) insted of 15v.

The measurement of volt as follows with digital meter:
The tr sec o/p-> 15.96v,
o/p-1 ->>5.2v
o/p-2 ->>19.5v


tnx

Can u explain the fact behind it and do i get ~15v.

 

[wwfeldman]

if the 230V is a little high, you'll get a little more on the 15 V side

15V RMS is 21.2 V peak
one diode drop through the bridge, a second diode drop through D1,
and a third diode drop on the return through the bridge yields 21.2 V - 3(0.7 V) = 19.1V

you need to put a load across C5 to see anything besides the peak voltage
diode D1 and capacitor C5 act as a peak detector, so you are seing the peak voltage

how much current should the 15 V supply provide?
connect R = 15V / (expected current in amps) across C5

you might want a three terminal regulator after C5 to provide regulated 15V
as you did for the 5V supply

 

[KlausST]

Hi,

Most transformer and power supply ratings are at full power load .... not without load.

Klaus