Re: Calculating DC Output Current From Unregulated AC Transformer


"Paul E. Schoen"


I find it easier to just simulate the circuit using LTSpice and adjust
values until they are close.

** Fine - but the OP was after a math formula and will never forgive you
for offering him a LTSpice solution.


I used a voltage source with 18 V peak at 60 Hz, and internal resistance
of 0.8 ohms, which is probably about right for a 20 VA transformer with
about 10% regulation at full load of 1.67 A. I added four rectifiers in
FWB, a 2000 uF capacitor, and a 12 ohm load. I got 1.71 amps RMS from the
voltage source, which dropped to 11.5 VRMS under load. The output power is
13.2 watts, with 12.5 VDC as you require, and a little over one amp. So in
this case the 20 VA tranny is derated by 13.2/20, or about 65%. That's
right in the range of 0.5 to 0.7 suggested by Phil.


** Amazing ......

But you will have 3 volts p-p ripple with that puny 2000uF cap.

Means the DC voltage falls well below 12, a lot.

Formula: I = C dv/dt ( where dt = 6mS )

Plus a 20 VA tranny has typically 15% regulation - not 10 %.

Worse if it heats a lot.


The ASCII file follows. You can take it from there.


** Never assume folk have LT Spice available or the ability to use it.



...... Phil



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