[sdiy] Power Supply Questions

[ASSI]

On Thursday 17 March 2011, Oscar Salas wrote:
> Talking of modern transformers... Do the manufacturers calculate that
> 80%?

Yes, and a lot of other stuff - but read on.

> It is the Myrra 44383

Not much of a datasheet actually, but it does provide one crucial 
clarification: it supposedly supplies 18V at full rated power into a 
resistive load and 20.8V unloaded, so the load drop is 15%.  From the 
tolerances on the primary voltage, the minimum peak voltage after 
rectification is 22.8V and the maximum 28.5V, which should give enough 
voltage headrom for the regulator, but will also need a good sized heatsink 
(up to 11.3W wasted just in the regulator).  Targeting 1V of ripple 
amplitude you will need 8300µF filter capacitance and gives you about 
15°...20° conduction angle on the rectifier.  Assuming rectangular load 
current to keep the math simple you have approximately 10A current pulses 
for 1/12th of the time.  To get to RMS current, square the current, average 
over one period and take the square root: sqrt(100A²/12)=2.89A.  Looks like 
your transformer is not going to keep cool, in a very literal sense - for 
this you'd want a beefy 105VA transformer.  Since the voltage headroom is 
large, you could go for more ripple: 2200µF capacitance gives about 3.8V 
ripple, a conduction angle of about 30° and a peak current of 5A for 1/6th 
of the time or an RMS current of about 2A.  You still need a 75VA rated 
transformer for doing this.

From your other post:
> Other question regarding power supplies is about the RMS current
> depending of the ripple after capacitors...

The more ripple, the smaller the RMS current (for the same peak current).

> For 1/5 cycle time charging capacitors: Irms=(1/5[5A]^2)^1/2=√5A=2.2A
> For 1/20 cycle time charging capacitors:Irms=(1/20[20A]^2)^1/2=√20A=4.4A
> Could someone explain this equation or explain why this happens?

RMS current is related to the Joule heat produced by it (which is why you 
can measure it with heat wire instruments).  Double the current means four 
times the heating power, so even if you only have it on for half the time 
you still get double the heat.


Achim.
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