Subject: Re: PSU's: Exceeding transformer rated current
Date: Fri, 10 Jan 2003 23:11:31 -0000
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"John Woodgate" wrote in message
> Ah, well, in that case, your original question meant 'can I get 20 W
> *into the load* from a 10 VA transformer, using a half-wave rectifier?'.
> The answer is that you can't even get 10 W, *because the rectifier
> circuit is nowhere near 100% efficient*.
> The question of whether you could get 20 W *out of the transformer* is
> more reasonable, but the answer is still 'no', and again you can't even
> get 10 W out.
"Don't know whether this forum is appropriate, but here goes.
In a half-wave rectifier circuit, a purely resistive load, no filtering, could you
double the rated secondary current? The transformer will not get any hotter than
being loaded at rated current output, but would the secondary windings be prone to
fail for overcurrent? TIA. Rich."
I probably did not pose my question very well. What the query is about is current
flowing into the primary for half the cycle rather than the full cycle. Before I had
thought it through I was thinking about the possibility, in a case of half-wave
rectification, of drawing double the current from the secondary, assuming the
secondary voltage being the same, with a view of supplying the rated transformer
wattage (as per FW rectification) to a load. Immediate thought was why not, since if
current is only flowing in the secondary for half the time, drawing twice the current
should not overload the tfmr, one rthing balances another. I think it turns out in
my not realistic example of perfect diode etc, that you could *as a simple matter of
ohms law* supply the same wattage to a load be there half-wave rectification or
full-wave rectification. However, there are reasons why this cannot be for a real
transformer, because in a real transformer you cannot just rely on ohms law because
you have core saturation issues etc. One thing also is that you cannot possibly
maintain, in the example I gave, the FW rated voltage. The rms voltage to the load
has to reduce with HW rectication.