Subject: Re: PSU's: Exceeding transformer rated current
Date: Tue, 7 Jan 2003 12:57:52 -0000
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wrote in message
> Richard wrote:
> > 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
> In general, no, though not all transformers will die when you do this.
> > get any hotter than being loaded at rated current output,
> > but would the secondary windings be prone to fail for overcurrent?
> Neglecting the core for a moment, you'll double the copper losses in both
> the primary and secondary windings.
> However, in practice you won't ever have this problem, because the core
> will bite you first.
> Any DC current in either the secondary or primary windings will drive
> the core towards saturation, reducing the maximum primary AC voltage
> that can be sustained without saturation causing massive heating and
> rapid failure. (minutes/seconds)
> I don't off-hand know the derating factor if you were to half-wave
> rectify the output of the transformer, I'd guess for most transformers,
> 10-20% of the AC rating would cause the same heating.
Thanks. What looks fairly simple at first turns out to be a tricky problem. I was
going to work out what the secondary current would be, maintaining the rated
secondary voltage (and I'm sure current would not be x2 rated) and then multiplying
that by the voltage to get the maximum load supplied, but I was tackling that without
taking into consideration the things you bring to light, like core saturation, copper
Current would be DC, but varying, almost like ac as far as the transformer is
concerned I think.