[sdiy] Power supply Xfmrs

[jh.]

> >What is the tolerance of the transformer's secondary voltage?
> >What if the ac line voltage is running low?
> >What about diode drop?
> >What about capacitor droop?
> >
> I knew someone would say this, but Don was first ;D
> If *everything* that COULD go wrong did, you wouldn't
> survive the average day. If everythig was running the other way,
> you probably wouldn't have a high enough voltage rating on the
> filter caps. Or, the voltage drop across the regs would be so high that
> you would overheat them at max current.
>
> i still say, given the 'average' 28v centrtap tranny, and half a dozen
> modules, all will be well. Most likely. Nobody can say more, for
> 'unknown' transformers & unspecified load.

Well yes, but OTOH I see my lights going dim when I switch on my
Studio, so I guess some temporary voltage drop isn't that unrealistic.

On the positive side, the transformer has a higher output voltage than
rated when the current is below its maximum. That's why I sometimes
*do* use 14V~ or 15V~ transformers for +/-15V. Especially
Wallwarts with 18V~ are hard to come by, so it's possible to
use a 14V~ wart at 20% of its rated power and still have some
"headroom". But nothing should be left to chance here - this
means measuring the resistance of the transformer winding and
doing transient simulations of the whole PSU under load.
(Winding resistance will have a different effect with FWR or
HWR, and unfortunately HWR is the method of choice when
you use an AC wallwart for +/-15V)

JH.