[sdiy] no turn on transient circuit on (torroid) trafos

[Martin Czech]

I think I have something to have an "idiot proof
transformer turn on transient protection circuit".
I need two very low voltage op amps (comparators), a
couple of switching transistors and two different RC
time constants.

The idea is that RC1 is fast and RC2 is slow.
Looking at the corresponding comparator outputs enables
one to determine if power is switched on or off,
and this is the key to trigger the power relay (5V or 12V)
at the right time.

The power can come up very fast, or very slow,
it can be toggled in any way, I think you can't fool that
circuit.

One thing is very important: if the fast circuit detects
low voltage, it must reset the slow RC at once.
Otherwise toggling would be an desaster.

The trigger levels and supply needs of the op amps have to be low enuogh
to prevent false triggering of the relay in the nowwhere land till say
about half the relay nominal voltage, this is were the circuit is just
about working. OTOH the circuit must not hopp on any brown out, so the
fast RC and fast level shouldn't be to "nervous". Otherwise the brown
out will switch the relay off, it gets even browner then ;->


So far I fumbled arround with a discrete 6 tranny opamp, that will
work with  2.5V   --> 30V. Maybe a LM3900 is also an option?
I must check the datasheet.

One trap must be avoided: if the circuit controls its own PSU
transformer the behaviour is load dependend. 
You can do this, but: A heavy load
will prevent the circuit to get from half on to on,
and this will soon burn out the transient avoiding series resistor,
since it normally needs only to allow for 1W or so.

If the load is completely unknown, a little cheap extra transformer
should be used only for the detection circuit. These guys are dirt cheap
and the load of the big transformer is absolutely unimportant then.

I still wonder why many fat PA amps still have no such soft turn
on thing inside, the torroid will blow any fuse up to 20A and
more. In such a situation you'll have to try and try, until
the caps get a little charge and
you turn the switch right at the mains zero crossing, this
might save you then. 

Been there, just before the concert I
coulnd't turn on that damned amp (it was a borrowed one, not
my gear). Sweat...


I'll try that out at home.
Another comparator would be usefull that flags if the -+15V is going
to be instable, this means pretty early after power off. In that case some 
switch/relay could short the line outs, anti plopp...


No matter if you play @ home with a 10W amp, but certainly
something usefull if a 1000W amp and/or headphones fully cranked up are used.
It is not only the plopp sound, when power is going down some synths
do random sweeps and chirps, of course pretty distortet and much
too loud. Pushing the wrong knob could kill the tweeters then....

In the audio lab we use a brickwall limiter for this kind of accident,
but the switch solution is may be simpler.



m.c.