When the cap gets too big...

[Martin Czech]

:::If I recall things correctly did some larger (classic type of powersupply
:::based) power amplifiers in the 80thies have a resistance and delayed relay
:::in order to bootstrap the caps with a series resistor and after some time
:::the resistor was shortened by the relay and you would see the full line
:::current (and thus voltage) available and the caps would load up from the
:::intermediate level. This method reduces the current-spike as you switches
:::the amp on.
:::
:::Actually, basically the same type of mechanism exists in electromechanical
:::motors which you also need to bootstrap in a similar fashion before the
:::rotation energy (analog to the capacitors stored charge) high enougth.

Sorry for the confusion, I was wrong.

The peak transformer current is basically determined by the transformer
equivalent resistance (two experiments to determine), and the load
resistance, and the type of rectifier circuit. This means: torroid
transformers may have excessive peak currents, with all possible problems:
magnetostrictive (sp) acustical noise etc.

But I think the duration of the current pulse is determined by the
cap. I have to read on, "Tietze Schenk" seems to be a good reading for
this subject.

My sin of youth is still on my writing desk, ie. lab power supply.
I used surplus trafos. I noticed excessive currents when turning on,
so I added a little 12V transformer (4 Euro), a rectifier and a cap,
this drives a relay, and this in turn shorts a 200 Ohm shunt resistor
in the primary trafo circuit. Problem: It works good for switching on,
but the relay doesn't open imideately after turning off, ie.  a toggle
sequence may again lead to excess current. I know this, I don't do this.
The good thing is that the little auxilary transformer is not shunted,
this means: turn on is guaranteed, no matter what the power trafo load is.
This could be a pitfall: if the turn on circuit will not come up,
the 200 Ohm resistor will explode. 

The excess turn on current is determined by the (empty) caps,
and also by the (non magnetical flux) magnetic cores.

A turn on circuit like that is strongly recommended for all linear trafo
power supplies with some VA, the turn on transient makes acoustical
noise, may crash your computer (yes, my computer had to be rebooted!),
and may damage your transformer windings (the force on the wires can
damage it so that they come loose, ie. acoustical humming noise all day).
It is even more recommended when using modern torroid trafos.

We had once a stage power amp we couldn't start, because the 16A/230V
magnetic fuse automaton would trip (evetually we managed to switch it
on during a zero crossing). This was a wellknown manufacturere.
Unbelievable!

m.c.