[Alex Pogossov]

"Patrick Turner" wrote in message
...
.... I've replaced the zeners with a far better
shunt reg based in a pair of series BU208P + MJE340 darlington
connected bjts on a heatsink because it seems that particular brand of
5W zeners is subject to failure after passing years. In fact the 75V
zeners had a zener voltage of around 82V when new. Their working
condition was with about 20mA in each, and 82V, and that's 1.64Watts,
and I'd cramped them between plate of aluminium and the aluminium
chassis with lots of heat paste to keep 'em cool as cucumbers.
Didn't stop them frying it seems.

A Hi-V Zener is a very good regulator, and if your 75V rated zeners had 82V
(!!!!????) drop -- they were faulty from the start.

Secondly, why on Earth do you run them at 20mA? 0.2mA would be quite enough,
but if you are "conservative", run 1mA through them. Of course in this case
they need to be followed by a darlington.

If a darlington is not an option, and the regulator *has to be* really
shunt, and to shunt large currents to ground -- here is a trick.
a) Take a TO-220 PNP uninsulated tab transistor;
b) Connect the string of Zeners between base and collector;
c) Bolt the TO-220 collector tab to the chassis -- good and easy heatsink;
d) Use the emitter lead as the "cathode" of the pseudo-zener. 0.7V will be
added to your regulated voltage

Zener current will reduce beta times. For better regulation connect 470R
between base and emitter -- it will give about 1.5mA stable initial current
through the zeners.

Zeners are very noisy though. It is always good to connect a capacitor
across a zener. For audio probably any electrolytic will do. For radio
applications -- 0.1uF ceramic with short leads close to the zener case.

Regards,
Alex