On Thu, 27 Jul 2006 16:19:08 +1000, Franc Zabkar
put finger to keyboard and composed:

Is it possible that your replacement part is made to less stringent
specs? For example, what if the zener reference were affected by self
heating?

On further examination, I think the power dissipation in the regulator
is fairly constant throughout the 153-180V range of input. This is
because the 120 ohm shunt resistor diverts the extra power. Therefore
there can be no significant variation in output voltage as a result of
self heating.


|-- 120 ohm --|
Vin o---|---- Reg ----|---o 135V load
500mA

The power dissipated by the regulator would be ...

V x 0.5 - (V x V)/120 where V = Vin - 135

By my reckoning, the dissipation at 153V, 163V, and 180V is as
follows:

153V 6.3W
163V 7.5W
180V 5.6W

- Franc Zabkar
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