"amdx" wrote in message
news
On 1/5/2017 4:14 PM, John Robertson wrote:
On 2017/01/05 8:38 AM, amdx wrote:
On 1/5/2017 10:02 AM, Bob Engelhardt wrote:
I have a Heathkit IP-27 bench supply that I got at the dump. Its
problem is that the current limiter doesn't work due to a bad
transistor. Unfortunately, the transistor is a germanium one and
replacements are $30!

I found a forum post where the guy had substituted a silicon transistor
(TIP32C) and he said a germanium diode in the base circuit should also
be replaced with a silicon one.


My question is: why does the diode need
to be replaced?

Because the current limiting is partially determined by the
relationship between the voltage drops of the diode (0.3V) and the
emitter / base junction of the transistor.
If you change only the transistor to (0.6V) you will probably never
develop enough voltage on the transistor to start it conducting.

Mikek


Mikek, would replacing the germanium diode with a silicon one fix the
voltage drop you speak of?

John :-#)#


There are twenty regulars here that could give you all the details with
the math, I'm not one of them.
If it were me, I'd change both diode and transistor to silicon and see
how it works. I think it will.

It looks like the diode generates a reference voltage - that needs to be
kept in step with the Vbe of the new transistor.

The silicon transistor still needs more Vbe to get it conducting, so I
suspect the sense resistors will also need some adjustment.