28 Aug
2010
28 Aug
'10
5:22 p.m.
On 2010 Aug 27, at 1:07 PM, Tony Duell wrote:
[HP9815 PSU]
Some further circuit analysis indicates that if +15 were lost the +5
chopper would end up biased off: there are intervening driver
transistors which need + drive V to turn on the chopper and the chopper
has a low-value R between B-E, biasing it off by default.
Regardless, the thing has been fine since it was manufactured over 30
years ago, including all the use it received during it's heyday.
Strikes me as a rather low-risk situation. To each his own. While one
part of me wants to correct it to the design intention, I'm not
worrying about it.
The capacitor in question is the 680uF unit on
the output side of the
+15V regulator. The regulator (723) is configured with current
limiting.
I have not analyssed the circuit in detail, but it appears that that
+15V
rail (oputput of the 723) supplies the +5V chopper control circuit.
It's
not clear what happens if that supply is missing, but since the chopper
transsitors is PNP, I could see it being turned hard on, which would
put
30V or so on the 5V line.
No, I am not goign to try this practically.