Re: Great Design
- From: alchazz <no.spam@xxxxxxxxxx>
- Date: Sat, 25 Apr 2009 19:52:19 GMT
On Sat, 25 Apr 2009 12:20:55 -0700, Tim Wescott wrote:
alchazz wrote:
I was amazed the other day. At my town's recycling center, I found two
variable power supplies made by HP, model 721A. I cleaned them up,
plugged them one at a time into a fused extension and turned them on.
Voila, they both worked!
The date code on the PCB is 12-15-59. These beauts are ~50 years old
and work like a charm. I tested them throughout their operating range,
0-30VDC, 0-225MA.
The pass transistor is a 2N375, with a P(max) of 45W and an Ic(max) of
3A. I wonder if it is germanium? I also wonder what the predicted MTBF
was of these guys? I know the current ones are targeted to 3yrs.
How you run a bit of power semiconductor has a huge effect on the MTBF
(more properly the MTTF) -- derate it so that it runs cooler, and it'll
last a lot longer.
The transistor is rated at 3A max. In this application it was current
limited to 225mA. The heatsink is the entire chassis. What is more
amazing is that the large filter capacitor has not failed. It may not be
effective. I'll have to check the ripple later to see what's what. And
that one is easily replaced.
Al
.
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