11 Apr
2012
11 Apr
'12
6:47 a.m.
On Apr 11, 2012, at 9:25 AM, Rob Jarratt wrote:
Looks like there is an MPSA55 and an MPSA55G. The
MPSA55 is a Darlington transistor, according to Farnell; I recently learned a little about
those repairing my RD53 motor control board, and my understanding is that the gain is even
higher with those. Given that it seems I need less gain than the MPSA55G provides, the
straight MPSA55 would seem unnecessary. However, Farnell will charge me a lot because it
has to ship them from the US to the UK. An Ebay seller here in the UK seems to have the
Darlington variety, so I could get some of those sooner and cheaper. The alternative seems
to be an MPS2907AG from Farnell
http://uk.farnell.com/jsp/search/productdetail.jsp?sku=1611212.
What would people advise?
Darlingtons are not 100% replacements for standard transistors. Among
other things, the voltage drop across the base will be twice normal,
which could screw up the biasing for something designed around 0.7V
drops. There's a reason (well, a few) that they're not used everywhere.
Looking at that page of the schematic, it looks carefully biased.
A casual examination leads me to believe that it's biased to trigger
the discharge of the 555 when its base voltage reaches a certain level
(i.e. a crude switching power supply), but I could be entirely wrong
about that; I only looked for a few minutes. In any case, given how
expensive 1% resistors were in 1978, I'd venture to say that you
probably wouldn't want to upset the bias point by swapping in a
Darlington pair.
- Dave