Equivalent of an A55 Transistor

-----Original Message----- From: cctalk-bounces at classiccmp.org [mailto:cctalk- bounces at classiccmp.org] On Behalf Of David Riley Sent: 11 April 2012 14:47 To: General Discussion: On-Topic and Off-Topic Posts Subject: Re: Equivalent of an A55 Transistor On Apr 11, 2012, at 9:25 AM, Rob Jarratt wrote: Darlington transistor, according to Farnell; I recently learned a little

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

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. 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

upset the bias point by swapping in a Darlington pair. - Dave