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Old 1st Apr 2021, 4:32 am   #16
TonyDuell
Dekatron
 
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,190
Default Re: Weir HSS100/4 PSU blowup

There is an RS trade counter that I can easily get to (I normally do collect my orders as I find it a lot more convenient than waiting around for a parcel) but I am not sure what they are doing during lockdown. But I never have a problem making my order up to over the 'small order charge limit', even if I just buy a few packets of nuts and bolts, reels of wire, etc. I am sure to need things like that in the future.

As for the circuit operation, I think that when the mosfet is off, the base of TR4 is held at 13V-ish due to R16/D9. Obviously under those conditions there is no collector or emitter current.

When the mosfet is turned on by the drive signal it pulls the emitter down. The emitter-base junction is forward-biased, the base drops below the zener voltage of D9 so the latter no longer passes any current. Current through T1 then causes more current to be fed into the base of TR4 via D11 (the current thrpugh R16 would be nowhere near enough, the hfe of TR4 is low according to Towers). This results in significant current through the collector circuit and T2 primary
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