Quote:
Originally Posted by woodchips
I have several of the Roband Varex PSU's that allow stacking to gain higher voltage, but also that the current limit will work over the stacked supplies ... Whilst not of obvious use, possibly the service manual might reveal how this magic is done.
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I don't have the manual for my supply, but there are manuals for other Kingshill supplies available online and in one of those they explain that the current limit works by adjusting the output voltage. So if I were to set the voltage to 300V and the maximum current to 100mA and then connect a 2kohm load the control circuitry would reduce the output voltage to 200V, limiting the current to 100mA despite what I'd set the voltage knob to.
In fact in my 3-stacked-supplies case the automatic voltage-slaving system should lock the Middle and Lower supply voltages to whatever the Upper one is. So the current limit only needs to act directly on the Upper supply.
The fault (cooked resistors) that I had didn't actually limit the supply's output via the normal current limit circuit. Instead it acted through a different current limiter - one designed to keep the current in the Middle and Lower supplies below 1A. There's no user control for this feature and I wasn't even aware of its existence until I traced out the circuit.
In other news I've discovered a mistake in my Layout circuit (the first picture in my earlier post). When I drew it I couldn't see the point of the extra three 8V secondary windings on the mains transformer, which I've shown wired in series with the two 80V ones in the Upper supply. There seems to be no good reason for that supply having a different 'raw' HT voltage from the other two. I've since measured the raw HT voltages and discovered that, even more strangely, one of the 8V windings is connected in phase with the two 80V ones, but the other two are connected
out of phase. So the total AC voltage is 80V+80V-8V+8V-8V = 152V ! It seems to me that two of the 8V windings just contribute extra ohmic resistance to the total, thereby heating the transformer more than is necessary. But maybe I'm missing something ...
Anyway, the corrected figure is attached.
I should also say that I have managed to replace 9 of the 18 damaged resistors with in-spec ones and the supply is now operating normally, at least as far as I have tested it, which was to 480V and 250mA (two 60W incandescent lamps in series).
Cheers,
GJ