On 2023-May-08, at 12:27 PM, Peter Coghlan via cctalk wrote: > However, Brent's calculations show that the current trip value for the -12V > line is as low as 1.3mA and I can't see any reason to disagree with his > calculations or his conclusion that this seems very low (except that another > tiny smidgen of current is available from the negative startup supply but > this won't really have any bearing on things). If this is really the case, > then placing something like a 5k6 resistor across the -12V line on the good > PSU should cause enough current to flow for the trip to operate. Finding > this level of leakage in the failed PSU is not going to be easy. > > On the other hand, if this test doesn't trip it, then please look very > closely at the resistors and connections to the inputs of E3d and verify > that they are as described on the circuit diagram. > > It seems very strange indeed to have a trip value as low as 1.3mA combined > with a shunt regulator whose method of regulation is to pull the voltage > down by drawing current from the supply line. Perhaps the shunt regulator > might be able to pull enough current to cause the trip to operate if the > -12V line was too high (in the negative sense) or if the shunt regulator > was under the mistaken impression that the -12V line was too high? > > (This is a bit unlikely but the 115V/230V switch is set correctly, isn't it?) > > On the other other hand, if the manual says that the -12V line is supposed > to be able to supply 150mA, then it doesn't make sense for the current trip > to operate at 1.3mA and we must be going wrong somewhere.
I thought I must be off somewhere by 10^n when first doing the calc. The 51Ω is 3 orders of magnitude away from the 0.01Ω on the other outputs, so a similar diff could be anticipated on the current sense. The low current sense might be explained as follows: This power supply does not follow the more-common design of flyback switching mains supplies. This supply is actually 2 (or 3) bucking regulators being fed from a mains-isolation transformer, with the bucking regulation going to the PWM oscillator then looping back to the driver on the primary side of the mains-isolation transformer. This is discernible in the presence of the freewheeling diodes beside the rectifier diodes of the +5 & +12 outputs. DEC seemed to like doing this as the VaxMate supply from jarratt last year was of similar design. A guess is it may have made the inductor design easier, separating the flyback operation out to separated inductors rather than in the all in the same mains transformer. I'm speculating the unusual shunt regulator in the -12V output is actually a 'controlled freewheeling diode' for a -12V bucking step-down. The choke would be stepping down the voltage from the -12 output secondary from something higher down to the actual -12V output. With a higher V from the secondary, a lower current is needed for the same energy throughput, and thus sense on a lower max current from the secondary. However, one would anticipate the regulation V sense point in such operation to be on the other side of the choke than what's shown in the schematic.
