9 May
2023
9 May
'23
7:28 a.m.
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.