Rob Jarratt robert.jarratt at
Sun Apr 5 16:53:32 CDT 2020

> -----Original Message-----
> From: cctalk <cctalk-bounces at> On Behalf Of Brent Hilpert
> cctalk
> Sent: 05 April 2020 21:18
> To: Rob Jarratt <robert.jarratt at>; General Discussion:
> and Off-Topic Posts <cctalk at>
> Subject: Re: VAXmate PSU
> Switching power supplies are, to coin a phrase, voltage/current-ratio
> translators.
> They will attempt to adjust the (cycle-averaged) input-current demand in
> inverse proportion to the input voltage, to meet the power demand of the
> When you load a switching supply, and run it with a low input voltage, it
> attempt to increase the input-current demand, either with increased peak
> current or increased duty-cycle (ON-time of primary switching
> Suppose you have a load demand of 100W. At 100V input the input current
> needed is 1A.
> At 10V input, the input current needed is 10A.
> If a supply is not explicitly designed for low supply voltages, it can
lead to
> excessive primary-side currents.
> This is why it is a bad idea to 'run up' switching supplies from a variac
> otherwise run them outside their specced input voltage range.
> You don't say what the observed duty-cycle (ON-time) is. What would be
> expected is it's running 'wide-open' because it's trying to get enough
> through the transformer to meet the load demand while gasping for
> from the input because the input voltage is so low.
> So from the scenario you've set up, it's difficult to discern whether the
> behaviour is normal or faulty (the scenario masks the otherwise-observed
> faulty behaviour).

I have to say that when I was thinking about this, I did wonder if the
problem was that it was trying to raise the output voltage with not a lot of
input, and that therefore the duty cycle would be too high. I will remove
the variac from the equation. For the record I was seeing a duty cycle of
about 50%. In later testing at 240VAC the duty cycle does seem a lot lower.

> All this is also dependant on how large your dummy load is (as a % of the
> max power output of the supply).
> If you want to run at a low input voltage, remove or very lightly load the
> From your schematic, there is a small load presented internally from
> voltage dividers around the outputs, although not all the R values are in
> schematic, so can't calc the current.
> If you still get the over-current SCR triggering, suspicion could lean
towards a
> short somewhere - a winding in the main transformer, secondary rectifiers
> caps - anything presenting an excessive energy sink to the main switcher,
> including over-sensitivity of the crowbar circuits on the secondary side.
> secondary crowbar circuit monitors the output voltages relative to a
> You could scope-monitor the gate of the SCR over there.

I have already done a ringing test on the main transformer, and I think that
it is OK. One of the windings does not ring very well, but I think it is one
that has few windings and supplies the on-going power to the primary side
once the PSU has started up. I have also done a bit of testing on the
secondary rectifiers, but not found anything so far. I will look at
secondary side again more closely.

> The spike you mention on the primary-side SCR gate without a corresponding
> spike on R13 does seem odd, seeing scope traces pic could be interesting,
> perhaps scope the anode, the gate and R13. Possibility of some odd trigger
> fault in the SCR.

I have obtained a scope trace as you suggest. R32 is still lifted so the
UC3842 is powered by the bench PSU, but I am using the full 240VAC (no
variac). The channels are:
1.	Ch1. 555 timer.
2.	Ch2. D19 Anode
3.	Ch3. D19 Gate.
4.	Ch4. Q1 Source.
The picture is here:

> There is a small amount of filtering on the SCR-gate/over-current voltage
> divider (C18/2.2nF) so you would expect to a slightly averaged version of
> voltage at R13 after the voltage divider (at the cap/gate).
> Aside: You have R27 & R28 at 20+20 ohms in your schematic. This is an
> low R for dropping the hundreds of supply V down to the 16V/low-current of
> 3842 supply. For schematic accuracy, you might double-check the value of
> those.

You are correct, I have mixed up the values with R30 and R31, the correct
value is 15K. I have updated the schematic, and rearranged it to look
diagrammatically more like a sample diagram in the TI datasheet for the
UC3842. The updated schematic is here:

> 10/10 for your tenacity in this repair attempt.

Thank you, I am sure it will be simple when I find the problem, and I am
learning a lot.

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