From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Brent Hilpert
Sent: 05 April 2020 21:18
To: Rob Jarratt <robert.jarratt at ntlworld.com>; General Discussion:
and Off-Topic Posts <cctalk at classiccmp.org>
Subject: Re: VAXmate PSU
Switching power supplies are, to coin a phrase, voltage/current-ratio
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
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
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
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
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
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
short somewhere - a winding in the main transformer,
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
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
divider (C18/2.2nF) so you would expect to a slightly averaged version of
voltage at R13 after the voltage divider (at the
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
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.