18 Feb
2022
18 Feb
'22
12:57 p.m.
On 2022-Feb-17, at 11:40 PM, Rob Jarratt wrote:
The 744 derives the +15 supply for the 723 from the same 20-30VAC main input supply, in
contrast to the 745 where the +15 is from another source.
With the 744, once your bench supply goes into current limit and drops its output voltage,
that's also dropping the +15 for the 723. Everything is going to be out of whack at
that point, the reference supply, the amp/comparator action, etc. I can't say at this
distance and limited info exactly what's going on, there's also the Q7 unijunction
shut-down circuit in there. It may be in some sort of oscillation 'trying to
start' or it may be latched up into an off state, because the supply voltage is below
some threshold. (I've never dealt with these supplies directly, I'm just observing
the schematics.)
This behaviour may be different for the 745 because the +15 ancillary supply is
separate/independant from the main input.
-----Original
Message-----
From: Brent Hilpert <bhilpert at shaw.ca>
On 2022-Feb-17, at 2:38 PM, Rob Jarratt wrote:
Presumably your high test load plus the initial cap-charge current is
pushing
-----Original Message-----
From: Brent Hilpert <bhilpert at shaw.ca>
20V on a 10 ohm load: current = 2A.
15V, 1.5A.
In this regulator design there is no path for more current than that
which the load draws, aside from temporary peak currents to charge
capacitors. If you're drawing 5A DC from the bench supply, something
beyond 'failure to start' is wrong.
That's interesting. On the H744s I have observed that if I have a high
load the bench PSU current limiter operates and the regulator cannot
output +5V, but if I start with a lower load and then add load, it can
continue to operate. Is the H745 different to the point that I
shouldn't expect this kind of behaviour? If it is the same, then why
do the H744s do this? I have tried waiting a few moments to allow the
input capacitor to charge up, but the regulator just does not start. the bench PS into current limit, that is, with a
high load there is less
available
current to charge the caps before the bench PS
starts current limiting.
This
would slow down the cap charge rate, so it would
take longer for the caps
to
charge. I can't say I see it 'stopping
starting', but it would lengthen
the time to
'start'. How long depends on the numbers.
There may also be some
dependance on how your bench PS responds in current limit.
But if that was the case shouldn't it just take a bit of time to get going,
once the input cap has charged wouldn't it start regulating? It shouldn't
take more than a few seconds, but it never seems to start. Anyway with the
H745 the problem seems to be elsewhere.