Testing H745 Regulators

[Brent Hilpert]

On 2022-Feb-17, at 11:40 PM, Rob Jarratt wrote:
>> -----Original Message-----
>> From: Brent Hilpert <bhilpert at shaw.ca>
>> On 2022-Feb-17, at 2:38 PM, Rob Jarratt wrote:
>>>> -----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.
>> 
>> Presumably your high test load plus the initial cap-charge current is
> pushing
>> 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.

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.