Testing H745 Regulators

[Brent Hilpert]

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.


On 2022-Feb-17, at 10:19 PM, Rob Jarratt via cctalk wrote:
> Regarding the rating I am not clear what the rating of the original part is, I haven't been able to find a datasheet for it, I have seen suggestions for both 20A and 35A, I do know that the H745 regulator is fed 20-30VAC from a transformer.
> 
> So presumably going for a 35A rating is the safer bet, and going for a minimum of 50V peak reverse voltage would be sufficient?


At 30VAC input, peak V is ~ 44V, you're probably better off with rectifiers higher than 50 PIV to provide some safety margin.