Testing H745 Regulators

[Rob Jarratt]

> -----Original Message-----
> From: Brent Hilpert <bhilpert at shaw.ca>
> Sent: 18 February 2022 06:30
> To: rob at jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
> <cctalk at classiccmp.org>
> Subject: Re: Testing H745 Regulators
> 
> 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.

> 
> 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.

I always forget that the VAC is the RMS and not the peak-to-peak. I will
look for a minimum rating of 100V.

Thanks

Rob