17 Feb
2022
17 Feb
'22
11:40 p.m.
-----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:
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.
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.
rectifiers
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 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