12 Jul
2011
12 Jul
'11
7:49 p.m.
there are two designs of the relay assembly on the H7140 fiche here,
the old ones parts list has a
Resistor (R1) 3 ohm 5% 7 W 13-14848-00
the new one has a sub assembly on that with thermistor listed as
Thermistor 7.0 10% dec part number 13-17198-00
Dave Caroline
On Tue, Jul 12, 2011 at 7:33 PM, Jarratt RMA
<robert.jarratt at ntlworld.com> wrote:
Actually I am not sure it does make sense, but then I
am not an electronics
expert, so I may have this wrong.
This resistor is not in parallel with the coil, but connected across the
terminals of the relay, ie the terminals the relay is designed to short when
it is switched on. The resistor is supposed to limit the current going to
the big electrolytic smoothing capacitors that come right after the bridge
rectifier and provide enough power for the circuitry to check that the mains
is "OK" and then switch on the relay when the capacitors have charged up and
connect them directly to the bridge rectifier instead of via the inrush
limiter.
I hope that makes sense.
I still need to know what the spec of these components is so I can find
replacements.
Regards
Rob
On 12 July 2011 15:03, dwight elvey <dkelvey at hotmail.com> wrote:
Hi
?I think you are talking about a dampenning resistor. This is
placed in parallel with the coil.
?The resistance is usually equal to or somewhat larger than
the coils resistance.
?Coils like to keep current flowing and don't like having the
current source broken quickly. The voltage would go up
until something arced or broke down.
?The choice of resistor would be determined by how quickly
one wanted the relay to release.
?Although, counter intuitive, a smaller value of resistance
would prolong the time for the relay to release. A higher
value would cause the release to be sooner.
?The higher value resistor would also create a higher voltage
across the resistor.
?At the instance the drive is removed, the same current that
was running through the coil is now running through the
resistor at that instant.
?This voltage adds to the source voltage and is the voltage
that the transistor ( if from a transistor ) has to hold off.
If the source voltage was 12V and it was an equal value
resistor, the transistor would have to take 24V when the
relay was turned off.
?Make sense?
Dwight
From: robert.jarratt at ntlworld.com
I used the wrong term I think, inrush limiter is the right one.
Rob
actually
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Rob Jarratt
Sent: 12 July 2011 04:10
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Identifying the Surge Suppression Resistor in PDP11 H7140 PSU
Work is continuing on repairing the H7140 PSU from my PDP11/24.
Now it seems that the surge suppression resistor across the relay has
failed.
> This resistor is marked as R1 on the printset (p66 and p69) but is two
(in-circuit),
components connected in series. The parts list on
p71 of the printset
lists a
3ohm 7W 5% resistor, but it is not 100% clear if
this is supposed to be
the
> surge suppression resistor, although it does seem likely.
> The thing is I measured the resistance of the two resistors one was
> open circuit and clearly the failed component, the other measured
10ohms,
it
> might be failing too I suppose.
>
> The two resistors are marked KCC 13-17198-00 8234. I have not been able
to
> find any information on them, so I can't
verify the spec and I don't
know
what
> to replace them with. Does anyone have any information on these
resistors?
Thanks
Rob