| [GMCnet] Onan, again [message #98944] |
Wed, 08 September 2010 08:17  |
Mike Kilroy
Messages: 80
Registered: July 2006
Location: Farmersville, OH (near D...
Karma:
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Hmmm.... Looking at the schematic again, I see the missing diode and
the drawing showing the suggested location of one... Putting the diode
across the K1 relay to help contact K2A live longer is a really sneaky &
cool thing to do! To supply a path for the inductive kick when a
contact opens, a diode in dc circuits is usually added ACROSS the
contacts themselves rather than where shown across the coil. But in
this case, the inductive kick causing the sparking is caused by the coil
itself.
That said, putting a diode across the other coils is ALSO a good
practice to reduce the voltage spike caused by their shutting off! A
diode across the other relay coils K2 & K3 would have the same
beneficial effect suppressing their small spike (this spike is likely
around 80vdc for arouind 5msec on these size relay coils).
These diodes do not effect the contact eating spark across the contacts
of K1. K1 appears to have the largest inductive load - the start coil
inside the starter; not the starter itself but the "K" coil inside the
starter. Seems like K1 would the most likely relay contact to fail due
to its high current switching load.
K2A has the inductive kick from relay coil K1 so its contacts would
benefit from diodes across them, even though its spike is being reduced
to a large extent by the added KH diode.
K2B doesn't appear to have any inductive load so would not benefit from
a diode across it.
K3 has the fuel pump coil & ignition coil so if one wanted to add a 3
cent diode across its contacts it wouldn't hurt.
So if one is going to go to the trouble of adding one diode, then they
should add a few more at the same time and help all that can be helped.
I normally use 1N4004's or 4007 since I have them around, they are
common, and cost only a few cents each.
>
> I've never been able to identify any external cause of board failures;
> generally it seems to be just general degradation, especially of relay
> contacts. There is one addition I recommend because I think Onan
> screwed up: At the left of the attached redrawn Onan diagram (marked
> 4kW but the same as 6kW), the coil of relay K1 shows a "KH Added"
> diode. That's to suppress the inductive surge through the contacts of
> relay K2 when its contacts open. USAF lost a couple of very high
> value Remotely Piloted Vehicles before I discovered a manufacturer's
> similar screw-up -- that's how I repaid the cost of 3 years of EE
> school. :-) Ken H.
>
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Mike (AC8V) & Vickie Kilroy
'73 Canyon Land 26' sidebath
455/ceramic filled crossovers
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