Home » Public Forums » GMCnet » Isolater vs Alternator (My experience)
| Re: [GMCnet] Isolater vs Alternator [message #326630 is a reply to message #326628] |
Sat, 02 December 2017 06:51   |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma:
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Senior Member |
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John,
That would certainly work if you use the two ac terminals as the two
battery terminals and the (-) terminal for the alternator input. That
would provide the necessary isolation of the batteries. At the (+)
terminal would appear the highest of the House and Chassis battery
voltages, less about 0.7 VDC. For those using electric windshield wipers,
that could be used as the +12 VDC supply, providing a speed reduction of
about 5%, which many would like. Or it could be used for any accessory for
which constant supply from either battery was desired (radio?).
The BIG question is whether the bridge, despite its rating, would survive
for very long. Certainly it should be mounted to the aluminum plate, or,
preferably, a finned heat sink, with thermal paste. I would NOT use the
100A version. Since they're designed for ac rectification, the average
current through each pair of diodes is only 1/2 of the bridge's rating.
Longevity may still be an issue.
Without knowing the internal construction of the bridge, it's difficult to
evaluate its probable longevity, but I suspect it's not as robust as the
isolators. Those have two diodes with their metal cases bonded to a common
aluminum plate to which the alternator input is connected; the other ends
of the diodes connect to the batteries. That assembly is potted with an
epoxy for environmental protection. I've never yet seen nor heard of a
shorted isolator. They predominantly fail by becoming open. I've cut,
broken, ground away the epoxy on a couple of failed ones and found the
"failed" diode (still good) separated from the aluminum plate. My theory
is that the different thermal coefficients of expansion of the diodes,
aluminum, and epoxy stresses the diode-plate junction, causing it to
eventually fail. Isolators are designed for the continually varying
current and environmental conditions to which we subject them, yet they
eventually fail -- too frequently.
I suspect that the subject bridges are intended for use in primarily
constant loads and may be more likely to fail in our application.
Thus, I prefer a robust combiner, which totally eliminates it as the cause
of the loss of feedback condition which causes runaway alternators. I have
had 2 of the Yandina C100 combiners fail. After one replacement under
warranty, the second time I upgraded (with warranty credit) to the C135,
which has given no trouble for 5+ years. It should be remembered that
Anna-Marie Foster, Yandina, inventor of the combiner, originally rated the
C-100 at 50A. Competitors offering almost identical units with 100A
ratings forced her to change her specification -- with no design change. I
consider the original 50A rating more appropriate.
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI & EBL,
Manny Brakes & 1-Ton, Troy-Bilt APU, etc., etc., etc.
www.gmcwipersetc.com
On Sat, Dec 2, 2017 at 5:10 AM, John Phillips
wrote:
> Sounds like the isolator to the chassis battery opened which increased the
> alt. output and cooked the house batteries. If the house battery load had
> been the problem the other side of the isolator would have opened and the
> house battery would have been saved.
> Looks like you we should oversize the isolator. We should be able to build
> our own isolators from diodes.
> Correct me if I am wrong but I think we could use half of a full wave
> bridge like:
> https://www.amazon.com/Baomain-Bridge-Rectifier-MDQ-
> 100A-Module/dp/B01JIKSHCA/ref=pd_sim_328_3?_encoding=UTF8&psc=1&refRID=
> AS5H1C1VNSSMRHE2WHZZ
> or
> https://www.amazon.com/Baomain-Bridge-Rectifier-MDQ-
> 200A-Module/dp/B01JILFZCO/ref=pd_bxgy_328_2?_encoding=UTF8&psc=1&refRID=
> 7VY1G9T5A8A558NREZJJ
>
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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