Re: Diode or MOSFET for back EMF?

Sep 05, 2003 1 Replies
T


I would like to protect ECU 200, and the relay contacts


> Would it be advisable to put in either a diode (such as between
> relay 155/terminal 87 and fan 172 (I'm not sure which fan and
> coil is the culprit, as stopped testing to prevent poss ECU
> damage [if not too late], but per diagram possibly fan 172 and
> relay 156 {thru closed relay 155}) or a MOSFET (where)?

You need to look closer at that Cartier J relay, specifically what happens between terminals 85 and 85A. I suspect (and/or hope) that 85 and 85A provide a diode'd OR function, where either terminal can energise the relay coil without feeding current back into the other terminal.


85A 85 _|_ _|_ \_/ \_/
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L

You're exactly right wrt the Cartier J 155 relay: it had a worn, but just discernable paper sticker w/schematic, (also) showing 2 diodes at

85 & 85a (as does the URL schematic, which I missed).

After seeing the diodes present I retested the circuit - it is indeed fan 37 and relay 155/85a causing the rundown emf

Thanks for the continued help. My hope in this project was to improve fan cooling... not disable it ;-), or wreak havoc to other sensitive components. I still don't fully understand the circuit, as I loop around tracing the 3 relays (altho simple at the base level, I suppose). But all appears well, and I've learned a few things

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