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Re: (ET) 36vdc relays


On Mon, 7 Aug 2006, Bill Kelly wrote:


RJ,

Those 36vdc relays from SURPLUS CENTER definitely have a diode across the
coil.  I purchase two of the relays and decided to bench test them.

Not worried about polarity, I placed 36vdc across the coil and heard a pop
and observed a puff of smoke coming from the relay.  I removed power and
performed a continuity check across the coil and found that it was shorted.

I decided to find out what had gone wrong so I removed the plastic cover 
and
found a small diode across the coil connections.  I snipped the leads and
removed the diode (failure mode - short).
Damn. :) Guess I should have bench tested one first too. I just never thought they would put a diode across the coil - that just seems to create a possible failure mode, that you don't have without the diode. 


I retested the relay and it worked so the blown diode did not ruin the
relay.


Hm, interesting. Still, why put it in there in the first place?



Further inspection of the relay had shown a "plus" sign painted on the
bottom adjacent to one of the coil terminals.
Huh? Where did you see that? Before hooking the first one up, I inspected mine thoroughly and didn't see any + sign anywhere. Is it only visible if you remove the case? 


So, if you leave the diodes in, you must observe polarity with wire 38 (+)
and wire 13 (-).  I removed the diodes.
I'm still not going to use these - for one thing, they're absolutely huge, and don't fit in there well. For another, the screwon terminals are a pain - I didn't want to hack up the original wires, so I made short adapters going from spade terminals to fork connectors to put under the screws. That just makes an even bigger mess to try squeezing in there. Last issue - the second relay I tried would actuate fine (apparently I managed to hook the polarity up correctly on that one), but one of the connection points inside stopped making good contact after actuating only twice. I'm going back to the original style relays. 

Mike