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I need a little terminology help so I can search for and obtain the correct relay.

The relay will control a 48v 32a circuit. It should be normally open unless a signal is present. If the signal is present, the relay will close to complete the 48v circuit . The signal current level is 175mA maximum.

The relay will not be on a PCB or rack of any kind, but probably mounted on a plywood board.

Does such a relay exist? How would it be described? Someone said I'd need to amplify the small signal in order to control large contactors. Is that true, and if so, how would I configure (or describe) that?

If someone has a specific brand or part to recommend, that would certainly be welcome.

EDIT: I have a 48v battery bank of LiFePO4 cells. The SOC cannot drop below a certain percentage or it will damage the cells. The battery management system (BMS) has a method to send a signal if the pack is okay to use. Here is what the BMS manual says:

An open drain digital on/off signal (is) used to signal to a load that the load can discharge the battery. This would normally be used to control a discharge contactor or to signal to a controller that discharge must be stopped if this signal is not present. This signal can be used as a backup to digital CAN communication with a controller. This is a signal current level (175mA max) and should be amplified for controlling large contactors or relays.

So it would seem that I need a relay, but there are so many types I'm having difficulty knowing how to narrow the search to what I need.

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The relay will control a 48v 32a circuit.

Your relay will require a coil voltage to suit your control circuit voltage. You haven't stated this as other than "signal".

The contact(s) will require a rating of ≥ 48 V DC and ≥ 32 A DC. (note capital 'V' for volt and 'A' for ampere.)

It should be normally open unless a signal is present.

Almost all relays feature a normally open (NO) contact. One with a changeover contact can be used. The normally closed (NC) contact would be left unterminated.

The relay will not be on a PCB or rack of any kind, but probably mounted on a plywood board.

You are looking for a panel-mount relay base.

Does such a relay exist? How would it be described?

Yes. Described as 48V DC plug-in relay with 32A DC NO contact.

Someone said I'd need to amplify the small signal in order to control large contactors. Is that true, and if so, how would I configure (or describe) that?

Please edit your question to supply details of the "small signal".

If someone has a specific brand or part to recommend, that would certainly be welcome.

Part recommendations and where to buy them are off-topic on this site.

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  • \$\begingroup\$ I edited the question to add info. \$\endgroup\$ – MorningCloud Jun 22 '19 at 21:20
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You need a relay that's rated for 48VDC or more, and 32A or more, with form A (normally open) or form C (transfer -- this is the sort that @transistor talked about). Note that relay currents get complicated -- AC current ratings are usually higher, and for DC, "making" or "holding" current is higher than "breaking" current.

If you're going to break a 32A circuit then there's a possibility of some serious inductive kickback, which would need a snubber circuit designed to match the load presented when the relay breaks. Just a bare relay could undergo some pretty serious arcing every time it opened.

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  • \$\begingroup\$ Any particular kind? Like above, is there a specific description for this snubber? When I search on snubbers, I get all kinds of results. \$\endgroup\$ – MorningCloud Jun 22 '19 at 21:23
  • \$\begingroup\$ The problem is that the snubber needs to match the load, and it's getting well into safety-related territory. If you can find a contactor that's rated for 48VDC, 32A breaking current I think that'll be enough (and don't blame me if you burn your house down). \$\endgroup\$ – TimWescott Jun 24 '19 at 4:53

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