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I've been trying to source a latching relay with a 5VDC coil and a 12VDC, 30amp contact rating. It's easy to find latching relays up to about 16A... but beyond that most are non-latching and the rest are astronomically expensive. So... if I were to use a DPST or DPDT latching 16AM relay, could I just split the current by using both switching contacts, effectively creating a 32A relay?

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  • \$\begingroup\$ I'm pretty sure this has been asked before \$\endgroup\$ – Voltage Spike May 10 '17 at 22:12
  • \$\begingroup\$ Have you looked at using a contactor instead of a relay for that sort of load? They cost but they're a good solution. \$\endgroup\$ – TonyM May 10 '17 at 22:54
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No, switch and relay ratings can not be doubled up like that. What happens is whichever opens last or closes first takes the brunt of the load so you really have to go with the single contact rating.

However, it is a fairly simple matter to turn a DPDT non-latching relay into a latching one. Simply use the second contact to drive the coil in parallel with whatever you are currently driving it with.

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  • \$\begingroup\$ Thanks, @trevor. Good to know. I wondered about that. It's milliseconds... but, yeah, probably enough to fry a contact. My objective in using a latching relay is to avoid current drain while the relay is latched. Leveraging the second contact as a means of 'latching' would cause an unacceptable power drain. \$\endgroup\$ – bcrimmins May 10 '17 at 21:46
  • \$\begingroup\$ @bcrimmins ya I wondered about that. Try and find a solid state one. \$\endgroup\$ – Trevor_G May 10 '17 at 21:47
  • \$\begingroup\$ @bcrimmins Or find a 12V one and drive it through an optocoupler. \$\endgroup\$ – Trevor_G May 10 '17 at 21:55
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You can easily find relatively inexpensive 100A SPST latching relays with 12V coil. A boost converter could be used to generate 12V from 5V. You can also buy them with a 5V coil, perhaps subject to minimum order quantity (MOQ).

Such relays are used for switching mains voltage to a small house, in smart meters etc. Life is typically 10K operations, so much less than conventional relays, but at your reduced current and voltage it will probably be much better.

You would need two of them to get the SPDT functionality, and break before make would not be inherently guaranteed.

Edit: The point has been raised that you cannot assume ratings are the same for DC and AC, and in fact the rating for a 100A 250VAC relay would be considerably less at 250VDC if it was suitable at all. However, experience (and the K100 data sheet, for example, in the link supplied, which indicates 28VDC/100A is within ratings) indicates that 250VAC rating can conservatively be taken to indicate rating at 12VDC and perhaps at somewhat higher DC voltages. Self-heating is the same at any RMS current, so one can never assume a higher current rating.

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  • \$\begingroup\$ Thanks, Spehro. Do you have a link to an example of such a relay you should share? \$\endgroup\$ – bcrimmins May 11 '17 at 0:27
  • \$\begingroup\$ @bcrimmins kgtechnologies.net/60-100a.php There are a number of others. Apparently prepaid energy is common in some countries- feed the meter or the power shuts off. Should be few dollars in volume. \$\endgroup\$ – Spehro Pefhany May 11 '17 at 1:11
  • \$\begingroup\$ Thanks, @spehro, those are interesting. If a relay is rated at, say, 250 VAC at 20 amps, how many amps can it support at 12 VDC? Seems like it should support more than 20 amps at only 12V. Is there a formula to calculate that? \$\endgroup\$ – bcrimmins May 11 '17 at 3:06
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    \$\begingroup\$ In general, unless the contacts are rated for D.C. Switching, do not assume that the DC current rating is anywhere near the AC rating. With AC, the arc is self-extinguishing when the voltage goes to zero every half-cycle. With DC, the arc may not extinguish, destroying the contacts. \$\endgroup\$ – DoxyLover May 11 '17 at 4:17
  • \$\begingroup\$ You can assume that the rating will be the same at 12VDC (limited by self heating). Here's one with ratings at up to 48VDC. ev-power.eu/DC-Contactors/DC-Power-Latch-100A-Coil-12V.html \$\endgroup\$ – Spehro Pefhany May 11 '17 at 6:54

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