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I have a board that drives three bistable relays for turning on/off three 12V loads.

To switch the relays, the board provides 6 outputs that connects a battery-negative pulse of 150ms at a maximum power rating of 10W at 12V. Each of these outputs drives one of the two coils (either on/off) of one of the 3 relays.

However the relays require a 150ms pulse of 36W at 12V, which is too much for the board to handle.

I plan to get 6 MOSFETs to increase the power rating of these outputs. I will connect each MOSFET gate pin to a board output, the source to the battery positive. This way, when the board attempts to switch a relay, it will apply a voltage between the gate and the source and turn the MOSFET on. The drain pin of the MOSFET goes to the relay coil (the other side being connected to the battery negative).

I think I need N-channel MOSFETs with a minimum voltage gate-source threshold (VGSTH) of ~10V and a maximum drain-source voltage of at least 15V. Considering how brief and infrequent the pulse will be I don't think the other specs really matter, but ideally the drain/source resistance when on (RDS-on) of as low as possible.

Will this work? Am I speccing the MOSFETs needed correctly?

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  • \$\begingroup\$ @mkeith yes they are bistable relays (latching relays) so they have 4 terminals for controlling the switch (a pair to turn on and a pair to turn off). I got a downvote and as I'm new to the site I am not sure if I have done something wrong? I would appreciate a comment explaining how to ask questions better in future. Thanks! \$\endgroup\$
    – Roger992111
    Oct 9, 2021 at 13:28
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    \$\begingroup\$ In my opinion, there is nothing in your question that justifies a downvote. I suppose it could be improved by adding a schematic diagram of what you propose to do. If you wish to add the schematic, you can use the built-in schematic editor. Edit the question then click on the button that looks like schematic symbols. \$\endgroup\$
    – user57037
    Oct 9, 2021 at 16:28

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Gate-source voltage threshold is the value when the MOSFET just barely starts to conduct. In other words, you don't care about that value for MOSFETs used as a switch. What you care about is the Vgs used to achieve the rated RDson.

You also do not need a MOSFET whos rated RDson is achieved at the 12V your board outputs. As long as the Vgs at which the MOSFET achieves its rated RDson is lower than 12V your board sends to it, and 12V is lower than the maximum tolerable Vgs, you are fine.

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