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MOSFET choice for relay switching


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I've got a fairly general electronics question, but you guys are the most helpful I know so I thought I'd ask here. I want to drive a 24V (0.5W) relay coil from a Tiva and need to pick an N-channel FET to do so.

 

I know some of the basics - checking it's a logic level FET, etc. but it's still difficult to narrow it down to a few hundred choices. Priority for me is probably SMT and few external components just to make assembly a bit easier.

 

This seems to be the best I found, but does anyone else have a go-to alternative. I think it should only need a pull-down on the gate to prevent accidental turn-on during power up. It should be rugged enough to not need an external flyback diode.

NXP BSS138BK

 

A good choice? Anything better out there?

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I don't particularly need to avoid a flyback diode, it's just that the one I linked to seems to have one integrated. I didn't read all the associated documents in detail but it looked like they have tested it with inductive loads. The threshold voltage is 1.1v and it's marketed for automotive and for relay driving. Maybe I've stumbled across a particularly suitable device - mostly by luck.

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I was going to suggest a IRLML2502, but I see they are only rated for 20V. The one you suggest has terrible drain current ratings because the RDSon is relatively high. You might need to look for something with an RDSon less than 100 milliohms, because the relays may draw quite a bit of current.  

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I don't particularly need to avoid a flyback diode, it's just that the one I linked to seems to have one integrated.

There is no flyback diode in this device.

 

All MOSFETS have a parasitic body diode, but it is no help for you because it is across the source/drain channel, not across the relay.

 

The NXP's BSS138 model has ESD diodes that protect the gate insulation, but again, these does not help you.

 

You cannot avoid putting a separate diode across the relay.

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Just tossing these out there:

stp62ns04z

People are using these to control high-impedance fuel injectors without external diodes, triggered by a 5V logic level signal but presumably can trigger with as little as 2V on the gate.

 

ISL9V3040 might be an option. They're used to control ignition coils in automotive uses and do not require external diodes.

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  • 2 weeks later...

FWIW, in this case I decided to go with a ULN2803A as I had a few relays to drive. I had a couple of through hole ones lying around for a quick prototype and it comes in SOIC18 for a finished version.

 

That's sidestepping my original question though - I also grabbed a few of BC817 (NPN transistor), BSS138 (N-channel) and LL4148 (general purpose diode for flyback) for the parts box. A similar thread to this started over on EEVBlog so some useful opinion there too.

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