• Announcements

    • bluehash

      Forum Upgrade   03/11/2017

      Hello Everyone, Thanks for being patient while the forums were being fixed and upgraded. Please see details and report issues in this thread. Thanks!
Sign in to follow this  
Followers 0
Fred

MOSFET choice for relay switching

12 posts in this topic

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?

Share this post


Link to post
Share on other sites

The 2N7000 would require 5 V to switch on completely, so it is not suitable for the Tiva.

 

Why do you want to avoid the flyback diode? This would require a high-voltage transistor, and those typically have higher gate threshold voltages.

Fred likes this

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

Is there a big difference between a MOSFET and IGBT for the purpose?

 

On the other hand... a diode meant for flyback / snubber purposes would seem appropriate depending on the frequency you'd expect it to work at.

Share this post


Link to post
Share on other sites

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.  

Share this post


Link to post
Share on other sites

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.

Fred likes this

Share this post


Link to post
Share on other sites

Hi,

 

Is there a special reason why it has to be a (N-channel) mosfet, @@Fred?

 

How about a plain old bipolar  (2N2904 npn) with base current limiting resistor; you'll need to use a flyback diode anyway.

So there's just one extra passive component   (diode + resistor).

 

Cheers.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0