MOSFET selection

[amgen]

they have these things..... SSR (solid state relay) 4 pin dip..... >1 amp, 60 v...... this is at Digikey but should be able to get most anywhere
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[Demon]

they have these things..... SSR (solid state relay) 4 pin dip..... >1 amp, 60 v...... this is at Digikey but should be able to get most anywhere...

Very interesting, I wasn't aware of these puppies. I have 5V 100mA mechanical relays similar to these:
https://partsource.ca/cdn/shop/produ...g?v=1686674311

I'll probably get a few SSRs from Digikey, just to have them on hand. But I really like MOSFETs; the AO3400A is only $0.0759 at JLCPCB.

Do you see anything wrong with the AO3400A for:
- max 800 mA load,
- 9V load,
- 5V trigger (or less),
- single burst use.

That weird spec for Vsd with Vgs of 0 threw me off:
Vsd, Diode forward voltage, 0.7V at Is=1A, Vgs=0V
I don't know what to make of that spec.

[amgen]

.. yes the 'AO3400A' are good for what you want, and the very low '0n resistance' means most all the voltage will be on the coil, which is what you want. The only thing about the SSR's is that the drive and load circuits are isolated from each other..... but sometimes that doesn't matter depending on supplies, grounds etc.

[Demon]

.. yes the 'AO3400A' are good for what you want, and the very low '0n resistance' means most all the voltage will be on the coil, which is what you want. The only thing about the SSR's is that the drive and load circuits are isolated from each other..... but sometimes that doesn't matter depending on supplies, grounds etc.

Excellent!

Yeah, that's why I considered the opto-couplers at first; to isolate the solenoid from the 5V circuit, but I'll be using separate sources:

- 5V from TPS56637 (for PICs, LEDs, and all other ICs),
- 9V upstream from TPS56637 (directly from wall adapter).

I just realized I should add a 9V limiter with a Zener diode, just in case a user substitues the 9V wall adapter for 12V, or even significantly larger. I don't want the solenoid to be propelled through the casing like a bullet.


EDIT:

After reading up on the Zener used as a voltage limiter, it's not a good option for nearly 1A of current.

I've since switched to a simpler approach. I already have a TPS56637 circuit; 7-12 Vin, 5V 5A out. This is what I used to power all ICs and such.

I'm going to make another using the same design (TI has an online tool to generate this); 9-12 Vin, 9V out (datasheet for TPS56637 says up to 28 Vin). I tested 5 Vin on my existing circuit; it drops off only 0.5V down to 4.5 Vout.

I just tested the solenoid again at exactly 8.5V and it works just fine.