Isolation - when is it needed?

[battpower]

Hi there,

I have a circuit where I need to operate a solenoid on a truck from a PIC microcontroller. The electrical environment on a truck is very noisy with the alternator having various loads (such as lights, air con etc) being switched on and off that can cause load dumps.

One side of the solenoid is permanently connected to +12v, and the other side needs to be switched to ground to operate it. I want to use a logic level MOSFET to pull it to ground. Is it a bad idea to drive the FET directly from the PIC, or should I use some form of isolation such as an optocoupler between PIC and FET? I'm concerned that the truck ground may be noisy and could affect my digital circuit. Would a relay be a better idea and not use a FET at all? I guess for true isolation I would need to use a DC-DC converter for the supply as well since it is powered by the truck +12v. ie a transformer isolated DC-DC converter.

This leads me to main main question; what situations is electrical isolation required? Is it just good practice, or are there specific conditions where isolation is mandatory? Apologies if this is a bit vauge.

Thanks

 

[pop]

As far as I know, you always isolate TTL voltages from higher AC or DC voltages. You're dealing with only 12V and if you're going to use a mosfet you're looking at a very low reverse current, so it could be ok if you connect the pic without isolation to the mosfet. Make sure though that in this case, the gate current at all times less or equal to 25mA. A safer way is just to use a 3VDC actuated relay.

Hope this helps somewhat

 

[Russlk]

There should be no problem driving the FET gate direct from the PIC but the ground must be common or ground noise could be a problem. Don't forget the diode across the relay coil to catch the backswing!

 

[Oznog]

Absolutely no reason to use a relay. There is no continuous gate current with a MOSFET and only a miniscule switching current.

Basically an N-channel pulling it down is ideal and there's no logical reason to complicate it. You do need the flyback diode across the solenoid. You also need a pulldown on the gate, since the PIC leaves the pins tristated until it's completed a powerup, the gate's floating so it may trigger accidentally.

Do get a MOSFET with a fairly high Vds in the "Absolute Maximum Ratings" section. 30V would be good, that should be able to take the worst of surges.

 

[zevon8]

Hi

I have had great success using the ST Micro series of Omni FET's for low side load driving in vehicles. They are logic level, well protected thermally and overcurrent. I have used them on motors, lamps solenoids, etc, and have had no problems. I have had no problems with noise or false triggering.

International Rectifier has a similar line, but I have not used them.

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