Multi-way 12v dimming

[dbuckley]

Thanks Tomo, thats a solution I hadn't considered. A quick browse shows the IRL520 might be a suitable component, or better still, a STP36NF06L.

 

A pull-off reiisistor - thats betwtixt gate and ground, yaas? Will a 1M there make a difference...?

[Tomo]

A pull-off reiisistor - thats betwtixt gate and ground, yaas? Will a 1M there make a difference...?

The STP36NF06L looks a pretty good choice for anything under 15A 'normal' load, driven from a 5V logic level - 0.75W dissipation at 100% duty cycle, probably no higher than 1.25W at 50% assuming reasonable switching curves.

 

You generally do need the 1M resistor, for an N-Channel MOSFET it's between gate and ground.

It's primarily for ESD protection - MOSFETs are notoriously sensitive to ESD, although new designs are pretty tough, especially in the sub-50A range.

 

1MOhm is usually plenty small enough - at 5VDC this is a current of 5 micro-Amperes so I suspect your PIC will be happy to drive it.

 

The small resistor between PIC and gate (Rg) is primarily a current limit, so you may want to go a little higher than 1 ohm to ensure current doesn't exceed max. peak drive.

 

You'll also need to watch out for inductive loads - most MOSFETs have an integral diode that'll protect it under most circumstances, so don't get one that doesn't have one. I've blown MOSFETs clear across the room by making a mistake with inductive loads...

(Admittedly this was >200A peak, with 750W motors)

 

That said, if you're driving under 20A or so most of that is basically irrelevant as there simply isn't enough energy to blow it up unless you are driving motors or deliberately big inductors.

- On the other hand, remember that the headline rating is impossible to achieve as it's for a junction temp of 25deg C - and you can't keep it there!