250mW LASER stuck on

[korky]

Hank,

Many thanks again.
I think I'd better leave this for today.

I couldn't find a pot to make a divider into the gate... until I realised there was one on the LM317's adjust line that I could 'borrow' for a bit.

There's me thinking that the gate won't take much current...
There's me thinking that the little 20 ohm trimpot will be fine.
There's me, connecting a 20 ohm trimpot straight across 12v.
There's melting plastic oozing out of the sides.

<sigh> Life is a learning experience.
Ah yes. The smell of burnt plastic makes me remember Ohms law. V=IR. A 12 volt drop across 20 ohms is... 0.6A

I'll find something other than a 20 ohm trimpot in the next few days, and see what the actual cutoff voltage is...

Keith.

[irving2008]

Ummm, think there is some confusion Hank...

With an N-channel FET a positive gate voltage with respect to the source pin turns it ON, by charging up the gate capacitor. A zero gate voltage will turn it OFF (eventually) when the gate charge dissipates.