250mW LASER stuck on

[HankMcSpank]

I reckon you need to establish what the cutoff voltage for your FET really is (as it's being help 'on' all the time)....like I say the 'actual' can differ somewhat from the 'spec'.

here's what I'd do...

('scuse the scribble ....back of an envelope - literally, & while cookinng the family bacon sarnies with my other hand!!)

...you'll need a variable resistor** (can be any value really)...just adjust the pot's wiper until the LED turns off & then measure your voltage at the gate. I reckon you'll find your fet needs more than +5V to turn it off. If so, you'll need a FET with a low cutoff voltage.


** if you don't have a variable resistor you could use a resistor chain to get different voltage presented at the gate. Eg a 5k + 5K resistor chain will give you 6V at the gate etc.

[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.

[HankMcSpank]

...you'll need a variable resistor** (can be any value really

I should have said... "can be any value above!" (sorry, I assumed when you said you knew a little that you would know not to go that low! Pots are obviously available in lots of resistance values, but 1k, 5k, 10k, 22k, 47k+ etc are the more common ones)

Yep, a 20 ohm trim pot would generate a bit of current with 12V across it!

Irving is right, a positive gate relative to source voltage turns the thing on (n-channel, p-channel moshfestch. schmoshfesh) But the test I poropsed is still valid, as you turn the wiper in the direction towards 0V with the pot wiper, eventually the FET should turn off (got to establish if it's possible to turn your particular fet off & at what voltage)

Cheers Irving ...I'll get me coat!