250mW LASER stuck on

[korky]

Kip, Good suggestion - except that it's on *all* the time. It's not reversed off-is-on and on-is-off.

No Eye Dear - yes, very good...

Reminds me of the LASER safety sign: "DO NOT LOOK INTO THE BEAM with your remaining good eye"

Irving
Thank you - the diagram is really useful.

The FET I'm using is an FDN359AN - a fiddly surface mount thing.
The Vgs max is +-20v, which looks fine.
The gate threshold for switch-on is 3v max, 1.6v typical.

The circuit I've used is *very* similar to the one you show - An LM317 rigged as a current regulator.
What I don't have though, is the diode into the Gate of the FET, or the resistor down to ground.

*Numpty question*
Are they important?

(I'm not sure if this changes anything, but I ought to admit that it's not a straightforward BOB it's connected to. It's a CRH Electronics 'System 3+' driver board. Reading the instruction manual, it says that the output I'm connecting to the FET is 'buffered and capable of driving up to 30mA')

Keith.

[irving2008]

the diode/resistor is to allow gate charge to drain away and allow the FET to turn off fully. A buffered logic output *should* pull down hard enough to do so, but I have experience of them not doing so...

If you have a test meter, what is the voltage swing at the gate? If you disconnect the BOB does the laser go off? It could be a dodgy FET of course... or reversed so the source-drain diode is conducting... (I know you've probably checked that, but I've lost count of the number of times I have checked and doubled checked and *still* managed to get it back to front!)

[irving2008]

It's a CRH Electronics 'System 3+' driver board. Reading the instruction manual, it says that the output I'm connecting to the FET is 'buffered and capable of driving up to 30mA'

To follow up this comment...

A FET is charge driven not voltage driven, the gate is a capacitor. The FET turns on when the gate charge exceeds a specific amount and off when that charge is depleted below a certain limit. Charge = current x time so to turn the FET on you need to pump enough charge in which means charging the gate capacitor. The datasheets helpfully give the charge required, in this case about 3.5 nanoCoulumb (nC) which means a pulse of about 1uS and 3.5mA will turn it on, and similarly a 3.5mA current should turn it off in about the same time. The actual gate voltage is not critical, its only needed to drive sufficient current into the gate capacitor and to maintain the charge as it leaks away.

Long and short of it is that a Bob output, buffered or not, should easily be able to drive and sink sufficient current to turn the FET on or off.

[korky]

Thanks for all the info and advice.

Irving, I've measured the voltage swing on the BOB output. 0.00 to 4.99 volts when switching the spindle off and on from EMC. Seems spot on.

Hank, I've taken the FET out of the circuit and carried out the check. With the multimeter on 'diode' setting, I got 0.17v accross source and drain when 'on'. As soon as I toucheda across source and gate, it read 'OL' (open).
That suggests the FET is working correctly, and suggests I'm not mixing up the pins.

I've then soldered the FET back into the circuit, and added the diode and 2k7 resistor to the gate for good measure.

Still no joy. LEDs light all the time. :sad:

The LEDs/LASER circuit draws 180mA (I've got it 'turned down' while I'm testing) whatever the BOB outputs. That's the same as it was drawing when I plugged the LED circuit straight into the 12v supply, without this FET switching malarkey.

More background:
The 12v supply is from a 'wall wart'. Transformer, 4 rectifying diodes and a capacitor.

The LASER driver circuit that came with the LASER looks like this...


Any more suggestions?

Cheers,
Keith.