250mW LASER stuck on

[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.