# Switching laser diode with BS170 MOSFET

#### Hendrik Bleus

Joined Feb 22, 2017
3
Hi folks,

Im trying to switch a laser diode on and off by using a square-wave signal with a max amplitude of 3.3V (can be lowered to match Vgs,thr). The max current throught the laser should be 350 mA, voltage over the diode is 4V. Is a Vcc of 5V enough or should it be higher? Which resistor values should I use? Can you guys help me to make this circuit?

datasheet BS170: https://www.farnell.com/datasheets/2304244.pdf

Thanks

#### ericgibbs

Joined Jan 29, 2010
15,530
Hi Hendrik,
To drop a 5V to 4V [across the Laser] is 2.8R [~2.7R] @ 350mA.
The BS170 has a high on resistance of > 1.2R to 5R at 200mA.!

What are the details of the pulse drive signal.?

E

#### dl324

Joined Mar 30, 2015
14,475
Im trying to switch a laser diode on and off by using a square-wave signal with a max amplitude of 3.3V (can be lowered to match Vgs,thr).
The threshold voltage is the voltage at which the device just starts turning on. If you're going to be operating at 70% of the maximum rated current, you should be driving the gate hard enough to have a lower on resistance.
The max current throught the laser should be 350 mA, voltage over the diode is 4V.
Is that the measured forward voltage? Or the maximum?
Which resistor values should I use?
Just use Ohm's Law.

#### crutschow

Joined Mar 14, 2008
29,810
a max amplitude of 3.3V (can be lowered to match Vgs,thr
Many seem to completely misunderstand the Vgs(th) value of a MOSFET.
It's the value where the MOSFET just starts to turn on at a low Ids current (typically <1mA) so, to turn on the MOSFET as a switch with a low Rds(on), you want the applied Vgs to be well (several volts) above the max Vgs(th) value.
For an applied Vgs signal of 3.3V, you want a logic-level type MOSFET (max Vgs(th) < 2V) so it fully turns on.