0
\$\begingroup\$

i want to be able turn off a led with a mosfet and a pushbutton. In order to do this i want the led to be in off state when voltage is applied to the gate of the mosfet. After researching for a while i found out that i probably need a p-channel mosfet in depletion mode for this task. When somebody could give an advice which Mosfet Model i could use for this purpose, i would be very thankful.

\$\endgroup\$
  • \$\begingroup\$ Depletion mode devices are quite rare and expensive. It is best if you find other methods such as using a NC pushbutton instead of NO pushbutton, or devise an inversion or latching circuit that uses more common enhancement mode parts. We would be able to help you more if you gave us more details (for example, does the button need to be held down? Or does it need to latch?) \$\endgroup\$ – DKNguyen Feb 24 at 0:23
  • \$\begingroup\$ Yes the button should be held down for the amount of time, i want the led to be off. Thanks for the fast reply. \$\endgroup\$ – abdussamed17 Feb 24 at 0:36
  • \$\begingroup\$ "i want the led to be in off state when voltage is applied to the gate of the mosfet." - why? \$\endgroup\$ – Bruce Abbott Feb 24 at 2:02
2
\$\begingroup\$

Depletion mode devices are quite rare and expensive. In general, it is best if you find other methods such as using a normally-closed pushbutton instead of a normally-open pushbutton, or devise an inversion or latching circuit that uses more common enhancement mode parts.


UPDATE:

You said you want the LED to be off for as long as the button is held (i.e. not a latch).

Just use a normally-closed pushbutton. If you can't find one, it's easy enough to just use a normally open pushbutton in parallel to steer the current away from the LED to turn it off.

You can divert current through a regular diode (D1) that has a lower Vf than the LED. This will produce a voltage across the LED that is not enough to turn it on.

You don't actually need D1 and can use a true short-circuit across the LED to keep it off. But your R2 has to be high enough resistance or high enough wattage to tolerate the heat. D1 just helps reduce current when the button is pressed.

schematic

simulate this circuit – Schematic created using CircuitLab

But if you must use a transistor, you can always do this. It's pointless, but you can do it.

schematic

simulate this circuit

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Thank you very much for your advice. \$\endgroup\$ – abdussamed17 Feb 24 at 0:59
  • \$\begingroup\$ Your workarounds are helpful. So p-channel mosfets in depletion mode are expensive, right? \$\endgroup\$ – abdussamed17 Feb 24 at 1:03
  • \$\begingroup\$ @abdussamed17 I don't think you can even find them. Also, see my edits about D1 in the post above. \$\endgroup\$ – DKNguyen Feb 24 at 1:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.