I'm trying to build a cheap motion activated LED light system.
Circuit is:
24V power –> LED –> LED –> resistor –> (drain) mosfet (source) –> GND
LED are 10V 10W on aluminium heat sink, resistor is 7.5ohm 5W
Flowing current is about 0.55A. I choosed to drive the LED at half its nominal current because I noticed that the light level does not increase proportionally to the current. Two LEDs at 0.5A emitt far more light than one LED at 1A, and heat less.
Due to the 3.3V output level of the PIR I choosed a FQP30N06L mosfet which should have a 2.5V max gate threshold voltage.
PIR sensor is powered with 5V, the GNDs of the 5V and 24V power suppliers are connected togheter.
Mosfet gate is connected to the output of the PIR sensor which can be 0V or 3.3V.
The system is working but the mosfet seems not to be fully on, the LED seems to be at 10%.
When manually activated, switching mosfet gate to GND or +5V, the circuits works great, LED is fully ON or OFF, so I think the problem is related to the 3.3V output level of the sensor.
I guess when powering with 24V, max gate threshold is not 2.5V anymore but raises, so 5V will work but 3.3V is not enough.
Whay would you do?
Should I search for another mosfet which will work in this conditions, or I've better build or buy a gate driver circuit?
TIA
Best Answer
You can easily increase the gate voltage by 1.2 .. 1.4 V using a pull-up resistor and a couple of diodes:
This will toggle gate voltage between 1.2 and 4.5V instead of 0 and 3.3V. There may be a small current flowing into the PIR when it drives its output high, which is undesired but which the PIR will most probably cope with.
A more elaborate solution using an extra BJT would be:
simulate this circuit – Schematic created using CircuitLab
This will drive the gate between 0 and 5V, as you did manually, so it should work.