Go Down

Topic: transistor gate does not respond to arduino (Read 1 time) previous topic - next topic

tummyacid

Hello everyone! I am building a small heating project -- think of it like a crockpot.  I have nichrome wire wrapped around a ceramic vessel to hold liquid at a specific temperature.  The crockpot uses about 3 amps at 12 volts and can boil 4 oz of water in about 30 minutes.

Anyway the trouble I am having is I want to use a transistor to turn the heater on an off.  I know a relay might be better but I was hoping to keep everything solid state.  I am using this transistor:

http://www.adafruit.com/datasheets/irlb8721pbf.pdf

The problem I am having is with using the gate pin on the transistor to actually turn off the heat. With the heater circuit wired up thru the transistor everything works fine and the transistor even stays pretty cool to the touch while heating the water.  If I touch the gate pin to the 12V+ rail, it turns off the way I want to.  If I try to drive the gate pin from the 5V on the arduino however, nothing happens.  The heat stays on.

12V is coming from an old laptop power supply I had sitting around while the arduino is being powered off my USB port. 
try not to lose that magic smoke

Tom Carpenter

You are going to have to post a schematic of how the transistor is wired.
~Tom~

CrossRoads

You need an open collector transistor to control the gate.
Use a pullup resistor to 12V for the high state of the gate.
Use the arduino to drive a 2N2222A type NPN transistor to bring the gate low.
(arduino drives base thru 270 ohm resistor, collector to the MOSFET gate, emitter to gnd).
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

tummyacid

Thank you very much! 

I was joking to myself that perhaps another transistor would do the trick but I guess that isn't such a bad idea after all.  I am pretty sure I've got some of these in my parts bin but just in case I am going to order these:

http://www.amazon.com/microtivity-Bipolar-Amplifier-Switching-Transistors/dp/B0092CRK5A/

think they will work?

I will also post a diagram as soon as I get home tonight.
try not to lose that magic smoke

Tom Carpenter

They'll do. Just connect the emitter to ground, the collector to the gate of the MOSFET (and a 10k resistor to +12v), and the base via a 1k resistor to the arduino.
~Tom~

Go Up