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[on:]

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. 

[Reply #1 on:]

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

[Reply #2 on:]

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).

[Reply #3 on:]

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.

[Reply #4 on:]

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.

[Reply #5 on:]

Common usage of "transistor" is usually to refer to a bipolar transistor (NPN or PNP) rather than a FET like the IRLB8721.
People say FET or MOSFET typically to distinguish.  However this may vary by culture.

[Reply #6 on:]

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.

That's a very high price for such a commonly avalible transistor.

1/10 the cost and free shipping:
http://www.ebay.com/itm/10pcs-Transistor-2N2222-KSP2222-TO92-NPN-/320760721563?pt=LH_DefaultDomain_0&hash=item4aaed4309b

If you don't like E-bay then

http://www.taydaelectronics.com/t-transistors/2n-series/2n2222a-2n2222-npn-transistor-0-8a-40v-to-18.html

Tayda sells lots of common electronics parts for low costs.

Lefty

[Reply #7 on:]

Ok I installed the transistor that was suggested but I still cannot get the Arduino to turn the heat off and on.  When I put the wire that goes to the Arduino on +12VDC it turns off, and when it is not connected (or connected to the Arduino) the heater turns on.

I have included a schematic of what I have wired up.  I am still learning to use this software but all the grounds are common and the two +12VDC are actually on the same rail.

[Reply #8 on:]

Have you really got a 1.11 Ohm resistor? If so that is way too low and will cause 12 Amps of current to be drawn through that bottom transistor.
Change that resistor to 1K.

[Reply #9 on:]

Show us your sketch...
Prove to yourself the sketch is working (use a LED with 220R resistor on the o/p pin)

[Reply #10 on:]

1.1ohm could be 1.1k? makes more sense..

[Reply #11 on:]

ahh I'm sorry it is a 1.1k Ohm resistor.  I had that but when I tried to export the image the program crashed so I had to redo all the text. 

The sketch I am using now is the blink example sketch that comes with the Arduino.  I tried hooking the 270 Ohm resistor to pin 13, ground, and +5 VDC to see if anything made a difference and nothing did.

[Reply #12 on:]

Take a good quality photo of your set-up so that others might analyse your situation.

[I looked at the specs of that FET (IRLB8721), I think that it should operate without any NPN driver before it.]

Try it with an LED + 1kΩ resistor in place of the heater.