Simple mosfet Q

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Coyoteboy

Senior Member
I'm a little confused i must admit. I thought I was pretty well up on interfacing these with components but ive struck something I cant figure out. I have an IRLZ44Z to power a small lamp (Overkill, will be used with different load later).
http://www.irf.com/product-info/datasheets/data/irlz44z.pdf
I've hooked it up through output 4, simple test pulse of 250ms on, 250 off. I am driving Pin1 of the IRL (gate) with output 4 (through a 4K7 for safety, and pulled low with a 4K7). Pin 2 of the IRL (drain) is linked to the load, and then obviously to the +12 rail. The source, pin 3, is connected to the 0v rail. When i do this the lamp doesnt light and after some time the Pic gets damn hot. I unplug all wires and it remains hot until i unplug and replug, whereby it remains cold and can be programmed again no problems. All outputs work with an LED/res combo. The IRL should switch with ~2v gate voltage and has protective resistors so i have no idea why the pic is getting hot and the lamp not lighting?!? I've rewired this twice now and i still have no luck.
 
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BeanieBots

Moderator
Well, it's either a wiring fault, dud FET or wrong value resistors.
Your pin connections look OK assuming pin 1 is to the left with the writing on top of the tab. The tab is also connected to the drain by the way.
Not sure about that particular FET, but many are very sensitive to static. It may have been zapped by bad handling.
Use a DVM to measure the resistance to ground at the PICAXE drive pin with the PICAXE removed. It should be about 9k4.
The 4k7 between gate and ground is a bit low but that's another issue.

Thinking more, can't be dud FET, the 4k7 would not pass enough current to make the PICAXE even get warm even if the full 12v was applied.
Has to be wiring fault. Shorted output, 12v going where it shouldn't or wrong resistor value.
Double check with DVM for short to 12v on any of the PICAXE pins.
 
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BeanieBots

Moderator
You should still find the fault though, because there is one!
Might be an output that (you think) is not connected to anything.
 
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Coyoteboy

Senior Member
Its something to do with the fet - if I touch the drain it turns on full (running it with PWM) and doesnt go off, gets hot rapidly. I wonder if I have internal shorts I cant see in my plastic proddy prototyping breadboard. Requires further investigation I think!

Thanks for checking my sanity!
 
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BeanieBots

Moderator
Do those checks now! 12v into a PICAXE will be instant death.
Very concerned about PICAXE getting hot. Not good! 4k7 shouldn't allow that to happen unless 12v is comming somewhere else.
Not surprised FET gets hot. You're only just turning it on with 4k7/4k7 from 5v. Replace 4k7 to 0v with 100k.
 
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Michael 2727

Senior Member
I'd be looking at the 4K7 output to gate and checking
that it isn't 4R7 or 47R, 47R will pass 106mA which the
picaxe output may survive a very short time and would
get quite hot until it died.

You don't need much gate to neg pulldown try 47K there.

Never trust Electronics Shop staff, when selecting
resistors etc, many of them don't know which way is up.
And lazy people put parts back in the nearest bin, not
always the correct one.
If you are not that good on colour code, take a
multimeter with you when buying parts.
 
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Coyoteboy

Senior Member
I'd certain on the resistor - no doubts at all there. The best operation has been achieved using zero gate resistance and a 22K pull-down. Boy i didnt know halogen lamps could whistle like they do when PWM'd!
 
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boriz

Senior Member
PICAXE manual 3. (Interfacing circuits) Page 7. No gate resistors required.

“through a 4K7 for safety, and pulled low with a 4K7”

This forms a divider that turns the +5v from the PICAXE output into 2.5v at the gate, unlikely to drive it fully on. The only possible reason for a gate resistor is to limit the gate current if the MOSFET fails with a gate-short-circuit to drain or source. But with a PICAXE, this is usually unnecessary.

“47R will pass 106mA which the picaxe output may survive a very short time and would get quite hot until it died.”

PICAXE output’s are internally limited to around 25mA. Even if you short them to ground.
 
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Dippy

Moderator
"..This forms a divider that turns the +5v from the PICAXE output into 2.5v at the gate, unlikely to drive it fully on." - precisely , and hence resistance of MOSFET is higher and hence it gets hot with a load.

Qu: assuming we don't have a gate resistor, what happens if the FET fails as drain-short-to-gate with a 12V supply and bulb??

Qu2: Why do you need a 22K pulldown? The I/o of the PIC is a far better/quicker pulldown.
I never use them and run some fairly fast FET pwm switching with just a 470R gate res.
However, I can understand a gate pullup res for P chan switching when using a higher Vsupply for load.
 
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Coyoteboy

Senior Member
Balls, I just plugged the power supply in the wrong way round (molex PC plugs but pins seem to poke out further than they should) and rammed an inverted 12v through the 'axe. It went pop pretty fast - didnt even get time to get hot. Looks like im ordering some more :( Now I have to bend over and suffer the unlubricated fist of the shipping and handling costs.
 
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