Driving a Mosfet (Wat am I doing wrong????)


Senior Member
Hi Guys, can someone help me please. Im trying to get a Picaxe 18A to trigger a Mosfet N - Channel (BUK456). But im only getting like 3v on the output, why? Heres how i hooked it up.

Drain - To 12V Siren
Source - (+)12v
Gate - Picaxe output 5v (no resistor)

I also have a resistor on the Gate going to gnd so it switches on and off correctly. So wat am I doing wrong? Can I not trigger the mosfet straight off the Picaxe output? Does the picaxe not supply enough current to it? Please Help, thanks alot


New Member
Download and look at the data sheet.

For the mosfet to turn on you need to connect the Source to ground or minus voltage, make the Gate positive by more than three volts (see the specs for gate threshold voltage)and connect your load between the positive supply and the Drain of the mosfet.

Then it will turn on . . . to make it work as a "saturated switch" (turn on fully) the gate should be made about 10 volts positive.

A picaxe running at 5 volts or less won't work or may work but drift out of conduction as the power supply voltage dropped or temperature changed - start working in the linear region not as a switch.

You can either supply the gate with more voltage from a second (+10-15V) power supply and something like an NPN transistor to switch that voltage to the gate (change the logic - since it would be turning the gate off when the transistor was turned on)

Or you can step up the voltage from the picaxe pin to drive the gate from the picaxe directly. I used a PWM output switching at 10KHZ 50% duty cycle to switch a npn 2N3906 with a 3 millihenry choke in its collector circuit and took the flyback voltage from that to drive a power mosfet with the 10+ volts it develops.

I posted a schematic in alt.binaries.schematics.electronic some time ago on how to. takes one transistor, inductor a couple of resistors, and some signal diodes and one 10V zener to keep the voltage down (I used an 8 volt zener and red led so I have an indication it is working).

220 K bleeder resistor on the gate to ground to turn off the mosfet and discharge the 10 volt supply.

I also developed a small blocking oscillator to do it that worked well - doesn't require a pwm signal from the picaxe and logic stays the same - but took one more transistor to do it. Also posted that schematic.

Plan B would to be to get a "logic level" mosfet and use that instead of the one you chose. LL mosfets have gate thresholds down around 2.5 volts or less - but check and double check the datasheet before you commit - not all of them will work over the temperature range you may need and your picaxe has to have the threshold voltage plus on its output pin.


New Member
The mosfet is not wired up correctly.

Source goes to 0V/Gnd
Drain goes to the negative of the siren.
And the positive of the siren goes to 12V.

If you want to drive the mosfet directly off a picaxe then you need one that has a 5V gate. There are lots of options - I like the BUK555 as its on resistance is so low for loads up to 10A that it doesn't even need a heatsink, plus you don't need any other support components.

Edited by - Dr_Acula on 10/08/2007 14:26:57

Michael 2727

Senior Member
An IRF530 or IRF540 should work also, many of the 10V gate MOSFETs should switch at 4V to 5V some may not be fully ON but will be close to it if drawing just a few amps.
You could pimp up a 5V regulator to get 5.5V or 5.7V which shouldn't distress the picaxe greatly. Every bit helps in some cases.


TEZARM, read all the above useful comments - especially as Flooby says READ the Data Sheet.

For basic switching you can think of N-Channel FETs like an NPN and use it as a Low Sided switch. And a P-Chan as a PNP and use it as a high sided switch.

BUT there are the Gate voltage issues too as mentioned above so care is required, but I have used numerous Logic level N-Chans power MOSFETs as low sided switches perfectly straight from a PIC.
A resistor to gate is good and sometimes a gate to gnd res is good (for N -chan). Sometimes you can get away without the latter as when you set an o/p low then it nearly grounds. It really depends on type and application.

I would also suggest you find some kind of basic introduction to transistor applications as sometimes things can get a bit complicated especially with mixed voltage power supplies.
They don't always behave like a mini-relay which is probably how you imagine them.


Senior Member
Hi Guys, thanks soooooo much for your help. Ok ok obviously I made a typo, sorry, I meant source to GND not positive etc. Ahhh, but the solution to my problem is my MOSFET is not LL. Dont use MOSFETS much so didnt really research it to well did I, SLAP!!!!!!!!!!!!
Ok, anyways thanks so much guys, u rock. TEZARM


Senior Member
Some Logic ready power N-Mosfets here: <A href='http://www.rapidonline.com/productinfo.aspx?tier1=Electronic+Components&amp;tier2=Discrete+Semiconductors&amp;tier3=MOSFETs&amp;tier4=TO-220+Logic+level+power+MOSFETs++N-Channel&amp;moduleno=77687' Target=_Blank>External Web Link</a>


New Member
Flooby, Dr_Acula, Michael2727, Dippy &amp; Boris...... U ROCK! Did you know that? Lol.

Did one of you blokes slap TEZARM? Or did he slap himself?

&quot;The Addict&quot;