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Author Topic: MOSFET N & P and relay  (Read 2031 times)
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HI everyOne,
Here is what i am doing, and..what is going wrong ..
I use a N channel Mosfet and a P Channel to switch on and off the motor of an electric screw driver.
I think there is a little problem in my schematic because when the pin of the ARDUINO is LOW, i still have 8,6 V on the Drain of the P channel Mosfet
When the PIN is HIGH, I have 10 V on the Drain, and the relay is working good and the motor is turning.
When the PIN is LOW, the relay and motor don't work, perfect ... but i still have something like 8.6 V on the DRAIN.
is it normal ?
Do i have to do something to avoid this ?
If i plug a 10mm led in place of the RELAY & MOTOR, with a 390 Ohm resistor to the ground, i can see a little activity on the LED ... the 8.6V.
PIN HIGH of the ARDUINO and the LED is fully On ..
Thank you for your advice ...
« Last Edit: March 23, 2011, 05:35:41 am by yopaii » Logged

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Maybe the 10K pullup on the PFET is'nt enough to keep it cut off..  try 1k .  Maybe leakage in the NFET?

What's the voltage on the PFET gate compared to it's Source??
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Out of curiosity, if you are driving a relay from the Arduino, why don't you just use a single bipolar resistor?
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Yes, why the high-side switch, that makes things much harder and doubles the components

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Ohh ... that's right ... I already had this circuit with mosfet and i tried to use it with my new setup.
the thing is, the electric screwDriver is for away (15 meters) from the Arduino.

Arduino > Mosfets    >>>>>>>>>>> 15 meters >>>>>>>>>>> relay + screwDriver Motor

You're probably right, i should be able to do it simpler.

If i follow this schematic
Arduino > pinout >>>>>>>>>>>> 15 meters >>>>>>>>>>> relay + screwDriver Motor
I will probably loose a lot of signal to trig the relay no ? that's way i wanted to use a 10V signal instead of the 5V outputPin

Any advice on which component i should use ?

thank you
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???

If you control FET with arduino, it have the power to drive your relay, 15m is not so long distance. Move that 10V to your arduino, and use it directly to be driven by FET.

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If i follow this schematic
Arduino > pinout >>>>>>>>>>>> 15 meters >>>>>>>>>>> relay + screwDriver Motor
I will probably loose a lot of signal to trig the relay no ?
Correct and you can't really drive a relay straight from the Arduino anyway.

GaryP has the idea, I'll just draw it out a bit

ARDUINO---TRANSISTOR/FET-----------------------------------------RELAY---MOTOR

You could probably drop the relay as well and just use a larger transistor but may get too large a voltage drop, depends on the motor and the wire.

You only need a single transistor or FET and switch the low side of the relay.

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What part number is the p-channel FET? - the number on the diagram doesn't exist according to Google.
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OK... i start to forget about mosfets .. and use transistor... here what i understood ..

I have a lot of this relay at home so i will use them ( http://pdf1.alldatasheet.com/datasheet-pdf/view/90292/SIEMENS/RT314012.html )
I am wondering if this transistor ( P2N2222A ) will do the job or do i need something stronger.
About the resistor value, ...
in the relay datasheet, they say the Nominal Coil current is 44mA for a 9V Nominal voltage, 33.1 mA for 12V .. i will have around 10 V.
If i take 40 mA as the Nominal Coil current, the current at the Base of the transistor should be IB = 0.04 / 75 = 0.53 mA.
So R = 5 / 0.00053 = 9434 Ohms .... can i use a 10 KOhms resistor or should i choose less that 9434 Ohms ?

I put 2 diodes around the relay ... near the Coil and one near the motor side ...
I am not sure if i did it right ...

thks
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That looks reasonable.

Quote
am wondering if this transistor ( P2N2222A ) will do the job or do i need something stronger.
I must off so haven't got time to look at the relay spec properly but it says "miniature PCB relay" or some such so I reckon the 2222 will be good.

The value of the base resistor is not critical in a switch circuit, anything around 4k7-10k should work I think.

You don't need the diode on the motor.

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« Last Edit: March 24, 2011, 07:30:13 pm by Graynomad » Logged

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Specifically the diode on the output of the relay is doing nothing.  You could put it across the motor terminals to reduce arcing on the relay contacts though (worth doing if a small relay and large motor - however it means the motor won't freewheel backwards.
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Another choice is opto-isolation.  Here is an 8-relay board for $25. DISCLAIMER: My Shop!

I've been looking for good relay boards and these are the best and best value so far:

=relay&s[title]=Y&s[short_desc]=Y&s[full_desc]=Y&s[sku]=Y&s[match]=all&s[cid]=0]http://arduino-direct.com/sunshop/index.php?l=search_list&s[search]=relay&s[title]=Y&s[short_desc]=Y&s[full_desc]=Y&s[sku]=Y&s[match]=all&s[cid]=0
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thank you everyone ...
MarkT ; about "the diode on the output of the relay" ... "You could put it across the motor terminals to reduce arcing on the relay contacts though"
that's what i wanted to do but .. i didn't put the diode at the right place ... i changed my schematic ... is it the right place ?



thk U Graynomad and Terry King for the relay board ... I will look at the product.
« Last Edit: March 25, 2011, 08:39:03 am by yopaii » Logged

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Yes - basically across any inductor and facing the way that doesn't short-circuit the supply!  There are other approaches to reducing arcing and big motors already include 'snubber circuits' (R+C) to reduce the problem.  If the relay is chunky and the motor is small the diode won't make much difference.
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