transistor problem

|500x313

Here is an image of my current circuit. (no H-bridge for now, i'm testing some stuff. And sorry, I made it on paint).

I don't understand why is the emitter pin of my transistor emitting 5v to the ground only. With 12v, the mosfet is supposed to be open enough to let flow 100+ amp (3rd graph in datasheet), but with 5v, it's only 20/30 amp, which is not enough. The motor can peak to 100 amps while starting, so I need the mosfet fully open.

And do I have to put a resistor on the green line? I have a few 10k that I could used.

Here the datasheet of the transistor:BC337-25 Here the datasheet of the mosfet:IRFZ44N Here the specs of the motor: EV Warrior I fried my arduino, so it's only outputing 5v flat, but if it would work, i would use the pin 9 and pwm.

Secondary/optionnal question: And why, if I put the motor after the mosfet (- of motor to ground, + to mosfet source, and the drain of mosfet to 12v battery), it will only received something like 9v when the mosfet have a 12v at the gate and only 3v when it's 5v at the gate? I really dont un understand, but I was planning to counter this problem with simple relay for the H-brige.

Thanks you everybody, I trying to learn and I really like electronics even if i'm bad at it. I would appreciate any answer of any kind, thanks a lot.

Bernard

Yes, well, you have the transistor in a common collector configuration, which has no voltage gain. To oversimplify a bit, whatever voltage appears at the base, will appear at the emitter but 0.7 volts less.

If you want to drive the mosfet with 0-12V, you have to use common emitter configuration. In which case, you will need the resistor you mentioned and another one as collector load.

As an electronics Engineer with 65 years of experience I strongly suggest you intially learn Ohms Law and down load some material on basic electronics and of electronics in general, how transistors work and their applications in running motors and other loads.

Without that basic knowledge you are going to end up with a box of "Fried Chips" To start with the Arduino boards have a very small power output capacity. a BC337 is not capable of driving an IRFZ44N FET to such large amperage use!

To drive a "100amp" motor with a single FET and a single driver is not really possible, an H Bridge of 4 FETs mounted on a large heatsink would be required. Coupled with a fully regulated power supply (PSU).

Again you give no information as to the application use, whether the peak current is starting or running current, with electric motors the "starting" current can be up to 10 times the normal under load running current, power source and application information are necessary to really give you assistance. Resistors and capacitors are also needed to control the voltages and current flowing in the cct and to removed any spikes or switching transients.

Be careful when using datasheets on transistors, whilst they may indicate High Current capacity in actual practise it is of much less. Also there are two uses, ICAS and CCS (Intermittent Commercial and Amateur Service and Continuous Commercial Service). The latter being an industrial engineering rating

To start with the Arduino boards have a very small power output capacity.

I connected the 5v out to my 12+ of the battery. Stupid accident, but it wasn't a too big current that break it.

BC337 is not capable of driving an IRFZ44N FET to such large amperage use

Interresting, can you say more about that?

My goal is to motorized a small vehicle. I read online that the load the continuous amp of a motor under load is about 30% of his stall current, which is about 100amps. I guess the starting of the motor is equivalent to the stall. Which make me deal with about 30 amps in continuous for the mosfet.

The I want to do an h-bridge, but I guess it wont be really more complicated than a single mosfet.

If you want to drive the mosfet with 0-12V, you have to use common emitter configuration. In which case, you will need the resistor you mentioned and another one as collector load.

How much resistance should I put?

Thanks both of you for your help! Bernrd

Yes use a 1K resistor in the base. Connect the emitter to ground. Connect the collector to a 1K resistor with the other end going to +12V. Now connect the gate of your FET to the collector of the transistor.

Sploddy: a BC337 is not capable of driving an IRFZ44N FET to such large amperage use!

Why would the current controlled by the fet be relevant here? That will have no impact on whatever's connected to the gate - it's a FET.

I don't think anyone mentioned that he'll need a diode across the motor, with the band toward the + side, to clamp the back EMF? You need that when driving a DC motor.

Grumpy_Mike: Yes use a 1K resistor in the base. Connect the emitter to ground. Connect the collector to a 1K resistor with the other end going to +12V. Now connect the gate of your FET to the collector of the transistor.

Yes, that's one way to swing the gate between ground and +12volt. Because this fet needs at least 8volts to properly turn on.

Disadvantage is that the fet is "on" when the Arduino is off (or starting up). A two-transistor NPN>PNP driver hasn't got this problem.

NPN transistor with 10k resistor between Arduino and base. Emitter to ground. Two 10k resistors in series between +12 and collector. PNP transistor with emitter to +12volt and base to 2x10k junction. 1K resistor between collector and ground. Gate to collector/1k junction. Leo..

Hi, This is a continuation of.

http://forum.arduino.cc/index.php?topic=324134.msg2252539#msg2252539

Tom.... :)

Hi,
Find attached the basic circuit for driving a motor from an arduino UNO.
Not seen in this diagram, 12Vdc(GND) is connected to arduino GND.

If you are going to use the large motor you have, then this would be too basic and under powered, but it shows you what is basically needed.

Your circuit is not really close to being usable, you need to read up on transistors and MOSFETs and how they work in a circuit.

Tom… :slight_smile:

I can't advise on transistory stuff, but can recommend ExpressSCH as an easy to use way to do schematics.

|500x197

So, is this going to worked? I can't test it right now, I dont have some PNP on hand.

Thanks everyone!

You could also use a decent FET, like the IRF3708PBF, which switches on logic level (I recommend that one because it works down to 3.3v, and has enough current handling capability for most any project), and then you don't need the BJT's

Then you can just connect the gate to source through a 10k resistor (pulldown, to make sure when Arduino is off, the fet doesn't turn on), and connect gate straight to the pin of the Arduino (some people will argue for a 220 ohm resistor between gate and arduino pin), with no BJT's needed.

Beurnii:

Yes, that will work.
You just have to add a kickback diode across the motor (cathode to +12volt/anode to drain).

The drawback of adding transistors between Arduino and mosfet is slower switching speeds (hotter mosfet).
You could use a small fet for the first transistor, e.g. a 2N7000 without gate resistor.
Or schottky diodes across the base/collectors (baker clamps).
But as DrAzzy pointed out, a beefy logic fet won’t need transistors, and can be connected straight to the Arduino pin.
A gate resistor can be omitted for smaller fets, but I think it’s not wise to leave the gate resistor out for bigger fets with a larger gate capacitance.
While ~130ohm is the theoretical minimum, 220ohm is easier on the Arduino, and still fast for the fet.
The bleed resistor to ground (in case the Arduino pin is tristating) can be any high value.
10k is ok, but a bit low. I would use 100k.
Leo…