Help needed reversing polarity through a IRFZ44N transistor.

Thread Starter

DanielHowden

Joined Nov 8, 2019
5
Hi, I have build a circuit like the following link...

How to control 12V LED and motor with 5V Arduino

I had some IRFZ44N transistors to use for this and it worked fine. However, I would like the output of the transistor to be positive and not negative as in the example. I have tried to logically do this but I have not been able to get this to work. Is it the transistor type I am using that is the problem?

I am very new to electronics and I am sorry if this is a very basic question.

The reason why I want to have a positive output from the transistor is that I want to signal an ignition coil to spark the plug at a timing of my choosing. While I am happy to program, the hardware part is a little more tricky for me.

Thanks, Daniel.
 

Alec_t

Joined Sep 17, 2013
10,446
Welcome to AAC!
I would like the output of the transistor to be positive and not negative as in the example.
I'm not sure what you mean. The FET source connects to ground and the drain switches between ground and some positive voltage. It doesn't go negative (below ground).
 

Thread Starter

DanielHowden

Joined Nov 8, 2019
5
Hi, thanks for the welcome. In this picture...

12V-LED-circuit-L-768x493.jpg

If I understand it correctly, the emitter is connected to the ground on the Arduino and the collector gives the earth connection to the light. This earth connection is controlled by the base getting the signal from the Arduino. The lights positive connection comes basically straight from the battery.

What I want to do is have the light directly connected to the earth on the battery (or negative) and the positive connection being made and broken through the transistor base and flowing out of the collector.

So in this example, I understand it to be that the emitter and collector are using a negative current when I want them to be using a positive current.

Thanks, Daniel.
 

AnalogKid

Joined Aug 1, 2013
8,140
Yes, you want to change to a p-channel power MOSFET. BUT - driving it will be a bit more complicated because if the source is at +12 V (same as the fan voltage), then the gate has to be driven or pulled up to +12 V to guarantee that the FET turns off when you want it off. Since the output of an Arduino swings only to +5 V, another transistor stage is needed. This is a much lower power stage, and can be done with a 2N7000.

ak
 

shortbus

Joined Sep 30, 2009
7,078
The reason why I want to have a positive output from the transistor is that I want to signal an ignition coil to spark the plug at a timing of my choosing.
Don't know what your ignition coil is or the timing your going to use, but that was one of the things back in the day of points ignition engines the was critical, not to leave the coil turned on too long. When doing a tune up back in the day you needed to be concerned with the "dwell angle", that was what controlled the amount of time the points were closed and power was on the coil.
 

AnalogKid

Joined Aug 1, 2013
8,140
Maybe. To make sure we know your intent, either change the transistors to the appropriate schematic symbols or add G, D, and S designations to the pins.

AND - Add a 10K resistor from the 9540 gate to source. This pulls the 9540 gate high (+12 V) when the 7000 is off, assuring a clean and firm turn-off.

Also, if the load is inductive (incandescent light bulb, motor, etc.) you should add a diode across it (cathode to GND) to protect the FET from the turn-off voltage spike.

ak
 

shortbus

Joined Sep 30, 2009
7,078
Also, if the load is inductive (incandescent light bulb, motor, etc.) you should add a diode across it (cathode to GND) to protect the FET from the turn-off voltage spike
That spike could be pretty high, since he is including an ignition coil in the mix.
 

DickCappels

Joined Aug 21, 2008
5,938
Just guessing but the IRF9540 probably does not have the ability to handle the negative-going pulse on the drain. You can try it but the part has a good chance of being destroyed by the experiment.

Do you know anything about the spark coil's characteristics, like the approximate inductance of the primary? You will likely need to put a capacitor across the MOSFET or spark coil's primary to limit the maximum voltage across the coil, much as the condenser in an automobile ignition system protect the points from arcing.

You are going to need something to turn off the P-channel MOSFET. As it is now, the N-channel MOSFET will charge up the gate capacitance of the P-channel MOSFET and when the N-channel MOSFET turns off, the gate of the P-channel MOSFET will remain charged for an indefinite period. R2 in the circuit below bleeds off the charge so that the P-channel MOSFET can turn off. But the circuit below (actually almost the same as I think you sketched probably would not perform well driving a spark coil (in place of "LOAD" in the schematic below.) if your goal is to make a spark.

1573313365521.png

To get a spark you need high voltage (yeah, we all know that) and to get high voltage the current in the has to collapse quickly, and only by turning off the switch -the P-channel MOSFET, can you achieve a high rate of current change.

To get that rapid charging and discharging of the gate capacitance you will need to pump a lot of current into and out of the gate, and the best way to do that is with a high side gate driver. See the Texas Instruments publication slua669a (attached) for some ideas.

By the way, to get this working you will probably find an oscilloscope to be very useful if not essential.
 

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Thread Starter

DanielHowden

Joined Nov 8, 2019
5
Thank you so much for the replies. I'm sorry I didn't mark up my circuit diagram correctly, I will get that right on the next one.

Did a look at the high side gate driver and, to me, it looks pretty complicated so I will do some research on that so I understand it
better.

If it's OK I'll post another diagram when I understand all the changes I need to make.

Thanks, Daniel.
 
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