Forward and Reverse on Low Volt DC motor using solid state electronics.

[Braeden Hamson]

Hey guys,

I'm wanting to be able to control a DC motor's direction with an arduino. But the arduino can't output the kinda power I need to run the motor. The motor runs on 7 VDC and <1 A. I know it can be done with a DPDT relay. But I have a bunch of MOSFETs already. Is an H-bridge a better way to go or should I save myself the hassle and use the relay?

 

[Doug3004]

The relay will not provide any proportional (throttle) control. It would just be zero % or 100%. If that mattered.

What mosfets do you have? They may not switch on with the 5 volts of an Arduino. Many need 10 volts to turn on completely, and you use a darlington or a "gate driver" chip to operate them.

Mosfets that don't have the full voltage to turn them on allow the voltage through like you expect, but only allow a tiny amount of current through. So if you have one that takes 10 volts to operate, 5 volts isn't going to turn it on 50%. 5 volts will maybe turn it on only 5%, or less. More like 1%, that I've seen....

The mosfet I usually use to have a 5v Arduino operate something directly is the IRLZ44.
5v to turn on, 60 volts max, 50 amps max
www.vishay.com/docs/91328/91328.pdf

 

[Braeden Hamson]

I have these guys. http://www.onsemi.com/pub/Collateral/NDD02N40-D.PDF

 

[Doug3004]

NDD02N40 -- those basically need 10 volts to drive them. Or a darlington or a gate driver chip.

Normally the RDS(on) MAX figure specifies the voltage that is needed to turn the mosfet completely "on".
For that one, the voltage is 10 volts (in the top right-hand side of the data sheet)

For the IRLZ44 I linked to, the RDS(on) (Ω) is given at VGS = 5.0 V.
So it only needs 5 volts to turn on all the way.

They're not all named this way--but usually a mosfet that has an "L" in the part name is a logic-level part.
And note that the feature above the RDS(on) says "Logic-Level Gate Drive".

 

[Braeden Hamson]

It seems buying MOSFETs is hard. I'm actually not using an arduino, I'm using a labjack which for all intents and purposes is basically an arduino. However it's max output is 2.5 volts. I'll be using these in a 12 volt system. So I need to be able to switch on 12 volts with 2.5. Is it reasonable to use these MOSFETs to do this? Also do you know a good learning guide to MOSFETs it seems I don't understand them.

 

[Audioguru]

Your Mosfet has a maximum on-resistance of 5.5 ohms when its input is 10V so with 1A though it its voltage loss will be 5.5V so the load gets only 1.5V and the tiny Mosfet melts with 5.5V x 1A= 5.5W of heat. Your Mosfet is rated at a very high 400V so its on-resistance is high. The IRLZ44 is rated at only 60V but its maximum on-resistance is 0.028 ohms when its input is 5V so with 1A it produces a voltage loss of only 0.028V and almost no heat.

A Mosfet that needs an input of 5V or 10V will do nothing with an input of only 2.5V.
There are many products made with tiny Mosfets that work with low input voltages. Maybe a search on Digikey will find them.

 

[duke37]

2.5V is plenty to turn on a junction transistor. If this is not enough, then you can use the transistor to turn on a powerful MOSFET with your 12V supply.

 

[Braeden Hamson]

Thanks for the info guys