Not enough power from L298N

I got an Arduino Mega 2560 for christmas, along with some L298N boards, identical to this one:

http://tronixlabs.com/robotics/motor-controllers/l298n-dual-motor-controller-module-2a/

I'm trying to use them to power a Lego 9V train (not InfraRed, just power straight to the rails). This should be really simple, and the sample Arduino sketch works fine.

But I'm not getting enough power through the Motor Controller. First I tried a 10V power brick wired directly, but this only put out 4.8V - not enough to power the lego motor. Then I tried the same power brick through the lego train controller, into the motor controller - this put out 6V, but still not enough to make the motor work. Then I tried a 12V power brick to the motor controller, and it's still not enough - 6.2V came out at the pins. I tried multiple Motor controllers, multiple Lego train motors and multiple pins on the controller and arduino, but the results are still the same.

The Lego train motors only draw 0.2A at 9V when running, so it seems unbelievable that there would be so much power drop across the L298N that the motors just hum without spinning. Has anyone else come across this? Do I need a more powerful power supply?

A motor that draws 0.2A running normally might pull 2+A at start up (stall current), far more than the chip can provide, leading to insufficient voltage, current and inability to overcome startup friction and provide extra torque to accelerate the train.

The L298N being a Darlington device will lose 2.5V or so on a good day, its not great technology. A high-current MOSFET H-bridge is probably the way to go - Pololu have some high current ones for instance.

If you don't need reverse then a single MOSFET would be enough. Logic-level n-channel to be precise, with a low Rds(on) (0.03 ohms or less I'd guess).

MarkT: The L298N being a Darlington device will lose 2.5V or so on a good day, its not great technology.

It loses 4.8V at 2A, if I recall the datasheet correctly.

Thanks for the comments guys. I'll look into the MOSFET solution, because I really don't want to build a dedicated PSU for this. It still seems weird that so much voltage gets sucked up though.

I'm completely new to this technology, so is there a particular MOSFET people can recommend for building an H-bridge? The guide I'm looking at right now is based on the IRF630.

Hi, I have used the L293/89 in my early bots, and yes the output transistors usually lose you a couple of volts, depending on voltage and the current being drawn.

So now I tend to use things like the DRV8833 and TBG4988 which give much higher voltage levels. As MarkT says Pololu have quite a range and I have used there DRV8833 with good results.

Regards

Mel.

Cactusface: Hi, I have used the L293/89 in my early bots, and yes the output transistors usually lose you a couple of volts,

They always lose 1.8 as minimum, regardless of volts and amps. Then it gets worse.

I can't really order things from Pololu as I'm in Poland, but my local parts supplier had some BUZ11 MOSFETS (50V 30A). One of those wired up to the original power supply was more than enough to make the Lego train motor work

Now I just need to find a circuit diagram for an H-bridge so that I can reverse the direction :D

Thanks everyone for the help so far.

Grrrmachine: I just need to find a circuit diagram for an H-bridge

That might be more difficult than you think. Search the Forum and you'll find lots of posts along these lines: Member A posts an H-bridge circuit and Members B-E all say it won't work, but for different reasons. Members F-K will post alternative circuits. Then Members K-P will rip those to pieces. Member A gets confused. Yet more members will tell you to get one off-the-shelf.... backToSquareOne.