# How to pull-down low voltage in L298N H-bridge?

#### fablau

Joined May 14, 2020
72
Hello everyone.

I am an electronic newbie, and I am replacing an L293D H-bridge IC with an L298N in a circuit controlled by an Arduino UNO driving two bidirectional coils, due to higher currents needs, but for some reason, I am unable to have a zero (or almost zero) voltage when the L298N H-bridge should be off (when all inputs and enable pins are set to LOW). I always get around 450mV from the outputs.

Here is the schematics of the circuit I am working on:

I have tried to add a 10k pull-down resistor at the inputs (5,6,7) and enable (10,11,12) pins, but no avail. Any tips to avoid having that pretty high voltage at the outputs when the H-bridge should be off?

Here is the datasheet of the L298N for reference:

https://www.mouser.com/datasheet/2/389/cd00000240-1795236.pdf

Any suggestions and thoughts are very welcome!

All the best,

Fab.

Last edited:

Joined Jul 18, 2013
21,612
Have you tried taking the inputs to GND with a direct connection? see what is the result.
Max.

#### fablau

Joined May 14, 2020
72
Have you tried taking the inputs to GND with a direct connection? see what is the result.
Max.
Yes Max, tried that, nothing changes! Isn't it weird?

#### sghioto

Joined Dec 31, 2017
1,553
The only thing I see missing on the schematic is a 100nf cap on pin 9.
Something doesn't look right where is the 5 volts for the Vss supply coming from?

#### fablau

Joined May 14, 2020
72
The only thing I see missing on the schematic is a 100nf cap on pin 9.
Ouch, good catch! I forgot to put it in the schematics, but I have it in the actual circuit. Thanks for noting that!

#### sghioto

Joined Dec 31, 2017
1,553
I added to my post concerning the 5 volt supply did you see that?

Joined Jul 18, 2013
21,612
These modules can be had for a couple of  and have the 5v supply built in for the Arduino.
Max.

#### Alec_t

Joined Sep 17, 2013
11,738
Have you tried monitoring the input pins of the L298 with a scope, to check there are no spurious oscillations?

Joined Jul 18, 2013
21,612
Just had a thought, if using different supply sources, are the commons bonded at a star point GND?
Max.

#### fablau

Joined May 14, 2020
72
I added to my post concerning the 5 volt supply did you see that?
Sorry, I missed that one. That 5v is coming from the Arduino. Why do you have concerns about that?

#### fablau

Joined May 14, 2020
72
Have you tried monitoring the input pins of the L298 with a scope, to check there are no spurious oscillations?
No, unfortunately, I don't have a scope... just a multimeter.

#### fablau

Joined May 14, 2020
72
Just had a thought, if using different supply sources, are the commons bonded at a star point GND?
Max.
Yes, both the Arduino and the supply source have common grounds (if that's what you are asking). Later today I am going to try a different L298N chip, maybe the one I was using is defective in some way?

#### sghioto

Joined Dec 31, 2017
1,553
Why do you have concerns about that?
I just didn't see any 5 volt supply shown. What are the actual readings on the output pins when OFF?

#### fablau

Joined May 14, 2020
72
I just didn't see any 5 volt supply shown. What are the actual readings on the output pins when OFF?
Good question: if I remove the 5v logic supply, the readings are still up to around 450mV. It looks coming from the 32v supply!

CORRECTION: In my first post above, I wrote that I read around 200mV on the output pins when in standby, whereas it is actually 450mV when the supply is at 32v! Sorry for the typo, I'll edit my post right away. I also replaced the schematics with the correct capacitor No.3.

Last edited:

#### sghioto

Joined Dec 31, 2017
1,553
I 'll rephrase, what are the voltage readings on the output pins?

#### fablau

Joined May 14, 2020
72
I 'll rephrase, what are the voltage readings on the output pins?
So sorry, for some reason I wrote mA instead than mV. Corrected!

And here is my definitive answer to your question: When the 5v logic supply is off, I still read 450mV. Something's wrong!

Also, please note, that when I take the readings, there's no load connected (the coils in the schematic above are NOT connected). I just read the output directly with the multimeter alone.

#### sghioto

Joined Dec 31, 2017
1,553
If both output pins read the same the current should be zero through the load.

Joined Jul 18, 2013
21,612
Well now with mV you can get a stray reading with a high impedance meter, I suspect you have nothing to worry about!
Max.