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L298 problem

I'm making a robot and I'm using an L298 to drive 2 DC hobby motors (specifically, the tamiya twin gearbox). For some reason, the L298 give out 3.4V on its outputs when applying logic low to the corresponding input. Does anyone know why this is? I am going by the example schematic in the datasheet, using 5V for logic power supply and 9V for motor power supply.

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Iranski's picture

Hello, I had the same problem. The pins number 1 and 15 have very misleading labels: Current Sensing A and Current Sensing B. When you look into the datasheet, you can see this pins must be grounded, even if you don't control the current.

Duane Degn's picture

Agreed. I just looked at the picture. The current sense pins need to be connected to ground. You also need some diodes on your L298N chip or you'll likely have problems with the flyback voltage.

A 9V makes a very very bad motor battery. It might work for a bit while it's fresh and when there isn't much a load on the motor but the little 9V are intended for low current applications.

birdmun's picture

by disconnecting the motor and checking the voltages that come out on the output pins. While I agree that a 9v battery is not meant for driving motors, I understand that you are trying to figure out why a logic 0 is providing 3.4v at that output.

Are all of your grounds tied together? I could see it being possible that your "ground" is floating if your grounds are separated.

ignoblegnome's picture

You can expect a 1.8 to 3V drop from the input voltage of the L298 to the motor output. Actually, it will be even higher at high current draw.

From what you describe and picture, I see a couple of possible issues.

  1. You are using a standard alkaline 9V, which can only deliver a limited current.
  2. The stock motors in the Tamiya twin gearbox only like about 3V.

So you are using to high a voltage supply, with not enough current. You may have gotten your motor to turn by directly connecting the battery, but I'm not sure it would turn under load.

Now, as to why your OUT2 of the L298 is at 3.4V, I only have a guess. Perhaps the internal resistance of the 9V battery is causing a voltage drop internal to the battery.

Try supplying the L298 with 4 AA alkaline batteries, giving 6V and better current capacity. See if that helps.

Maxhirez's picture
Can anyone else see the image? Next question, just from the description, Are you tying the enable high or Sending it a PWM signal?

Yes the battery is fine, I connected it directly and it runs fine. Heres the link to the image: http://imgur.com/NSCkf

Also, enable is tied high.

Maxhirez's picture
If you're certain the battery is fine, try this: hook up your supply battery directly to the motor without the HBridge and see how long it runs.
Maxhirez's picture
An "N" battery isn't going to cut it. Remember, voltage is pushed and current is sucked-You need more amps than it can have drawn from it whatever the voltage reads. Try it with at least 4 AAs feeding the motors and chances are your problems will disappear.

I'm not using an "N" battery. I'm using a brand new 9V. And for right now, I only have half the driver running, or one motor. 

Maxhirez's picture
If your battery looks like this: SCBAT9VOLT.jpg then forget it. Also, your verbiage here is a little confusing. Each motor should have 3 inputs corresponding to it-two logic and one which is logic or level (PWM.). If the logic is "high" or "low" on all three channels then no-go.