3
\$\begingroup\$

I have an old RC car. I removed all electronic from it, just left the plastic and the DC motors. It has 2 DC motors(1 for moving, 1 for left/right). I decided to attach arduino to it, while controlling the motors with the L293D H-Bridge. I wired it like this:

enter image description here

I noticed that motors are not running at full potencial(much slower with the h-bridge, then if I wire it directly to the battery). Also the L293D temperature goes insane(maybe 70-80C in 2-3 seconds) while motors are running. I rechecked all connections and spoils few time, they look good. I'm using a 7.2V 700mAh battery and since I didn't replaced the motors I'm not sure at what current do they work, tho they should work fine with voltage up to 12V.

My first thought was that maybe the L293D cannot handle the motors current, that's why it's overheating. After little research I found

The L293D is rated for 0.6A continues and 1.2A peak per 1 channel.

but my battery is 700mah so I think that's not the problem.

What might be causing this overheating and why my motors aren't running at full speed?

\$\endgroup\$
  • \$\begingroup\$ The battery capacity is only tangentially related to the current draw. \$\endgroup\$ – Ignacio Vazquez-Abrams May 24 '14 at 22:57
  • \$\begingroup\$ See this electronics.stackexchange.com/questions/108686/… \$\endgroup\$ – Andy aka May 25 '14 at 11:11
  • \$\begingroup\$ Set idle constant to driver IC, can't holding motor position with full power ! Claim load and define acceleration ramp, otherwise your driver/motor always run with full power. Full power not equal to full speed, can use full power on motor idle. \$\endgroup\$ – dsgdfg Apr 17 '16 at 13:51
2
\$\begingroup\$

Many people have posted about this in the past, but check the voltage drop of the L293D, which can be as high as 3.6 V (1.8 V lost on the high side, 1.8 V lost on the low side).

enter image description here

This means your motors are only seeing half the voltage of the battery (7.2 V - 3.6 V), and the remaining is wasted as heat in the L293. Also, the capacity of your battery doesn't have much to do with your current draw. In order to measure the dynamic current draw of your setup, you need to physically measure the current (for example, in series through a multimeter) with the voltage that you expect to see on the motor (that is, accounting for the voltage drop in the h-bridge - for example, 3.6V). I suspect the answer will be surprising, the motor will draw quite a bit more current than 600mA, and it will also not run very fast at only 3.6 V.

\$\endgroup\$
  • \$\begingroup\$ Thanks for the answer! Will I always have this voltage drop using H-bridge? Can I use any other H-bridge without this voltage drop? Can you suggest any other H-bridge? \$\endgroup\$ – Deepsy May 24 '14 at 23:11
  • \$\begingroup\$ The voltage drop is caused by the transistors within the L293D, and are specific to this part. A MOSFET based H-bridge (with built in gate drivers) will be as easy to use but have much lower internal voltage drops: The DRV8837 seems to be recommended: electronics.stackexchange.com/questions/108686/… \$\endgroup\$ – Zuofu May 24 '14 at 23:14
  • \$\begingroup\$ Thanks again! Just 1 more question. If I decide to satisfy 1 direction, will I have big voltage drop with TIP120. In the most arduino tutorials this transistor is recomended. I want to reduce the voltage drop as much as possible. \$\endgroup\$ – Deepsy May 24 '14 at 23:40
  • \$\begingroup\$ It will depend on your actual current, but the TIP120 has fairly high saturation V_CE (on the order of 2-4 V). For low voltage, high current applications, you are typically better off using a MOSFET. Bipolar based devices can operate at high currents, but typically require higher voltages as well. If you do not want to deal with the small backage of the DRV8837, you can buy a breakout board (that will still be about the same size as a L293 or TIP120): tindie.com/products/BBTech/drv8837-motor-driver-module \$\endgroup\$ – Zuofu May 24 '14 at 23:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.