Electronic – Will connecting Vcc and enable together cause problems

dc motordiodesh-bridge

The following chip seems to be acting funny.

http://www.ti.com/lit/ds/symlink/sn754410.pdf

I have one DC motor hooked up to 1Y and 2Y and the other to 3Y and 4Y.

Could the behavior be because I connected the VCC1 (logic supply voltage) and 1,2EN and 3,4EN all together? NB. No resistor between them either (see page 6 of link). What might happen?

The weird behavior is the following:

  1. When driving a DC motor in one direction (1A = 5V, 2A =0V), I'd expect 1Y to be VCC2 (output supply voltage) and 2Y to be 0V. Instead with a VCC2 of 9V, 1Y is around 7V and 2Y is over 1V. Making the voltage drop between the DC motors leads only 7-1 = 6V. What is happening here?

  2. When activating two motors at the same time. They start to drive as normally and then slow down to a stop and twitch. This might be a software issue, but I'm not sure…

  3. Also is it normal that the inputs of the H-bridge chip are floating values. Up to 1.3V-ish if I don't hook them up to the controller and set them to zero from there. I would expect them to take the 0 from ground

  4. Finally, what is the purpose of all the diodes on the page 6 graph? I have the outputs hooked up directly to the DC motor.

Page 6 graph:

Page 6 Graph

Best Answer

The device you have chosen isn't meeting your expectations because it doesn't work very well driving up to 1A motors. Here's a the circuit: -

enter image description here

The output stage is really old hat and poor for driving an amp or more on low supplies. The upper transistor in the output stage will barely be able to get to within 0.7V of the supply and the lower transistor will probably just about get to 1.4V of ground. This is the main problem.

If you look at page 4 it tells you this: -

  • Low level output voltage can be as high as 1.8V (not 0V) for a 1A load
  • High level output voltage can be down at Vcc - 1.8V for a 1A load

As for the inputs floating up this is to be expected given the input circuits used within the device.

Regards the circuit diagram in the question - I believe the external diodes are added because it is driving a two phase motor and this might create extra problems that the internal diodes can't cope with.

Also be aware that the continuous output current for this device has an absolute max value of +/-1.1A so be aware of that when choosing motors. There are far better devices than this old dinosaur by the way.

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