L293D Question [SOLVED]

Hi everyone,

I measured the resistance between the connectors of a 6V motor and found it to be about 4 ohms. I power the motor with 4 LR6 batteries, so 6 volts.

When I switch on the power to the motor, and just before it starts to turn, the momentary current going through the L293D chip is, I suppose:

I = V/R = 6/4 = 1.5 Amps

The L293D data sheet says the absolute maximum current permitted is 1200mA, and that for less than 100 microseconds. So 1500mA doesn’t seem like a good idea.

Should I put a small resistor (say, 2 ohms) between the L293D’s output pin and the motor? That would make 6 ohms in all with the 4 ohms of the motor’s windings and so 6V/6 ohms = 1 amp (<1.2 amp so OK).

I ask because I just googled “L293D circuit” and apparently no-one has ever thought it necessary (over 100 circuits and not a single one with a current-limiting resistor).

Conclusion: there’s something I don’t understand.

Does anyone know what I’m missing?

Thanks.

The L293D typically drops 2-2.5V so the voltage at the motor is probably nearer 4V if you're lucky.

Steve

There are much better motor drivers available today. Pololu has a wide selection of modern and inexpensive drivers for smaller motors.

The L293D data sheet says the absolute maximum current permitted is 1200mA, and that for less than 100 microseconds. So 1500mA doesn't seem like a good idea.

It is good that you took the time to research that -- most people starting out don't, then post on the forum asking why their circuit doesn't work.

The L293D is not suitable for that motor, and it is extremely inefficient in any case, with a large fraction of the battery power simply wasted as heat. This dual or this single motor driver from Pololu would work well and lead to more motor power from the same batteries.

Good morning!

slipstick:
The L293D typically drops 2-2.5V so the voltage at the motor is probably nearer 4V if you're lucky.

I think you mean with the motor running.

I connected an L293D to the 6 volt (motor) and 5 volt (logic) power supplies and hard-wired the enable pin and one of the inputs to 5 volts, and the other input to the ground rail common to both power supplies. I read 5.83 volts (old batteries) on the voltmeter between the outputs to the motor (without the motor connected, otherwise it would spin and I'm concerned about just before it starts to spin), so, unlucky!

The problem would disappear with the Pololu drivers so I won't be buying any more L293D's. Thanks for that, I never thought of looking there.

However, for the moment I'm stuck with my L293D, so before marking this thread "Solved", and for anyone else who might be wondering, am I right in thinking that, as things stand, I need a 2 ohm current-limiting resistor to protect the L293D?

Thanks again.

kayel:
However, for the moment I'm stuck with my L293D, so before marking this thread "Solved", and for anyone else who might be wondering, am I right in thinking that, as things stand, I need a 2 ohm current-limiting resistor to protect the L293D?

As soon as the motor tries to draw current the voltage will drop, which was the point of my earlier comment. Since the voltage across the motor will be lower then the current will also be lower. No extra voltage drop across a resistor is likely to be needed.

Steve

@slipstick

I haven't seen a single L293D circuit with a current-limiting resistor, so the whole of the internet apparently agrees with you.

I won't be using the L293D again so the question has now become purely academic.

Thanks to the three of you for replying.

HAPPY CHRISTMAS!!!