Let's Make Robots!

L293/SN754410 h-bridge schmatic



I am a technology education teacher who teaches an industrial design course. This previous year we used the L293D chip to make a motor shield that works withe Arduino. The schematic that i was using works very well with the L293D chip but will not work with the L293 or the SN754410. Attached is a schematic of my motor shield with diode protection. i hope the diodes allow me to use the L293 and the SN754410. If any one is familiar with this sort of motor shield i would love to have my schematic looked over and any possible suggestions made. 

I did post this on the Arduino forum, but i thought it my be more usefully on here.

H-bridge_2_.jpg71.29 KB

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Since on this small scale high-side, low side, and regenerative braking are not worthwhile it's nice to only have 2 bits (Direction & PWM) per motor.  I also never like the possibility of shoot-through.  This is if pin 2 and 7 go high it will by-pass the motor and probably fry the chip.

I got a couple of these chips and was going to make a small PCB like this:

Just my 2 cents bits

I just wanted to clarify that shoot-through refers to both the high side switch and low side switch on the same half of an h-bridge being turned on at the same time.

Your post says "This is if pin 2 and 7 go high it will by-pass the motor and probably fry the chip."

Actually, pin 2 controls the high and low side switches on one half of the H-bridge, and pin 7 controls the high and low side switches on the other half of the H-bridge. So applying a logic high to both pins 2 and 7 at the same time will connect both sides of the motor to the positive supply voltage. If the motor was stopped at the time you raise pins 2 and 7 high, nothing will happen. The chip will not be damaged. It is wise to inhibit the outputs (pin 1 = ground) any time you are changing pin 2 or pin 7.

I made some changes, i don't know if this is a good idea or not. I am powering EN 1 and 2 and VCC1 from the regulated Arduino power. I am powering Vcc2 with an un-regulated 6 volts. I added a 10k resistor between the regulated Arduino power and enable pins. ( i saw that on the data sheet for the sn754410).


Any sugestions?


all your logic will be fine running on the Arduino's 5V. That's how it Should be. Vcc2 is motor power only. Make sure you have a good ground connection between the Arduino and the motor controller. You should have one ground for everything.

I was trying to regulate the voltage through the resistor. i will change it so that it pulls 5 volts from the arduino. Can I tie the L293/SN754410 ground to the arduino ground?

I was under the impression that the SN754410 was not diode protected.


ESD protection diodes are in the SN754410 but snubber/kickback protection diodes are not. The difference is not completely apparent to me either, but all the application schematics in the datasheets show additional diodes added and I`m not one to argue with the guys who made the chip.

but I think the diodes to protect against ESD are snubber diodes. But I see what you're saying about the app notes:

That bank of diodes says to me it's a "naked" chip too. I (take my shortsightedness as you will) never got that far in the datasheet lol. I only made it to this diagram:

... and had favorited this seemingly knowledgable post at AVRfreaks...

"The short answer is you don't need to add diodes, The long answer is, if you look at page 2 of the TI data sheet, the diagram that says "typical of all outputs", you'll see TI has thoughtfully provided the chip with its own built-in back EMF protection diodes to the right of the transistors. On page 4 the parameters "Vokh" and "Vokl" are the characteristics of these diodes, showing that the back EMF will never go more than 2.5V beyond the supply rails at a current of 1 amp. You can be sure 1A is not a maximum - these diodes will be rated for at least the maximum output current of the chip."

Diodes in parallel to the power supply, it looks good to me. Either way I'll be testing it soon. I need some more drivers for my smaller upcoming bots and I feel stingy with my diodes :) I'll be sure to post if blue smoke rises. 

I have used the SN754410 without additional diodes with no problems. Gareth's "Picaxxo" uses the chip without external diodes. The only time I would bother with external diodes is if you were driving something like a 24V motor as the chip is rated up to 36V. For little bots running on 12V and less I think your wasting diodes and PCB space.

Okay, i am going to go ahead and build the circuit without the diodes and see how it goes. I will post the outcome.

i am not the best at PCB design but here is the finished PCB. I am still going to add several small circuts for the photoresistor sensor system i use. But here is my idea.