Open collector operation with an SN754410 H-driver?

Thread Starter

graybeard

Joined Apr 10, 2012
54
I am trying to repurpose a PCB with an SN754410 H-bridge driver to control a motor that needs a pair of simple open collector drivers. I am thinking that I can just leave the Vcc2 pin disconnected and the SN75441 will work fine as an open collector driver.

Has anyone ever done this before? Can anyone think of any issues that I have not foreseen?

Thanks in advance for your thoughts.

Mark
 

Thread Starter

graybeard

Joined Apr 10, 2012
54
I lifted the Vcc2 pin on the SN754410 and tested it both with nothing connected to the Vcc2 pin and with the 6V used to drive the motor connected to Vcc2 and it worked in both cases. Unfortunately, the SN754410 ran pretty hot driving the motor, so I guess I will have to leave the MOSFETs in the circuit. I was hoping to drive the motor directly. That means a new PCB for this product rather than repurposing an old one. The current draw for the motor is within the specs of the SN754410, but I hate running an IC like that hot to the touch.

But my question still stands, is it good design practice to run an H-bridge driver in an open collector configuration?
 

ElectricSpidey

Joined Dec 2, 2017
2,177
A few thoughts...

Without a complete schematic there can be no definitive answer to your concern, but the concept seems sound. (at least for these drivers)

As far as overheating...did you actually measure the temperature or just judging from touch?

Are they the DIP package? Did you consider placing heatsinks on the chips? (or do they have them already)

I won't question how you are driving these chips, I will just assume you know what you are doing there.
 

Thread Starter

graybeard

Joined Apr 10, 2012
54
Thanks for the reply ElectricSpidey. I didn't bother with schematics because the question was so simple.

I only touched the DIP IC, and it got too hot to touch. The SN754410 max current spec is 1A continuous/ 2A peak. With the 6V motor, it draws pretty close to 1A, too close for comfort. I pot these products in Silicone2, and they live behind the dashes in cars and trucks, so I like them to run cool. I am not sure what the ThetaJA is for a glop of Silicone2, but I assume it is not very good. When I drive the motor with a pair of MOSFETs, the MOSFETs stay cool to the touch. I think it would be cheaper to use the MOSFETs than it would be to try to heat sink the IC.

The driver is being driven by a PIC16F1508 and has worked well for years driving up to a couple hundred mA air core gauge loads and small stepper motors.

I am currently working on a product to drive 50s vintage hot rod tachometers. The tachometers are mechanical speedometers (With RPM scales) driven by stepper motors synchronized with the engine speed. (I was shocked to see a stepper motor from the 50s.) The current draw of the motor is much higher than smaller modern ones. Some of the tachometers have 6V motors and others have 12V motors.

Back to the design practice question, I forgot to mention that the frequencies involved are all audio frequencies.

After thinking about it, in order to snub the output using the SN754410's internal diode, I need Vcc2 connected to the motor power. If I don't connect Vcc2 on the IC, I would need to add an external diode and have a wire to the motor power in that case as well.

Unless someone can think of anything I have missed, I think I have answered my own question. Using an H-bridge driver will work to drive the motor. The pull-up part of the H-bridge won't do any work, but the snubber diode will protect the pull-down transistor. My particular motor draws too much current for this IC, but the design practice should be ok.

Thanks again for your reply, ElectricSpidey.
 
Last edited:

ElectricSpidey

Joined Dec 2, 2017
2,177
Actually I was referring to the driver schematic, not your circuit schematic.

So, I had another look at the TI datasheet and sure enough the part of the internal schematic I needed was right there on page 8 showing the snubber diodes, I missed that page for some reason when I checked the data sheet the first time...good call on those.
 
Top