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Author Topic: dumb question about SN7544 h bridge  (Read 1484 times)
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So I think I have been frying some of my h-bridges. Be easy guys Im completely new at this. So I have the SN7544 1A h-bridge. Everything is wired correctly. I have been alternating power sources because I am working on a project that will drive 2 24v steppers but they seem to work just fine at a lower current (12v). I have not however been checking the amperage of the different power sources I have been connecting into the h-bridge. Ive connected it to a 12v 1.5A and a 15v 1.1A power source and it seemed to crap out on me. Should I keep the power to the h-bridge down to the suggested 1A to keep these bridges from frying or does the bridge regulate the output current to a 1A maximum?

Thanks, and sorry if this was a dumb question!
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1A is too much for a SN754410 unless you cool it with a strong fan or put an ice cube on top of it. The limiting factor for most motor drivers is heat, not the current that's published in the datasheet (see our Motor Driver Myth application note).

The amperage of the power sources doesn't determine the current flow through the driver, it is the motors that determine the load current. A 12V 1.5A supply means that the supply *could* provide 1.5A if the motors demanded it.

The bridge by no means regulates the current. If you hook up a high-current motor to the bridge and you allow this high current to flow, the driver will be destroyed unless it is a driver with overcurrent protection (which the SN754410 does not have).

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thanks! im starting to understand now. I now remember whenever I had the motor on a higher rpm thats when the driver started to crap out. So does the 12v 1A power source cut the amperage off at 1 amp or can it still supply more for the motors needs?
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It really depends on your power source. Some will start to drop their voltage as you exceed their current, some will catch on fire. What are you using?

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a basic ac adapter
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Is it "heavy" or "light"?

The older "heavy" (linear) supplies are basically nothing more than a transformer, bridge rectifier, and filter capacitor. It probably puts out 17V-18V at no current, sinking down to 12V at 1A or so. If you keep drawing current above 1A, the voltage will just keep sinking down below 12V until it catches on fire (I doubt there's any kind of fuse in there).

The newer "light" (switching) supplies will just keep a steady 12V over the 0A to 1A range. Beyond that...it depends who makes it. It could shut itself down in an orderly manner, its voltage could start to go down, it could catch on fire.

In any case, it's never a good idea to exceed the maximum current rating of a power supply.

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