For an H-bridge, also known as a Full Bridge, the diodes are placed in parallel with each transistor as shown in n3wb's schematic.
To help you determine the diode rating required for inductive kickback here are two good rules of thumb:
1) The minimum repetitive surge current rating that will be required for the diode can be determined by the steady state current of the motor used at the highest supply voltage. Naturally if the diode is rated for a continuous current equal to or greater than the current of your motor then the brief inductive ringing will not destroy the junction.
2) The minimum reverse voltage rating for the diode is, of course, the highest possible supply voltage in your design.
Other things to consider:
3) Keep traces on the PCB short and use no vias in the output signal path - this reduces inductance.
4) Snubber capacitance should be placed close to the driver pins.
5) Forward and reverse recovery time. Schottky diodes are very fast (nano second range). In all honesty, most datasheets will NOT specify forward recovery time.
I mentioned earlier that it is good design practice to use external protection devices because the internal diodes are rated for ESD only. It was mentioned that the SN754410's internal diodes are BANNED, but after looking at it again and comparing it to the L293D, I think there is something missing.
It is not clear at all from the datasheet and previous TI comments shed no light on the matter besides "we say so." I put in a call today to see if I can get characterization data or talk to a silicon designer. If there truly is a limitation with the 754410 then it must be device related given the silicon process used.
Need help with sn754410 motor driver
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Please be positive and constructive with your questions and comments.
- ImaginaryAxis
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Re: Need help with sn754410 motor driver
That helps a lot. I am extremely new to all of this and still get stuck on reading data sheets. I am not seeing anything stating any sort of minimum reverse voltage though which is why I am confused.
Considering my power supply is 6.5V, is there anything with a reverse rating low enough to have an effect on this?
My local shop here carries most NTE parts, so here is the range I am looking at:
http://www.nteinc.com/Web_pgs/schottky_barrier.html
I'm not asking for an exact answer, but more of a how/why. It almost seems like the levels I am dealing with are too low to matter for these diodes, unless I am missing something huge in the data sheets.
Considering my power supply is 6.5V, is there anything with a reverse rating low enough to have an effect on this?
My local shop here carries most NTE parts, so here is the range I am looking at:
http://www.nteinc.com/Web_pgs/schottky_barrier.html
I'm not asking for an exact answer, but more of a how/why. It almost seems like the levels I am dealing with are too low to matter for these diodes, unless I am missing something huge in the data sheets.
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Re: Need help with sn754410 motor driver
If I use PWM on the enable line to control the speed of the motor, does anyone know what the frequency of the PWM signal should be?
- adafruit_support_bill
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Re: Need help with sn754410 motor driver
That will depend, to some extent, on the motor. In general, lower frequencies give more torque. Higher frequencies are less audible.
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Re: Need help with sn754410 motor driver
Are we talking in terms of a few hundred Hz, or several kHz here?In general, lower frequencies give more torque.
- adafruit_support_bill
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Re: Need help with sn754410 motor driver
I have seen them run as low as a few hundred Hz, but most controllers operate in the range of about 1-100 KHz. IME, small 'can' motors do better in the low KHz range.
For a good description of the principles and some of the math behind it, look here: http://homepages.which.net/~paul.hills/ ... sBody.html
For a good description of the principles and some of the math behind it, look here: http://homepages.which.net/~paul.hills/ ... sBody.html
Please be positive and constructive with your questions and comments.