4 replies to this topic

[GlenS]

Hi,

 

I am designing a system to control 4 low voltage dc motors using a couple of SN754410 ic's. I have read different opions on the use of Schottky diodes to protect agains reverse voltage from the motors, some say use them, others say they are not necessary. I'm fairly new to Micro electronics, software being my bag, so what is the opions of all you experts on here. Thanks for any feedback, and Happy New year to you all

 

Glen

 

[supra]

@GlenS

 

It is not necessary to used diode for motors whenever you used 754410 or any other IC chips.

[Arbiter]

Schottky dodes are fast switching with a low forward bias. They're fast switching because they have a zero recovery time - there is nothing to recover from because they don't use a charge carrier depletion region at the junction (which is how a PN diode works).

 

Over-engineering is always better than under-engineering in a prototype or a one-off. If you don't fit them you'll probably get away with it. If you do fit them you can be certain there won't be backlash, assuming they're big enough. Your next step is to fret about how big is big enough. Are you going to add input protection? An over voltage zener shunt? A current-limiting thermistor? Where do you stop? You might think, "Not in the prototype, I'll get it right and then add all that bunk." but in fact you are more likely to blow up your prototype than a finished product.

[CW2]

I am not an expert, but it is considered a good practice to use flyback diode when switching an inductive load, such as a motor coil; but in certain motor types or driving topologies it might not be desired or could require additional components (e.g. center tapped DC steppers driven by switching to ground). When using a specialized driver IC, refer to the device datasheet, usually there is a sample application schematic. SN754410 has internal clamping diodes on each output (datasheet page 2), but you might still consider using external flyback diodes if the characteristics of the internal ones do not meet your project criteria, e.g. when you need a specific switching time.

[GlenS]

CW2, Arbiter and supra,

 

Thanks guys for the response. TBH, I had missed the diodes in the datasheet, and will probably not use the external ones in this project. I'm attempting to create a controller for this robot arm (sold by Maplin in the UK) using the Netduino and a couple of joysticks. The next step will then to hook it up to a RC transmitter, to replace the joysticks