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Topic: Motor doesn't like switching high side. (Read 447 times) previous topic - next topic

stoopkid

I have a couple medium lego power functions motors than I want to control speed and direction with the arduino. I am ordering the SN754410NE to control them. The problem is that I am playing around with them, switching them with a PSMN2R0-30PL. I am just doing one pin at a time right now so only speed, no direction. When I use the mosfet to switch the low side of the motor it works fine. But when I switch the high side it goes much slower.

If I do anything like add a bit of resistance to the high side it stops working. But if I power it with a clean 5v, down from its usual 9, it runs fine. Just slower. Is this normal? How can I make sure this motor will work with that motor driver chip?

Thanks.

CrossRoads

"How can I make sure this motor will work with that motor driver chip? "
Connect N-channel & NPN to the low side.

Connect P-channel & PNP to the high side.  That's how.

The 754410 makes those connections internally, see page 2.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

stoopkid

So the high side specifically need to be driven by a P-channel? I had assumed that was simply to use only one signal to pull one low and one high. Is that my problem?
(I should clerify that I do not have a P-channel on me so I can't just try it)

Grumpy_Mike

Quote
So the high side specifically need to be driven by a P-channel?

Yes if the voltage you are switching is greater than the voltage driving the switch. Otherwise the output on the high side can never be greater than the switch voltage which in this case is 5V.

stoopkid

That sounds familiar, hopefully I remember this time. Thank you this solves my problem completely.

MarkT

High-side MOSFET driver chips exist that synthesize a bias voltage to allow them to switch n-channel MOSFETS on the high-side - an external capacitor (and sometimes diode) are needed for the charge-pump circuit.   Since n-chan MOSFETs are about 3 times superior in performance to p-chan MOSFETs (all else being equal) these are very handy.  The popularity of this technique is why p-chan MOSFETs aren't very common.
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