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Author Topic: The H-Bridge Schematic has confused me  (Read 709 times)

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Offline MaltaeronTopic starter

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The H-Bridge Schematic has confused me
« on: September 12, 2010, 10:07:58 PM »
Looking at http://www.societyofrobots.com/schematics_h-bridgedes.shtml and seeing that the top two MOSFETs are supposed to be P-channel and the bttom two N-channel, if A were pulled high and B pulled low, wouldn't the top-right MOSFET open up because B is low and is P-channel, and then the bottom-right open up because A is high and N-channel? Thus causing a short circuit? Looking around, I typically see A on one side -- the left -- and B on the other, such that if A and B are high, then the circuit still does nothing just as if both were low.

Besides the note you should "Use PNP transistors for the top two MOSFETs, and NPN MOSFETs for the bottom two (the ones connected to ground). Otherwise bad stuff could happen," it looks like the schematic would work if only N-channel MOSFETs were used, such that the behavior would be as described in the article.

Could I do that and expect it to work fine? I have IRLU3802 N-Channel MOSFETs and only need to move 1.5A at 5V.

Thanks.

Offline Soeren

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Re: The H-Bridge Schematic has confused me
« Reply #1 on: September 13, 2010, 11:59:02 AM »
Hi,

Besides the note you should "Use PNP transistors for the top two MOSFETs, and NPN MOSFETs for the bottom two (the ones connected to ground). Otherwise bad stuff could happen," it looks like the schematic would work if only N-channel MOSFETs were used, such that the behavior would be as described in the article.
You're absolutely correct. It was drawn for N-ch. devices all 'round and the note about P-channel at the top was added quite recently, after a discussion here, but it takes another design as well.
Don't build it like it's shown, but add some logic to avoid shorting the battery (or control each gate separately).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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