Bob is right... you will need to find (or generate) a higher voltage than the pic supplies to turn on the upper FET's in the bridge. I usually couple with an opto-isolator to the upper gates. It is cheap and clean. Another thing to be aware of: The FETS will not turn off immediately, so put a little dwell between turning the motor off in one direction and on in the other. A pic can reverse the outputs in well under a microsecond. It is unlikely that the FETs will turn off that fast, unless you have a really stiff driver circuit, so you end up with a short circuit (maybe two) across the motor supply. If you don't pay attention to this you will have either big current spikes on the motor line or the sweet smell of combusting FET. Also, the motor will act like a generator when coasting. Depending on how you do the driving, you can end up with some weird bias on the FET switches until the motor stops. Clamp each FET with a diode S-D to deal with this issue. When you reverse a motor running full speed, you will get a current spike of about twice the locked rotor current. This can be a bunch of current! Size the H-Bridge FETs accordingly. You can figure out the locked rotor current by measuring the resistance of the motor when at rest and dividing that number into the motor voltage. I dislike H-Bridges, and only use them when I have to. The problem with driving them with a PIC is dealing with the power-up transient before the PIC initializes the ports. Be clever and take into account the power-up state of the PIC output lines when you design the circuit. If I am not dealing with a gazillion motor reversals, I reverse with a DPDT relay and use one FET for PWM. It is a lot safer and often cheaper than a full FET H-bridge. In short....H-Bridge design is a non-trivial art... Edward Gisske, P.E. Gisske Engineering 608-523-1900 gisske@offex.com ----- Original Message ----- From: "Bob Blick" To: Sent: Wednesday, November 19, 2003 11:20 AM Subject: Re: [EE:] Driving an H-bridge > Jinx said: > > I'm ready to power up an H-bridge made with 4 N-ch FETS > > for motor control. Any advantage, recommendation or advice > > as to whether the PWM is on the top or the bottom or does it > > not matter ? > > You've left something out. > > Four N-MOSFETs does not equal an H-bridge, you need more parts! You will > need to shift levels to drive the MOSFETs. > > Cheers, > > Bob > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics