hi, m using a H-bridge for 12v,1.6A motor in which upper two mosfet r p- type n lower r n- type. i m using pic o/p for h bridge so using 4 transisters with 4 mosfets.pnp with gate of n mosfet and npn conncted with gate of p-mosfet.i m connecting the gates of alternate opposite mosfets ie one p type with opposite n type mosfet together. my problem is that te both p type mosfet are always high irrespective of the signal on gate(either high o low) and one of n type mosfet sourc is not high when its getting hig signal on gate. pls advice
The reason it does not work, is that a MOSFET is a voltage controlled device (transconductance). A voltage on the input (gate-source) controls a current on the output (drain-source). The voltage on the gate must be above the threshold voltage (4.5 to 7 volts) of the MOSFET in order for the MOSFET to turn on.
If he's using the 8V that he cites that should be plenty of voltage for all but the oldest of MOSFETs. If he's using logic-level FETs then he shouldn't have any trouble at all.
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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Ur stl f splng s unclr. Please remember that you have a full keyboard, including vowels and a shift key for abbreviations like 'MOSFET'. Please also remember that you are not being charged by the character, so you may type those vowels without paying extra at the end of the month. This is a newsgroup, not a cell phone text message ring.
As near as I can decipher what you are trying to say, the gate of each P-type MOSFET is connected to the gate of it's opposite N-type MOSFET. This should work with some nasty current spikes at the transitions, so I'm not sure just _how_ things are connected.
If you can, draw up a schematic of your circuit and post it either to a web site, to alt.binaries.schematics.electronic, or use something like AACircuit (google for it) to draw one as text to post here. Alternately, spell out a detailed net list (Collector of Q1 to Gate of Q3...).
If you have the pins for it on the microprocessor (I assume that 'pic' means "PIC Microprocessor") then the following circuit will drive each gate adequately. You'll need to use four pins on the processor, and a software bug will short your power to ground which is always exciting, but the circuit will work:
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Could be. I would expect that most 12V MOSFETs of any part number would still work, and that only a batch that was on the fringe wouldn't.
I can't make heads or tails of his circuit description as I indicated in my other post, so it's hard for me to say what's really going on -- hopefully he'll clarify for us.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
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