H bridge MOSFET gate - Page 3 - Electronic Circuits Projects Diagrams Free

TheOne · 13th December 2004, 02:30 PM

Quote:

Originally Posted by fabbie

i just tested the hbridge i posted previously and it works fine. the main reason it works ok might be due to the low voltage applied(12V) and also without any pwm signal.

Now see, it wasn't that bad

You should always try and evaluate something before throwing in the towel!

Oznog · 13th December 2004, 06:06 PM

Quote:

Originally Posted by fabbie

Lastly, i dont get it why the turn on voltage for NPNs connected to the +5v rail has to be 5v + vth. This sounds like a BJT to me.I thought that as long as the gate voltage overcomes the threshold voltage, it would turn on immediately and have all the voltage drop across the g-s.

NMOS (not NPN). Say you need vgs=5v for the desired level of turn-on. Your source for a NMOS is connected to the load, and when turned-on as intended the load will be at the same voltage as the +5v rail. So to get vgs=5v, vg must be +10v.

fabbie · 14th December 2004, 09:31 AM

hey when my motor was running under the pwm signal, it started to make strange screeching noises. Why is this happening? isit because of the pwm frequency? the frequency im using is 2000Hz.

one more thing. I think you guys were right when u said the p channel might short the entire circuit. Everytime when changing from forward and reverse using the PIC, the current would suddenly rise up from 2A to 3.5A. Lucky thing im using a low supply. Isit a wise decision to add a DELAY subroutine when changing from forward to reverse?

Styx · 14th December 2004, 11:57 AM

no you just need some interlocks between firing the N-type and P-type FET in a leg

fabbie · 15th December 2004, 08:39 AM

when u said interlocks, what do you mean by that? add resistors?

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14th December 2004, 09:31 AM	#33
fabbie	hey when my motor was running under the pwm signal, it started to make strange screeching noises. Why is this happening? isit because of the pwm frequency? the frequency im using is 2000Hz. one more thing. I think you guys were right when u said the p channel might short the entire circuit. Everytime when changing from forward and reverse using the PIC, the current would suddenly rise up from 2A to 3.5A. Lucky thing im using a low supply. Isit a wise decision to add a DELAY subroutine when changing from forward to reverse?

14th December 2004, 11:57 AM	#34
Styx	no you just need some interlocks between firing the N-type and P-type FET in a leg

15th December 2004, 08:39 AM	#35
fabbie	when u said interlocks, what do you mean by that? add resistors?