One thing that you can do, in case that you *accidentaly* turn on two
transistors of the same pair is to put 2 lamps (60W for example) in
series between each other and both lamps in series with the +V going
from the rectifier to the H-bridge.

Save me 2 times....

Best regards

Mauricio D. Jancic
Janso Desarrollos - Microchip Consultant
www.janso.com.ar
info@janso.com.ar
(54) - 11 - 4542 - 3519
Lugones 3238 "B" - C1430ECN
Capital Federal
Republica Argentina
(54) - 11 - 4542 - 3519
MSN: jansodesarrollos@hotmail.com

>>-----Original Message-----
>>From: pic microcontroller discussion list=20
>>[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Russell McMahon
>>Sent: Mi=E9rcoles, 19 de Noviembre de 2003 05:32
>>To: PICLIST@MITVMA.MIT.EDU
>>Subject: Re: [EE:] Driving an H-bridge
>>
>>
>>> 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=20
>>> matter ?
>>
>>
>>As a rule the low side drivers are liable to be better for=20
>>PWMing. This is because the high side is more liable to have=20
>>limited bandwidth due to whatever you use for isolation=20
>>purposes. If you use opto couplers (as your circuit does)=20
>>then the high side is ver likely to be appreciably slower.=20
>>Whether this matters depends in part on the PWM frequency.
>>
>>You can check this easily by running the circuit with a light=20
>>resistive load and applying PWM to high and low successively=20
>>and DC drive to the other half. Look at the FET gate drive=20
>>waveforms as you increase the PWM frequency. The 3 transistor=20
>>FET drive circuit that you are using will provide pulses down=20
>>to around 1 uS in length without too much design effort. The=20
>>opto driven high side version us not liable to get anywhere=20
>>near this but YMMV.
>>
>>        RM
>>
>>--
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