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[DPRG] help needed in h-bridge

Subject: [DPRG] help needed in h-bridge
From: jdrumm9015 at aol.com jdrumm9015 at aol.com
Date: Mon Jul 11 12:00:05 CDT 2005

Horng Woei Chang,
  If you just switched the NPN and PNP transistor locations without changing the base connections
expect some "magic" smoke to be emitted.  If the NPN was on the bottom with the emitter connected
to ground the PNP was on the top with the emitter connected to +5VDC you would have both transistors
on for much of the time when switching.
  The original circuit will work for low voltages.  The base - emitter junctions of the transistors are
exposed to the full reverse bias of the motor supply voltage, if the bases are driven to the same
voltages as the motor supply.  If that voltage gets much over 7 volts expect more "magic" smoke to be emitted.  Most biploar transistors only have about 7 volts reverse bias allowed on the base emitter
before the junctions breakdown.  Without a doubt follow Rick's comments about using some 
resistors to limit the base current if you are going to use this circuit.
  A very poor circuit to start with, I would suggest continuing your search and find a better
John Drummond 
-----Original Message-----
From: Rick Bickle <rbickle at swbell.net>
To: 'Horng Woei Chang' <chwoei at gmail.com>; dprglist at dprg.org
Sent: Mon, 11 Jul 2005 10:29:36 -0500
Subject: RE: [DPRG] help needed in h-bridge

I believe that this schematic intends for you to connect something to
the bases of the transistors. Be sure to put some resistance in series
with the transistor bases to limit the base current.

This particular schematic looks to have a problem. I believe that the
NPN and PNP transistors are transposed here. Put the NPN transistors at
the bottom, and the PNP transistors at the top for better function.
Remember that for an NPN BJT, the emitter voltage cannot be higher than
the base voltage - 0.7V. For a PNP BJT, the emitter voltage cannot be
lower than the base voltage + 0.7. Therefore it is more helpful to have
the NPN transistors sink power and the PNP transistors source power.


-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org] On
Behalf Of Horng Woei Chang
Sent: Monday, July 11, 2005 12:21 AM
To: dprglist at dprg.org
Subject: [DPRG] help needed in h-bridge


     As i seen H-bridge tutorials in this web, the difference is the
base is not connected but in my case the base is connected , so i would
like to ask if this H-bridge is working ,the picture is attached, and
what if we vice-versa the polarity.


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