DPRG mailing list archives: [DPRG] Queries about the McManis H bridge

Mon Oct 9 09:43:06 CDT 2006

Vimal,
  The 10K resistors bring the base to ground when the switch is open.  There is base to emitter capacitance if you did not bleed this charge off the transistor would stay "on" until this capacitive charge discharged through the transistor.  The resistor gives a path to bleed this charge off and NOT allow any other capacitive coupling to turn your transistor back on when you didn't want it to.

  Leaving ANY input floating is an invitation to Mr.  Murphy to come wreck havoc with your circuit.  Always control all inputs in all states and you don't get any unintended behavior.

John Drummond 

-----Original Message-----
From: vimalsshankar at yahoo.com
To: dprglist at dprg.org
Sent: Sat, 7 Oct 2006 3:48 PM
Subject: [DPRG] Queries about the McManis H bridge

I have a few queries about the McManis's bridge :

1. In the explanation at :

http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-impl.html

Can someone please explain to me why the 10k pull-up
resistor insures that the transistor is "off" (or not
sourcing any current) when the switch is open.
Same for "In the sink circuit the 10K resistor
connects the base of the transistor to ground, which
forces the transistor off when the switch is open."

As far as I know, the transistors will turn on only
when the base is correctly biased. It will ALWAYS be
"switched off" otherwise - so why the pullup / pull
down resistors ?

2. Can I use the 1N4001 in place of the 1N4936 ?
Also, how do I know the specs of the inbuilt
freewheeling diodes in the TIP122/127 ?

3. I am using the TIP122/127 in place of the
TIP102/107. Will it matter much (I promise to stay
within the amperage mentioned in the specs of the
TIP122/127)

4. What about using the CNY17-2 inplace of the
menioned optoisolators ?

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