Alt-BEAM Archive
Message #11880
To: beam@sgiblab.sgi.com, beam@sgiblab.sgi.com
From: Terry Newton wtnewton@nc5.infi.net
Date: Wed, 15 Mar 2000 09:25:15 -0600
Subject: [alt-beam] Re: H-Bridge
Hey all,
A little tip regarding smoke proof bridges..
simply connect a diode from the base-side of each input
to the motor out opposite of the one that goes low when
the input is high... for example using a popular
4-transistor smoking bridge...
+ >----*-------------------.
| e
e .-----R------b PNP
PNP b--|---R-----. c
c | | |
| | | |
*----*--motor--*----*
| |
c c
in >--R------b NPN NPN b----R---< in
#1 e e #2
_|_ _|_
Add 2 diodes to make smokeproof...
+ >----*-------------------.
| e
e .-----R------b PNP
PNP b--|---R-----. c
c | | |
| | | |
*----*--motor--*----*
| | | |
c | | c
in >--R--*---b NPN| |NPN b--*-R---< in
#1 | e | | e | #2
| _|_ | | _|_ |
`----------|-|>|-----' |
`------|<|----------'
Note - use 1N34 diodes that have a lower Vf than
the transistors (or put extra diodes in series with
npn bases). This makes it impossible for both bases to
go high at once, whoever gets there first latches the
other one out. There may be some struggle and current
drain, but darn better than smoke!
The idea for this was gleened from one of Wilf's designs,
I think the 6 transistor one, I noticed that about the only
difference from a standard bridge was latching diodes.
Terry Newton
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