Alt-BEAM Archive
Message #01428
To: beam@corp.sgi.com
From: "Ben Hitchcock" beh01@uow.edu.au
Date: Wed, 10 Mar 1999 23:46:38 +0000
Subject: [alt-beam] Re: Surface mount H-bridge chip
>>
>>Actually, a non-smoking version doesn't need extra parts. You can
>>do it like this:
>>
>> http://www.xs4all.nl/~sbolt/Other/Hsmt-bridge.gif
(7961 bytes)
>
>That'll work but at a cost - it always draws current, even standing
>still and if powered up without a driver then poof... for walkers or
>something with a switched power supply where the constant drain isn't
>a problem, it looks like an easy way to go.
>
What I do for a solar powered reversing circuit is to have an enable
transistor that switches the power on to the H-bridge when the motor needs
to be run. I use the same H-bridge circuit as Steve.
Here is the H-bridge in ASCII:
+ volts rail -----------------------o---------------o---------
| |
R3 P P R2
.--/\/\/---n n----/\/\/\--.
| p p |
| | | |
| o------Motor----o |
| | | |
| | | |
| R4 n n R1 |
input 1 ----o--/\/\/---p p----/\/\/\--o-- input2
N N
| |
0 volts rail -----------------------o---------------o---------
The one extra transistor only consumes a few extra mA when the motor is
running, and stops the current drain when you don't want the motor to run.
For this to work, however, you need to make sure that the outputs from the
74HCXX chip are doing the right thing:
eg. If we switch the +Vcc line to the H-bridge, then when the motor isn't
running then we need to make sure the outputs from the 74HCXX chip are both
low. Having both high will drain current.
Or, if we switch the 0V line to the H-bridge, then the outputs must both be
high.
It's up to you which configuration you use (0 extra transisors, 1 extra, or
2 extra, depending on your design specs and how tight space is.)
Ben
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