On Fri, Apr 09, 2010 at 02:17:28PM +0200, Roland Jollivet wrote:
> A DPST will perform the function fine, unless you want DPDT for some other
> reason.
Hmmm, maybe you meant to type SPDT? Even leaving one side of the motor
connected to the H-bridge output, we have to switch the other side of
the motor away from the H-bridge to a shunt resistor. I've collected
components, and am starting to build that up. (The relays to hand are
only SPDT, after all.)
Now, should I connect a pair of back-to-back hefty zeners or transorbs
across the motor, to clip the hundreds of volts of back-EMF which could
be generated by interrupting 10A flowing in the motor winding? The shunt
resistor is not in circuit until the relay armature completes its
travel. That could be enough to let the magic smoke out of the 7i40's
H-bridge FETs, unless there's very robust protection there.
Alternatively, using Andy's bridge rectifier idea reduces the number of
great fat zeners required. I then just have to add one common
overvoltage shunt across the supply rail, close to the four
motor-stopping relays.
Thanks for the suggestions. (I'd hate to blow up a 7i40 by guessing
wrongly while trying to increase protection against mishaps.)
Erik
--
In all affairs it's a healthy thing now and then to hang a question
mark on the things you have long taken for granted.
-- Bertrand Russell
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