> A customer let the smoke out of a H-bride motor driver I made for 
> him,
> so now I want to add reverse-power protection. My standard 
> anti-parallel
> 1N4004 won't do much good when connected to a lead accu, so I am
> considering a series diode (1N4004, or a schottky). But now the 
> back-emf
> from the motor, which is fed to the power rail by the 4 diodes, has
> nowhere to go. What is the solution to this? A big zener on the 
> power
> rail, for let's say 150% of the maximum power supply? If so, which
> wattage would be needed? Or maybe a consensator? If you, what 
> capcity
> and what type? Or are there other solutions, maybe with a polyfuse?

MOSFET in series with line.
If N Channel, place in negative lead with drain to battery, source to 
load negative lead. (Unintuitive connection. Has to do with body 
diode. Works).
gate goes to positive battery connection. Gate divider may be needed 
depending on battery voltage.

If P Channel place in positive lead, keep source towards load, gate to 
opposite supply line, proceed as per N channel.

When battery is correctly connected the gate is biased positive 
relative to the source and the FET is enhanced. Operation is in a 
quadrant not usually used but the FET doesn't mind.

When battery is connected backwards the gate is biased negative 
relative to source and FET is off. Looking at the arrangement in this 
state you see why you have the FET "backwards" to usual as the body 
diode now also blocks.

A low Rdson FET allows very low voltage drop when on.

This is a brilliant idea and decades  old. some turkey claimed to have 
patented it quite recently.

In you case it is even more brilliant than usual as the FET is a 
bidirectional channel and will pass motor back en=mf into the load.
Some design may be required with the gate circuit to provide a small 
"hang" time delay. Once you have the general principle the rest is 
easy to adapt to your needs.



        Russell McMahon
 

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