On Sat, Apr 10, 2010 at 12:42:14PM +0200, Roland Jollivet wrote:
>
> What about this;
>
> http://www.fotothing.com/photos/1f5/1f5ff1cf0801bfc786bfbb8663578306.jpg
Hey, yes, that looks like it'll just about do it. I figured that at 30V,
47 ohms would dissipate 19W, so a 20W resistor would do. [1] At 10A
motor current, load dump would then spike to 470V, though.
If we put 1000 uF non-polarised in series with the resistor, then 10A
can flow for 6 ms for a 60V rise on the cap. The relay armature
could have made it to the shunting contact by then. Now the resistor can
be much smaller, reducing the spike. (Keep it under 20 ohms, and the
spike stays well under the 7i40H voltage protection threshold.)
Rafael's snubber design find will help me optimise that. (A good find!)
That'll do the job, but without the caps, the motor is stressed with
nearly 500V. If I make the non-polarised caps by connecting 2200 uF
(or larger) electrolytics back-to-back, then thats 8 caps + 8 power
resistors + 4 relays, for 4 motors.
Before building that, I'll just go over Peter Wallace's alternative
again. (I've just realised that it should dynamically brake the motor
too, if I'm thinking straight.)
Grateful thanks for helping me to a workable solution.
Erik
[1] If EMC quickly shuts down the 7i40 H-bridge, then smaller would do,
because the dissipation is transient. But I only know of shutdown on
watchdog bite or loss of controller connection.
--
The important work of moving the world forward does not wait to be
done by perfect men.
-- George Eliot
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