On Wed, Oct 2, 2013, at 10:47 AM, andy pugh wrote:

> I guess I could use a 250W resistor, which is 30s to 5V, or a 1kW
> resistor, which is 7.5 seconds to 5V. But both seem wasteful, and I am
> not sure I have the space.
> (Also quite expensive:
> http://uk.rs-online.com/web/p/panel-mount-fixed-resistors/7014059/ )

For intermittent duty (as you are using it - connected only on power down),
the cheapest high-power resistors are heaters.  For example:

$8 gets you 38.4 ohms, 1500W intermittent (or continuous, if wet ;-):
http://www.ebay.com/itm/200960173435
That has more then enough thermal mass to handle 20,000uF at 300V.
I've used 2 of those in series to discharge 24,000uF at 600V (4.8 times
the energy).  The flange makes it fairly easy to mount.

$13 gets the same ohms and watts, but can run continuously if needed:
http://www.ebay.com/itm/131001379245
Just don't put anything meltable nearby.  I've seen these used as DB
resistors for spindle drives - mounted in a mesh box on top of the 
machine so the heat can escape.


-- 
  John Kasunich
  jmkasun...@fastmail.fm

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