> The capacitor is what gives the low impedance, and anything over
> 1000pF is plenty.

JT> Yeah, but that's before the output limiting resistor. Why muck
JT> around? Just put a tesla coil behind it, and let the five foot
JT> spark burn the dust to a crisp. :-)

 The limiting resistor does bugger-all, becasue the corona current
is so small. I think we used to stick 100K in series with silicon
diodes to save them, if someone shorted the 10KV lead on a 12" TV.

> Resistors connected to the mains are supposed to be "safety"
> resistors defined in AS 3250 able to withstand 150% nominal
> voltage and 10KV from 1000pF without changing value more than 50%
> (from memory). In practice, any spiral-cut 1/2W resistor will
> pass.

JT> Till it gets past the warranty period. We were taught to check
JT> the charge on power supply bleeders even if they looked good.
JT> Many used to go open and you'de never know till you touched it.

 That's the problem... they go open-circuit and no one knows. I keep
getting belts off my food processor, that way. Bastard! Those little
power supplies they hang on the mains using a 0.1uF as a dropping
capacitor are real mongrels. They use a 2.2M resistor to bleed it, but
it takes a second or so to drop down to 30V (which you don't really
feel) and if you pull the plug and your finger gets across the pins...
whap!

> I used to design vacuum cleaners. Pye was big is vacuum cleaners.

JT> No Bob, you're going senile, you used to design television
JT> sets. :-)

 Whenever some engineering problem needed "brains" the TV Lab ended
up with it. I did a fan, too. They wanted to know why the fan motor
was so noisy and hot... they were running the bloody iron saturated! I
told them to put more turns on. I knew bugger-all about motors, so I
treated it like a power transformer. It worked really well...

> How did we move from engineering to PR?

JT> When you implied that vacuums were better for dust allergies.

 I only said that ion generators were useless, and a vacuum cleaner
was better than nothing.

> Yair... so vacuum once a week and get some benefit.

JT> You don't know what dust allergies are like, do you?

 You were the one that ran off with allergies, let alone dust
allergies, but it so happens that I *do* know about allergies, and
sensitisation. Once you become sensistised to an allergen, nothing
at all is far too much. Your body tunes into it, in the same way that
a dog tunes into a smell. The allergen molecule has a receptor
purpose-built for it.

> So, use a bigger motor. Back in the 70s, a 600W motor was *big*.
> Now the entry level is 1600w!

JT> Actually no, it's better to use a low power unit for better
JT> fine dist collection. When I used to be a field techo, I
JT> carried a vacuum to suck up toner from laser printers. We were
JT> all given low power units because the photocopier guys (who
JT> used them more than us) found the high power units used to suck
JT> too much toner through the bag, into the motor and seriously
JT> reduce the motor life.

 Weren't you talking about an effective filter restricting airflow
too much? Now you seem to be recommending a lousy filter.

> Yes. We used to weigh the bag tested against an unmodified
> machine on the same carpet. The head makes the biggest
> difference, and the really hard part is to get it to work right
> to the edge.

JT> They never do, that's why they give you that small nozzle
JT> thingy.

 That's the chicken way out. The head makes an enormous difference to
the way that the dirt is picked up. They stopped me before I could get
right into it.

Regards,
Bob
          

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