"DoN. Nichols" wrote in message
...
On 2008-12-17, RogerN wrote:

If I understand your circuit correctly, there is current flowing through
the
resistor only during the half cycle that the rectifier is conducting.

Yes -- but very little current once the capacitor is charged.
Its primary function is to keep the initial charge surge from frying the
selenium rectifier, which were rather more sensitive to excess current
even for short times than the Silicon ones. I kept it once I saw how
much of a spark plugging in the grinder caused with the full wave bridge
and the 2000 uF capacitor. I figured that would also shorted the life
of the switch if I didn't keep the resistor to lower the surge current.


I'm
guessing that with the original circuit, half cycle conduction, resistor,
and 8uF capacitor, the voltage at the chuck was probably closer to 120VDC
than 170VDC.

I think that it was closer to the 170 VAC. The chuck never felt
warm, even after a long time holding a workpiece (unless I was grinding
too heavily, which was uncommon.)

Without knowing the resistance of the mag chuck I'm only
guessing but a 20 ohm 10 Watt resistor could drop up to 20V for 50% duty
cycle giving a 150V peak and the 8uF capacitor most likely won't hold
this
voltage very well during the reverse half cycle. Just guessing I'd
figure
the resistor, diode, and capacitor combination probably supplied close to
a
120VDC equivalent.

O.K. Time to check the resistance of the chuck. Back in a few
minutes ... how about 36K? I don't think that there will be much
ripple, or drop in the resistor once the cap is charged.

That would be 4.7 mA at 170VDC -- or a bit less than 10mV drop
across the resistor. If it were drawing enough to drop half of the
voltage across the resistor, we would need a 360W resistor or so. :-)

But for the short cap charging period, even with the cap bumped
from 8 uF to 2000 uF, the high current is short enough so the average
power in the resistor is well within specs.


Wow, I would have never guessed the chuck resistance to be that high.
Another poster said his chuck was rated for 115VDC at 1.8A. His chuck is
over 200W and your is 0.8W. I was guessing your chuck to average maybe 0.7A
or so. If I'm calculating correctly, you have less than 1 mW power
dissipation in the 10W resistor, they could have used a 1/2W resistor and
still been 500X oversized.

The design you made sounds more like a DC power supply. The half wave
rectifier and 8uF capacitor sounded like something that was suppose to
deliver less than the peak voltage. Thinking about it a little bit here,
are you sure that wasn't 0.36K Ohms?

RogerN