According to Gunner :
This is a bit embaressing. Sigh..its been a looong time since I needed
to figure this sort of thing out.....Im a board changer/wrench
turner/relay and plc type...
I have an elderly electromagnetic chuck that Ive been sitting on for a
couple years until I found a decent usable 6x8 surface grinder..which
I recently did (Covel), but it has no power supply. Rated 110vts DC.
O.K. The mag chuck, I presume you mean, is rated 110 VDC?
So I dug out a bridge rectifier, added a power cord, and powered up
the chuck with the output. Chuck does indeed work, but its a smidge
weaker than I think it should be..or would like it to be. Its half
wave DC..so Im only applying a magnetic field half the time, right?
Huh? How do you get half-wave with a bridge rectifier? That
should give you full-wave rectification.
Now if I added a simple capacitor to the output, it should give me a
longer signal time as it fills in the gaps.
Yes.
This should result in a
slightly stronger effective magnetic field. Right?
One that is not forming and collapsing 60 times a second
Assuming Ive not **** in my messkit at this point..what MFD cap should
I use? Cringe.....blush
Current draw is about 1 amp
Well -- it could be calculated to determine what the minimum
capacitance needed would be -- but I simply used a 250V 1000 uF computer
grade electrolytic. (At least 200V is needed because the peak voltage
from that bridge should be on the order of 170 V, and with minimum
ripple, the voltage across the capacitor will be that pretty much all of
the time.
You can use larger capacitance if you want, or significantly
less. The 200V is a minimum, and higher won't hurt.
You also want a resistor to limit the surge current. Let's see,
with 1 Amp current, a 5 ohm resistor would drop 5 volts, and will need
to be capable of handling 5 Watts. Probably a 10 Watt wire-wound would
do nicely.
Clean DC would be nice..but its only a magnetic chuck....
Shrug..
Now the last thing....some electo chucks tend to magnitize over
time..or bleed off the field over a number of seconds.
Right.
If I put a maintained On/off/Momentary On toggle switch in...I should
be able to run a momentary bit of 110vAC though the chuck to
demagnitize it, right?
Yes -- if you have the switching right, so you don't apply that
while the capacitor and bridge are connected to the chuck.
I have a schematic of one which I designed to replace what was
in the base of my Sanford grinder (about a bit smaller -- 6x4 inch,
IIRC).
http://www2.d-and-d.com/PROJECTS/SAN...DER/index.html
an click on the link for the pdf version of the drawing. It is easier
to read when printed. The .gif is probably not as good, and certainly
the reduced size schematic tends to have lines vanish, depending on the
browser in use.
The switch which I used has a center off position, and is stable
in either on position, though a momentary to the "degauss" position
could be used. I consider the switch which I have better, as you get
better demagnitization of the workpiece if it is removed while the AC is
on -- akin to moving a tape head demagnetizer away from the head before
switching off the power, which could otherwise switch off during a peak
and thus leave more residual magnetism.
The chuck had a plug which fit into the side wall of the base of
the grinder, and that is what is shown in the schematic -- not the chuck
itself.
Note that when using this design, you *don't* want to use
coolant. If you want to do that, add an isolation transformer between
the power line and the bridge rectifier.
Good Luck,
DoN.
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