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.

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?

Now if I added a simple capacitor to the output, it should give me a
longer signal time as it fills in the gaps. 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

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.

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?

Thanks

Gunner, hanging his head in shame...


"A prudent man foresees the difficulties ahead and prepares for them;
the simpleton goes blindly on and suffers the consequences."
- Proverbs 22:3

Gunner wrote:
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.

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?

Now if I added a simple capacitor to the output, it should give me a
longer signal time as it fills in the gaps. 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

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.

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, you do need a capacitor. Size depends on how much current you are
drawing, it can be calculated (depends on how much ripple you can take)
but something like 5,000 uF per amp is probably about right. Make sure
you get one with the correct voltage rating. 4,700 uF should be fine
with 1 A.

The AC should demagnetise it OK.

Leon

Should have said: the DC voltage across the capacitor will be about 1.4
x 110 V. I'd get a 200 V one.

Leon

On 13 Mar 2006 03:02:33 -0800, "Leon" wrote:

Should have said: the DC voltage across the capacitor will be about 1.4
x 110 V. I'd get a 200 V one.

Leon

Yes of course. That much I knew...G

Ive got some big assed caps in cans...600 vots, 15,000 mfd if I
recall, but Ill have to check. I found out they were too high a value
for balancing caps for an RPC..so they are sitting on the shelf.

Too much?

Gunner



"A prudent man foresees the difficulties ahead and prepares for them;
the simpleton goes blindly on and suffers the consequences."
- Proverbs 22:3

"Gunner" wrote in message
...
On 13 Mar 2006 03:02:33 -0800, "Leon" wrote:

Should have said: the DC voltage across the capacitor will be about 1.4
x 110 V. I'd get a 200 V one.

Leon

Yes of course. That much I knew...G

Ive got some big assed caps in cans...600 vots, 15,000 mfd if I
recall, but Ill have to check. I found out they were too high a value
for balancing caps for an RPC..so they are sitting on the shelf.

Too much?

Gunner

A motor start capacitor around 100uF should work just fine, also consider
putting a current limiting resistor in series with the chuck to cut down the
inrush current and don't forget to put a bleeder resistor across the cap, i
usually just use a incadesent pilot lamp for 110v bleeders.

Best Regards
Tom.

Gunner wrote:
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.

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?

You're getting a full wave recitified DC outout, not half wave with a
bridge rectifier.

Now if I added a simple capacitor to the output, it should give me a
longer signal time as it fills in the gaps. This should result in a
slightly stronger effective magnetic field. Right?
One that is not forming and collapsing 60 times a second

120 times a second with a full wave rectification, 60 with half.

Assuming Ive not **** in my messkit at this point..what MFD cap should
I use? Cringe.....blush

Current draw is about 1 amp

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.

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?

That should work.

"Gunner" wrote in message
...
On 13 Mar 2006 03:02:33 -0800, "Leon" wrote:

Should have said: the DC voltage across the capacitor will be about 1.4
x 110 V. I'd get a 200 V one.

Leon

Yes of course. That much I knew...G

Ive got some big assed caps in cans...600 vots, 15,000 mfd if I
recall, but Ill have to check. I found out they were too high a value
for balancing caps for an RPC..so they are sitting on the shelf.

Too much?

Gunner

"A prudent man foresees the difficulties ahead and prepares for them;
the simpleton goes blindly on and suffers the consequences."
- Proverbs 22:3


Gunner are the caps bipolar (ie for AC)? I'm note sure but I think you may
have to derate the max voltage value for DC use. Someone over at
sci.elec.design will know.

No warranties but after checking the voltage rating I'd try the caps. The
average voltage output may increase. The higher capacitance value may also
result in more inrush current when you first turn it on and charge the caps.
A slow blow fuse may help with this.

rob

On Mon, 13 Mar 2006 11:30:55 GMT, the renowned Gunner
wrote:

On 13 Mar 2006 03:02:33 -0800, "Leon" wrote:

Should have said: the DC voltage across the capacitor will be about 1.4
x 110 V. I'd get a 200 V one.

Leon

Yes of course. That much I knew...G

Ive got some big assed caps in cans...600 vots, 15,000 mfd if I
recall, but Ill have to check. I found out they were too high a value
for balancing caps for an RPC..so they are sitting on the shelf.

Too much?

Gunner

Gunner:

Try the caps out of an old PC supply. They're usually rated something
like 330uF 200V (they are used as a voltage doubler in the 120VAC
input mode, so they are in series). One of two of those should suffice
330uF o(r 660uF @ 200V with two in parallel). While your in there
scavenging, grab the input surge supressor (NTC thermistor) and put
that in series with the input (looks like a disk capacitor and gets
warm). That'll keep from killing your bridge with the surge to charge
the caps.



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

A bridge rectifier will give you full wave DC pulsed at 120 cps (htz). Each
bottom half wave is inverted to the top, so to speak. You just need a bit
of fill between the peaks, as you said. If the chuck gets a bit warm
because the feed is a bit hot from the direct DC-ed 120vac, I'll send you a
little variac that will easily allow adjustment. That would also allow a
safer degauss cycle.
Respectfully,
Ron Moore

"Gunner" wrote in message
...
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.

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?

Now if I added a simple capacitor to the output, it should give me a
longer signal time as it fills in the gaps. 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

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.

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?

Thanks

Gunner, hanging his head in shame...


"A prudent man foresees the difficulties ahead and prepares for them;
the simpleton goes blindly on and suffers the consequences."
- Proverbs 22:3

Gunner wrote:

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.

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?

Now if I added a simple capacitor to the output, it should give me a
longer signal time as it fills in the gaps. 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

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.

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?

Thanks

Gunner, hanging his head in shame...


"A prudent man foresees the difficulties ahead and prepares for them;
the simpleton goes blindly on and suffers the consequences."
- Proverbs 22:3


The problem with using AC to demagnitize is that when you turn it off
you may turn it off
when the voltage is high and you remagnitize it. You need to slowly
reduce the AC current.

Bill K7NOM

Sounds like a great application for a Solid State Relay. I'll throw one of
those in, too.
Ron

"Bill Janssen" wrote in message
...
Gunner wrote:


Now the last thing....some electo chucks tend to magnitize over
time..or bleed off the field over a number of seconds.

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?

Thanks

Gunner, hanging his head in shame...


"A prudent man foresees the difficulties ahead and prepares for them;
the simpleton goes blindly on and suffers the consequences." - Proverbs
22:3

The problem with using AC to demagnitize is that when you turn it off you
may turn it off
when the voltage is high and you remagnitize it. You need to slowly reduce
the AC current.

Bill K7NOM

I had a thing like that once that was intended for running portable power
tools. The rectified/boosted power really made portable AC/DC tools run
strong. It was intended to compensate for the drop in a long extension cord.
After you get this thing built, you might try plugging in a couple of power
tools, just for fun. Mine was stolen, and as far as I know they are no
longer made.

On Mon, 13 Mar 2006 10:45:00 GMT, Gunner
wrote:

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.

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?

Now if I added a simple capacitor to the output, it should give me a
longer signal time as it fills in the gaps. 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

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.

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?

Thanks

Gunner, hanging his head in shame...

A bridge rectifier (if connected correctly.... ) produces full-wave
rectified DC. The RMS value of this DC will be about the same as the
applied AC voltage. If the chuck was rated at 110 VDC I don't think
you want any caps, just fullwave rectification. The inductance of the
chuck winding may well smooth out the current ripple quite a bit.
Using guesses of 100 ohms for coil resistance and 500 mH as
inductance, I get 1 amp of avg current with 0.127 amps RMS of 120 Hz
ripple current.

Adding capacitance can raise the DC voltage closer to the peak voltage
(165 volts) which may overdrive (overheat) the chuck.

"Gunner" wrote in message
...
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.

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?

Now if I added a simple capacitor to the output, it should give me a
longer signal time as it fills in the gaps. 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

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.

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?

Thanks

Gunner, hanging his head in shame...


"A prudent man foresees the difficulties ahead and prepares for them;
the simpleton goes blindly on and suffers the consequences."
- Proverbs 22:3

Surprised no one pointed this out. 110 VAC is rms voltage. The output of a
bridge rectifier/capacitor combination will be about that times the square
root of two, practically somewhere around 150 volts.

You probably need a variac in front of the rectifier if you really want 110
VDC.

Jerry

On Tue, 14 Mar 2006 00:31:26 GMT, the renowned "Jerry Foster"
wrote:

Surprised no one pointed this out. 110 VAC is rms voltage. The output of a
bridge rectifier/capacitor combination will be about that times the square
root of two, practically somewhere around 150 volts.

You probably need a variac in front of the rectifier if you really want 110
VDC.

Jerry

Act-ually, the first thing Gunner should try is really E-Z, and that's
to put an inverse parallel rectifier across the coil*
(REVERSE-biased). As it sits now, there's no path for the current when
the current drops below the peak, until the diodes avalanche, which
means a whole bunch of energy is likely going into the diodes and not
staying in the electromagnet.

* care to polarity or all 5 diodes will release their smoke


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Mon, 13 Mar 2006 20:00:58 -0500, the renowned Spehro Pefhany
wrote:

On Tue, 14 Mar 2006 00:31:26 GMT, the renowned "Jerry Foster"
wrote:

Surprised no one pointed this out. 110 VAC is rms voltage. The output of a
bridge rectifier/capacitor combination will be about that times the square
root of two, practically somewhere around 150 volts.

You probably need a variac in front of the rectifier if you really want 110
VDC.

Jerry

Act-ually, the first thing Gunner snip

Sorry, forget it, the bridge clamps it to two diode drops on top of
the input supply anyway. dunh

But there is a potential problem when the input connection is broken
with no capacitor on the input or output of the bridge.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Mon, 13 Mar 2006 20:00:58 -0500, Spehro Pefhany
wrote:

On Tue, 14 Mar 2006 00:31:26 GMT, the renowned "Jerry Foster"
wrote:

Surprised no one pointed this out. 110 VAC is rms voltage. The output of a
bridge rectifier/capacitor combination will be about that times the square
root of two, practically somewhere around 150 volts.

You probably need a variac in front of the rectifier if you really want 110
VDC.

Jerry

Act-ually, the first thing Gunner should try is really E-Z, and that's
to put an inverse parallel rectifier across the coil*
(REVERSE-biased). As it sits now, there's no path for the current when
the current drops below the peak, until the diodes avalanche, which
means a whole bunch of energy is likely going into the diodes and not
staying in the electromagnet.

* care to polarity or all 5 diodes will release their smoke


Best regards,
Spehro Pefhany


Blink blink....huh??

Gunner



"A prudent man foresees the difficulties ahead and prepares for them;
the simpleton goes blindly on and suffers the consequences."
- Proverbs 22:3

On Tue, 14 Mar 2006 03:04:49 GMT, the renowned Gunner
wrote:

Blink blink....huh??

Gunner

Pls. see correction. ;-)



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Mon, 13 Mar 2006 22:15:48 -0500, Spehro Pefhany
wrote:

On Tue, 14 Mar 2006 03:04:49 GMT, the renowned Gunner
wrote:

Blink blink....huh??

Gunner

Pls. see correction. ;-)



Best regards,
Spehro Pefhany


Ah...I didnt understand the correction either.......

Now ask me about Welding, or ballistics, or skinning and curing hides,
or other Stuff.....

G

Gunner



"A prudent man foresees the difficulties ahead and prepares for them;
the simpleton goes blindly on and suffers the consequences."
- Proverbs 22:3

In article ,
Gunner wrote:

On Mon, 13 Mar 2006 20:00:58 -0500, Spehro Pefhany
wrote:

On Tue, 14 Mar 2006 00:31:26 GMT, the renowned "Jerry Foster"
wrote:

Surprised no one pointed this out. 110 VAC is rms voltage. The output of a
bridge rectifier/capacitor combination will be about that times the square
root of two, practically somewhere around 150 volts.

You probably need a variac in front of the rectifier if you really want 110
VDC.

Jerry

Act-ually, the first thing Gunner should try is really E-Z, and that's
to put an inverse parallel rectifier across the coil*
(REVERSE-biased). As it sits now, there's no path for the current when
the current drops below the peak, until the diodes avalanche, which
means a whole bunch of energy is likely going into the diodes and not
staying in the electromagnet.

* care to polarity or all 5 diodes will release their smoke


Best regards,
Spehro Pefhany


Blink blink....huh??

The problem is that the coil of the magnetic chuck stores energy in the
magnetic field. How much energy is stored per amp of coil current is
proportional to the inductance of the coil.

The problem is that when the coil current is abruptly interrupted, the
energy in this magnetic field comes back out, attempting to keep the
coil current flowing in the same direction as the original current
flowed. This is seen as a high-voltage spike, reaching to hundreds or
thousands of volts, and will puncture any attached semiconductors (like
the rectifier diodes).

The classic solution is to put a rectifier diode in parallel with the
coil, oriented so that under normal conditions no current flows through
the diode. When the coil current lead is opened, the magnetic field
energy goes into this rectifier diode, preventing the voltage spike.

If one mis-orients this rectifier diode, it will be a dead short across
the power supply, and all five diodes (4 in the bridge, 1 across the
coil) will blow, unless the fuse manages to blow first.

Joe Gwinn

If one mis-orients this rectifier diode, it will be a dead short across
the power supply, and all five diodes (4 in the bridge, 1 across the
coil) will blow, unless the fuse manages to blow first.

Come on now, we all know that the third law of electronics, states that, the
$300.00 picture tube is supposed to blow first to protect the $0.20 fuse!

Peter

On Mon, 13 Mar 2006 21:59:09 -0500, Spehro Pefhany
wrote:

On Mon, 13 Mar 2006 20:00:58 -0500, the renowned Spehro Pefhany
wrote:

On Tue, 14 Mar 2006 00:31:26 GMT, the renowned "Jerry Foster"
wrote:

Surprised no one pointed this out. 110 VAC is rms voltage. The output of a
bridge rectifier/capacitor combination will be about that times the square
root of two, practically somewhere around 150 volts.

You probably need a variac in front of the rectifier if you really want 110
VDC.

Jerry

Act-ually, the first thing Gunner snip

Sorry, forget it, the bridge clamps it to two diode drops on top of
the input supply anyway. dunh

But there is a potential problem when the input connection is broken
with no capacitor on the input or output of the bridge.


Best regards,
Spehro Pefhany

With the coil fed by a bridge rectifier, when the AC input
is disconnected, the inductive overshoot drives all four diodes
into forward conduction and this safely dissipates the stored
energy in the resistance of the magnet coil. A protective
capacitor is not necessary on the input or output of the bridge.

With no output capacitor the DC output voltage will be the
MEAN value of the input voltage - 0.9 x RMS. less the diode
drops.

With a large output capacitor the output approaches the
PEAK value of the input voltage, again less the diode drops.

Jim

On Tue, 14 Mar 2006 08:50:37 -0500, Joseph Gwinn
wrote:

In article ,
Gunner wrote:

On Mon, 13 Mar 2006 20:00:58 -0500, Spehro Pefhany
wrote:

On Tue, 14 Mar 2006 00:31:26 GMT, the renowned "Jerry Foster"
wrote:

Surprised no one pointed this out. 110 VAC is rms voltage. The output of a
bridge rectifier/capacitor combination will be about that times the square
root of two, practically somewhere around 150 volts.

You probably need a variac in front of the rectifier if you really want 110
VDC.

Jerry

Act-ually, the first thing Gunner should try is really E-Z, and that's
to put an inverse parallel rectifier across the coil*
(REVERSE-biased). As it sits now, there's no path for the current when
the current drops below the peak, until the diodes avalanche, which
means a whole bunch of energy is likely going into the diodes and not
staying in the electromagnet.

* care to polarity or all 5 diodes will release their smoke


Best regards,
Spehro Pefhany


Blink blink....huh??

The problem is that the coil of the magnetic chuck stores energy in the
magnetic field. How much energy is stored per amp of coil current is
proportional to the inductance of the coil.

The problem is that when the coil current is abruptly interrupted, the
energy in this magnetic field comes back out, attempting to keep the
coil current flowing in the same direction as the original current
flowed. This is seen as a high-voltage spike, reaching to hundreds or
thousands of volts, and will puncture any attached semiconductors (like
the rectifier diodes).

The classic solution is to put a rectifier diode in parallel with the
coil, oriented so that under normal conditions no current flows through
the diode. When the coil current lead is opened, the magnetic field
energy goes into this rectifier diode, preventing the voltage spike.

If one mis-orients this rectifier diode, it will be a dead short across
the power supply, and all five diodes (4 in the bridge, 1 across the
coil) will blow, unless the fuse manages to blow first.

Joe Gwinn


Ah!! Ok. Makes perfect sense.

Gunner



"A prudent man foresees the difficulties ahead and prepares for them;
the simpleton goes blindly on and suffers the consequences."
- Proverbs 22:3

On Tue, 14 Mar 2006 08:50:37 -0500, Joseph Gwinn
wrote:

In article ,
Gunner wrote:

On Mon, 13 Mar 2006 20:00:58 -0500, Spehro Pefhany
wrote:

On Tue, 14 Mar 2006 00:31:26 GMT, the renowned "Jerry Foster"
wrote:

Surprised no one pointed this out. 110 VAC is rms voltage. The output of a
bridge rectifier/capacitor combination will be about that times the square
root of two, practically somewhere around 150 volts.

You probably need a variac in front of the rectifier if you really want 110
VDC.

Jerry

Act-ually, the first thing Gunner should try is really E-Z, and that's
to put an inverse parallel rectifier across the coil*
(REVERSE-biased). As it sits now, there's no path for the current when
the current drops below the peak, until the diodes avalanche, which
means a whole bunch of energy is likely going into the diodes and not
staying in the electromagnet.

* care to polarity or all 5 diodes will release their smoke


Best regards,
Spehro Pefhany


Blink blink....huh??

The problem is that the coil of the magnetic chuck stores energy in the
magnetic field. How much energy is stored per amp of coil current is
proportional to the inductance of the coil.

The problem is that when the coil current is abruptly interrupted, the
energy in this magnetic field comes back out, attempting to keep the
coil current flowing in the same direction as the original current
flowed. This is seen as a high-voltage spike, reaching to hundreds or
thousands of volts, and will puncture any attached semiconductors (like
the rectifier diodes).

The classic solution is to put a rectifier diode in parallel with the
coil, oriented so that under normal conditions no current flows through
the diode. When the coil current lead is opened, the magnetic field
energy goes into this rectifier diode, preventing the voltage spike.

If one mis-orients this rectifier diode, it will be a dead short across
the power supply, and all five diodes (4 in the bridge, 1 across the
coil) will blow, unless the fuse manages to blow first.

Joe Gwinn

Addendnum. Im using a 50 amp (or bigger) bridge rectifier out of a
Hardinge HC control box.

Gunner



"A prudent man foresees the difficulties ahead and prepares for them;
the simpleton goes blindly on and suffers the consequences."
- Proverbs 22:3

After reading your post, I started wondering about the mag on my Arter
Grinder. It has a clockwork rotary switch that one has to turn 270 degrees
then it locks on. To turn it off, one turns the switch a few degrees and it
unlocks and turns off slowly. I think it pulses the mag.

On Tue, 14 Mar 2006 16:00:28 +0000, the renowned
wrote:

On Mon, 13 Mar 2006 21:59:09 -0500, Spehro Pefhany
wrote:

On Mon, 13 Mar 2006 20:00:58 -0500, the renowned Spehro Pefhany
wrote:

On Tue, 14 Mar 2006 00:31:26 GMT, the renowned "Jerry Foster"
wrote:

Surprised no one pointed this out. 110 VAC is rms voltage. The output of a
bridge rectifier/capacitor combination will be about that times the square
root of two, practically somewhere around 150 volts.

You probably need a variac in front of the rectifier if you really want 110
VDC.

Jerry

Act-ually, the first thing Gunner snip

Sorry, forget it, the bridge clamps it to two diode drops on top of
the input supply anyway. dunh

But there is a potential problem when the input connection is broken
with no capacitor on the input or output of the bridge.


Best regards,
Spehro Pefhany

With the coil fed by a bridge rectifier, when the AC input
is disconnected, the inductive overshoot drives all four diodes
into forward conduction and this safely dissipates the stored
energy in the resistance of the magnet coil. A protective
capacitor is not necessary on the input or output of the bridge.

Yup, agreed. Two pairs of diodes in series/parallel across the coil.
8-(

With no output capacitor the DC output voltage will be the
MEAN value of the input voltage - 0.9 x RMS. less the diode
drops.

Yes, & the heating value will be the RMS input voltage (ignoring diode
drops), the valley magnetizing current will depend on the coil
inductance. If something held near (or with a piece at an angle to)
the chuck buzzes, the inductance is too low to be ignored.

What is normally used to power these things? Just a bridge or
something else? From using bridges to power DC relays and solenoids
from AC, I know the pull-in strength can be significantly compromised
by not filtering the supply. I recall this issue has come up before
here.

With a large output capacitor the output approaches the
PEAK value of the input voltage, again less the diode drops.

Jim

Specifically Vavg ~= sqrt(2)*Vin - (0.5* Iout*(1/120)/C + 1.2)

So with 660uF, 120V in, 1A out, about 162V. With 330uF, about 156V.

That will cause about 75% more temperature rise in the coil than with
unfiltered full-wave rectified power. If that's too much, a suitable
series resistor could be used, preferably between the bridge and the
capacitors (or a variac, of course).


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Tue, 14 Mar 2006 16:00:28 +0000, wrote:

On Mon, 13 Mar 2006 21:59:09 -0500, Spehro Pefhany
wrote:

On Mon, 13 Mar 2006 20:00:58 -0500, the renowned Spehro Pefhany
wrote:

On Tue, 14 Mar 2006 00:31:26 GMT, the renowned "Jerry Foster"
wrote:

Surprised no one pointed this out. 110 VAC is rms voltage. The output of a
bridge rectifier/capacitor combination will be about that times the square
root of two, practically somewhere around 150 volts.

You probably need a variac in front of the rectifier if you really want 110
VDC.

Jerry

Act-ually, the first thing Gunner snip

Sorry, forget it, the bridge clamps it to two diode drops on top of
the input supply anyway. dunh

But there is a potential problem when the input connection is broken
with no capacitor on the input or output of the bridge.


Best regards,
Spehro Pefhany

With the coil fed by a bridge rectifier, when the AC input
is disconnected, the inductive overshoot drives all four diodes
into forward conduction and this safely dissipates the stored
energy in the resistance of the magnet coil. A protective
capacitor is not necessary on the input or output of the bridge.

With no output capacitor the DC output voltage will be the
MEAN value of the input voltage - 0.9 x RMS. less the diode
drops.

With a large output capacitor the output approaches the
PEAK value of the input voltage, again less the diode drops.

Jim

Yup, the diodes will snub the coil on turnoff.

Most DC voltmeters do respond to average value. However, neglecting
diode drops, the RMS value of the rectified DC must be the same as
the RMS value of the AC by definition. The rectified voltage
waveform is the same as the AC wave except that alternate
half-cycles are reversed in polarity. RMS means root mean squared,
and the squaring operation makes polarity irrelevant. Therefore, the
RMS values (neglecting diode drops) are identical.

On Tue, 14 Mar 2006 08:50:37 -0500, Joseph Gwinn
wrote:

In article ,
Gunner wrote:

On Mon, 13 Mar 2006 20:00:58 -0500, Spehro Pefhany
wrote:

On Tue, 14 Mar 2006 00:31:26 GMT, the renowned "Jerry Foster"
wrote:

Surprised no one pointed this out. 110 VAC is rms voltage. The output of a
bridge rectifier/capacitor combination will be about that times the square
root of two, practically somewhere around 150 volts.

You probably need a variac in front of the rectifier if you really want 110
VDC.

Jerry

Act-ually, the first thing Gunner should try is really E-Z, and that's
to put an inverse parallel rectifier across the coil*
(REVERSE-biased). As it sits now, there's no path for the current when
the current drops below the peak, until the diodes avalanche, which
means a whole bunch of energy is likely going into the diodes and not
staying in the electromagnet.

* care to polarity or all 5 diodes will release their smoke


Best regards,
Spehro Pefhany


Blink blink....huh??

The problem is that the coil of the magnetic chuck stores energy in the
magnetic field. How much energy is stored per amp of coil current is
proportional to the inductance of the coil.

The problem is that when the coil current is abruptly interrupted, the
energy in this magnetic field comes back out, attempting to keep the
coil current flowing in the same direction as the original current
flowed. This is seen as a high-voltage spike, reaching to hundreds or
thousands of volts, and will puncture any attached semiconductors (like
the rectifier diodes).

The classic solution is to put a rectifier diode in parallel with the
coil, oriented so that under normal conditions no current flows through
the diode. When the coil current lead is opened, the magnetic field
energy goes into this rectifier diode, preventing the voltage spike.

If one mis-orients this rectifier diode, it will be a dead short across
the power supply, and all five diodes (4 in the bridge, 1 across the
coil) will blow, unless the fuse manages to blow first.

Joe Gwinn

Draw the circuit, Joe. There is a path for the inductive current thru
the diode bridge. There won't be any spike on turnoff.

Gunner wrote:

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.

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?

Now if I added a simple capacitor to the output, it should give me a
longer signal time as it fills in the gaps. 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

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.

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?

Thanks

Gunner, hanging his head in shame...


"A prudent man foresees the difficulties ahead and prepares for them;
the simpleton goes blindly on and suffers the consequences."
- Proverbs 22:3




If you dont have a transformer for isolation I would check for a short
to ground in the magnet. As far as needing a cap or ballast resistor,
it is not necessary. Most of the magnetic chucks Ive repaired dont have
any. Your diode bridge may be the problem. I would never reuse a bridge
diode unless you have a very good method of testing it.

This method wil work.

Put a light bulb across it and see if you get full brillance on the
bulb. Then put a diode in series with the light bulb and see if you get
half brillance. Reverse the diode and do the same thing. This will
check the complete bridge diode. Many diodes get "weak" they get an
abnormal voltage drop across them under any load but test good with a
diode tester. I've changed more of them than I can remember that had
abnormal voltage drops. In avionics the KN-65 DME was notorious for
bad diode bridges.

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.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On Tue, 14 Mar 2006 11:35:08 -0600, Don Foreman
wrote:

On Tue, 14 Mar 2006 16:00:28 +0000, wrote:

With the coil fed by a bridge rectifier, when the AC input
is disconnected, the inductive overshoot drives all four diodes
into forward conduction and this safely dissipates the stored
energy in the resistance of the magnet coil. A protective
capacitor is not necessary on the input or output of the bridge.

With no output capacitor the DC output voltage will be the
MEAN value of the input voltage - 0.9 x RMS. less the diode
drops.

With a large output capacitor the output approaches the
PEAK value of the input voltage, again less the diode drops.

Jim



Yup, the diodes will snub the coil on turnoff.

Most DC voltmeters do respond to average value. However, neglecting
diode drops, the RMS value of the rectified DC must be the same as
the RMS value of the AC by definition. The rectified voltage
waveform is the same as the AC wave except that alternate
half-cycles are reversed in polarity. RMS means root mean squared,
and the squaring operation makes polarity irrelevant. Therefore, the
RMS values (neglecting diode drops) are identical.

Just a bit mo-

With no output capacitor the DC output voltage will be the
MEAN value of the input voltage - 0.9 x RMS. less the diode
drops.

The RMS value of the output voltage is equal to the RMS value
of the input voltage (less diode drops)

However it is the MEAN value of the applied voltage that
determines the DC component of the output current and it is the
value of this current that determines the current drawn from the
supply and the power dissipated in the magnet.

If,as is usual with this type of electromagnet,the
inductance is large, the magnet current with be almost pure DC
with only a small ripple component. This means that (neglecting
the effect of the residual ripple component) the waveform of the
current drawn from the supply will be a square wave of amplitude
equal to the DC output current.

Jim

On Tue, 14 Mar 2006 11:38:54 -0600, Don Foreman
wrote:

Draw the circuit, Joe. There is a path for the inductive current thru
the diode bridge. There won't be any spike on turnoff.

Sorry, but there is. The coil is running on rectified DC, even if
it isn't filtered. And when you cut the power and the magnetic field
collapses there is a Counter-EMF spike coming out of the coil in
reverse polarity - a big pulse of negative energy coming out the
positive connection.

It can't dissipate in the incoming diode bridge - the CEMF pulse is
going the wrong way to go back through the bridge. If you don't
provide a spark gap, diode, varistor or neon lamp to dissipate it, you
have a lot of volts there.

Been there, bitten by that, sometimes hundreds of times in a day.

(Strapping the DTA on a Step phone switch, hot moves and changes on
the mainframe, etc. You learn not to jump.)

-- Bruce --

--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.

On Wed, 15 Mar 2006 19:15:02 GMT, Bruce L. Bergman
wrote:

On Tue, 14 Mar 2006 11:38:54 -0600, Don Foreman
wrote:

Draw the circuit, Joe. There is a path for the inductive current thru
the diode bridge. There won't be any spike on turnoff.

Sorry, but there is. The coil is running on rectified DC, even if
it isn't filtered. And when you cut the power and the magnetic field
collapses there is a Counter-EMF spike coming out of the coil in
reverse polarity - a big pulse of negative energy coming out the
positive connection.

It can't dissipate in the incoming diode bridge - the CEMF pulse is
going the wrong way to go back through the bridge. If you don't
provide a spark gap, diode, varistor or neon lamp to dissipate it, you
have a lot of volts there.

The collapsing field will try to maintain current flowing in the same
direction that it was, which was and will be thru the diodes. When
mains power is cut, the polarity of the voltage on the coil will
reverse. The polarity of this voltage induced by collapsing field
is correct to make the diodes conduct. If you draw the circuit, you
will see that there are two paths thru the bridge for this current to
flow if the line side of the bridge is disconnected.

If you still disagree, before posting further argument please try this
on the bench with an inductive load, a bridge rectifier and a scope to
observe what happens.

Been there, bitten by that, sometimes hundreds of times in a day.

If you open the circuit between the bridge and the coil, yes.

In article , Don Foreman says...

If you open the circuit between the bridge and the coil, yes.

The real worry is that the first positive-going spike will exceed
the prv ratings for the bridge. Then you do get smoke.

Hardinge found out about this on their potted feed motor drive
units, they were smoking quite regularly. The newer versions
have MOV snubbers on any inductive thing that gets switched, and
I suspect they boosted up the ratings on their active devices as
well.

Jim


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