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Posted by Bushy Pete on March 14, 2006, 9:45 am
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>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
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Posted by Gunner on March 14, 2006, 11:09 am
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On Tue, 14 Mar 2006 08:50:37 -0500, Joseph Gwinn
>
>> On Mon, 13 Mar 2006 20:00:58 -0500, Spehro Pefhany
>>
>> >On Tue, 14 Mar 2006 00:31:26 GMT, the renowned "Jerry Foster"
>> >>
>> >>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
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Posted by Gunner on March 14, 2006, 11:11 am
Please log in for more thread options On Tue, 14 Mar 2006 08:50:37 -0500, Joseph Gwinn
>
>> On Mon, 13 Mar 2006 20:00:58 -0500, Spehro Pefhany
>>
>> >On Tue, 14 Mar 2006 00:31:26 GMT, the renowned "Jerry Foster"
>> >>
>> >>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
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Posted by Don Foreman on March 14, 2006, 12:38 pm
Please log in for more thread options On Tue, 14 Mar 2006 08:50:37 -0500, Joseph Gwinn
>
>> On Mon, 13 Mar 2006 20:00:58 -0500, Spehro Pefhany
>>
>> >On Tue, 14 Mar 2006 00:31:26 GMT, the renowned "Jerry Foster"
>> >>
>> >>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.
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Posted by Bruce L. Bergman on March 15, 2006, 2:15 pm
Please log in for more thread options On Tue, 14 Mar 2006 11:38:54 -0600, Don Foreman
>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.
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