Diode on a solenoid

Clay B.

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Jun 2005
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Concord,NC
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Does anybody know why you would have a diode on a 24 vdc solenoid coil.

I am installing some sensors and a new DL-6 PLC on an existing peice of equipment and this diode blasted a couple of outputs before I found it. I reversed polarity going to these solenoids when I installed the new PLC (Old PLC sinking , new PLC sourcing outputs)and thats when the trouble started. Basically I shunted all of my current across this diode which in turn shorted out the output so when I turned this output off I still did not have any current. Once I saw the diode I realized what was going on. None of the solenoids use an indicating LED either.None of the other solenoids on the machine have a diode built into the plug.

It has been awhile since I sat thru an electronics class and I can not think of one good reason for this being here.


Thanks for any info or ideas

Clay
 
quick question: how many volts in your car battery? ...



quick answer: about 12 ...



quick question: how many volts to make the spark jump across the gap in your spark plug?



quick answer: at LEAST 20,000 volts ... more is better ...



quick question: where does that high voltage come from? ...



quick answer: from the ignition coil ... the DC voltage from the battery builds up a magnetic field around the coil ... when the “points” open, the field quickly collapses ... the field “cuts” through the conductors of the coil ... and the high voltage is generated as a result ...



now to your solenoid ... when the PLC energizes the DC coil, it’s basically building up a magnetic flux field around the coil ... and when the PLC “opens” the circuit, an extremely high voltage can result ... just like in the ignition coil circuit of your car ...



so the diode is usually put there to provide a “short circuit” path for the high voltage ... of course it needs to be installed in the CORRECT direction - so that the original “control” current is NOT shorted - but the resultant “collapsing” current IS shorted ... if you’ve changed the positive/negative polarity of the coil, then the diode probably needs to be reoriented too ...
 
The diode is there in order to provide a safe path to shunt the reverse current that occurs when the coil is de-energized and it's magnetic field collapses. Without some sort of reverse path, the collapsing field can generate extremely high voltages in an attempt to dissipate.

It's cheap protection for the solid state output's of your module.
 
my typing is getting better all the time ...

race.JPG


but it looks like rdrast did better with the actual "nutshell" answer ... as he said:

It's cheap protection for the solid state output's of your module.
 
Last edited:
then again a fuse is also "chaep" but could have save thos outputs.
ron & rdrast, you guys type with the speed of light...lol.
 
The technical term for the diode is flyback diode or freewheeling diode. It gives the colasping magnetic field in DC coils a rapid discharge path to both clamp the possible high voltage generated and to allow the relay/solenoid/contactor to open faster.

You can use 1N4007 diodes in most applications. Put the cathode to the positive power connection. When you reversed the PLC outputs is when you got into the problem area.

A lot of older equipment had 15 or 20 amp diodes across bigger coils and they will wreck havoc on today's puny amp PLC outputs when connected backwards.
 
Leadfoot said:
The technical term for the diode is flyback diode or freewheeling diode. It gives the colasping magnetic field in DC coils a rapid discharge path to both clamp the possible high voltage generated and to allow the relay/solenoid/contactor to open faster.

You can use 1N4007 diodes in most applications. Put the cathode to the positive power connection. When you reversed the PLC outputs is when you got into the problem area.

A lot of older equipment had 15 or 20 amp diodes across bigger coils and they will wreck havoc on today's puny amp PLC outputs when connected backwards.

yea Havoc is the right word. Imagine my suprise when my DC power just dropped off from the rest of my circuit.
 
1N4937 is Better

Leadfoot said:
You can use 1N4007 diodes in most applications.
For the most part, a 1N400x series will work fine. A diode takes time to switch. The1N400x series is fairly quick, but I prefer faster switching. Use 1N4937 instead.
 
MOV or TVS are not polarized and so are perfect for those who mix sink and source outputs....

1N400x are quite old but who can complain when they can be bought as cheap next door any time you need them...
 
Leadfoot said:
The technical term for the diode is flyback diode or freewheeling diode. It gives the colasping magnetic field in DC coils a rapid discharge path to both clamp the possible high voltage generated and to allow the relay/solenoid/contactor to open faster.
The free-wheeling diode will delay the release of the solenoid armature since it serves to maintain current through the coil as the magnetic field decays.
 

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