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I always feel safer using a reverse biased diode to protect against the collapse of any magnetic field from a coil. The reverse polarity can be ramped up considerably. The floating pulse can cause problems. (this is on DC, with AC you might entertain a snubber circuit).

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Joe what is your opinion of using MOV's insead of diodes across the inductive coils?

Hi

 

Just one thought on this.

A diode will normally fail open circuit where a MOV will normally short circuit when it fails.

This would cause a problem with your power supply.

I use diodes on all coils now after having problems with interference (thanks again to Joe and Keith) for great advice at the time.

 

Regards

Denis

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I'm not Joe, but my thoughts are as follows:

 

The diode solves the problem by conducting normally, via its forward path. 

 

The MOV solves the problem by conducting "abnormally" via its breakdown path.  A MOV can only sustain a finite number of breakdown events before failing.

 

To me the diode is an inherently more robust method for protecting against this type of spike that you know will keep occurring. The diode is more of a preventative measure rather than a protective measure.  A MOV is more typically used on applications where you can't prevent the spike from occuring, but you must limit the damage it causes.

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Don't use an MOV to absorb the flyback pulse.  An MOV is designed to provide a temporary short circuit when a certain voltage is reached; a tiny conductive hole literally forms in the MOV.  When the high voltage goes away the hole "heals" but that area will not conduct again.  Eventually you run out of area to make holes in and the MOV no longer works.  An MOV also has a Joule rating, which is the maximum amount of energy contained in the spike it can dissipate; if you exceed that they explode.

 

I don't think you'll be be blowing up any MOV's in your application but you'll find they'll work for about three months and then gremlins like the PLC watchdog timer causing a fault will start occurring.

 

Use a reverse-biased diode for DC applications.  Use an RC snubber network for AC applications.  Both methods are designs for a small spike that will ALWAYS occur, not the powerline transient that will rarely occur.

 

Joe T.

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I am looking for an opinion.

 

My panel assembly shop has 5 times now managed to install a 1N4007 diode backwards and take out the transistor output of different controllers.  I have found some bi-directional Zener diodes that I believe while not as quick as a 1N4007 (and at a slightly raised voltage) will do Ok in the job and take away my shops ability to damage outputs by incorrect installation (all outputs are 24 VDC).

 

The part number is SA28CACT-ND and is advertised as a transient volatage supressor.

 

http://www.littelfuse.com/~/media/electronics/datasheets/tvs_diodes/littelfuse_tvs_diode_sa_datasheet.pdf.pdf

 

is the data page for the device.

 

Let the opinions flow :-)

 

 

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Keith -

 

The back-to-back zener you called out is what we use in our shop for that very reason.  I use a thirty volt version.  I always talk about using diodes on the forum because they're more readily available and their usage for this purpose is well documented on the Internet.

 

I did a little digging on this part number and it's specific to Digikey.  If you take off the CT-ND and go with the straight Littelfuse part number it becomes harder to find.   It crosses to an P6KE33CA which is more readily available.  Digikey has them for $0.49 each, unless you want to buy a reel of 4,000.  Then they're much cheaper.

 

I've often threatened to buy a foam bat to hit the person who doesn't pay attention to detail when assembling a panel and blows something up.  Too bad it's illegal to charge them for the IO module.....

 

Joe T.

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Would The good folks at Unitronics consider it plausible to make an optional terminal block for the outputs that incorporates a 30V TVS or something?

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I have always worked on the principle that it is much better to clamp as close as possible to the source of the issue, like Kratmel's photos. 

So the question arises, how much better is this than doing it at the outputs?  Has anyone actually done a careful analysis of different distances and effectiveness?

cheers, Aus

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21 minutes ago, Ausman said:

I have always worked on the principle that it is much better to clamp as close as possible to the source of the issue, like Kratmel's photos. 

+1

I have looked at this pulse on a scope.  For a 24V DC control signal, it varies between 200 and 700 volts in amplitude (depending on the size of the coil)  and is 1-5 microseconds wide (that's 200 KHz to 1 MHz). 

If you clamp the flyback pulse all the way back at the output module, then all the wiring between the device and the module becomes an antenna, transmitting this garbage into your other wiring.

Joe T.

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