Surge Suppresors

A question came to my mind, and i would like to share it with you guys..

When To use surge suppresors with PLC outputs? with what kind of output? with what kind of actuators? what is its effect on the cost the cost of PLC systems? what is the ......

Surge suppresors

I assume you mean when driving a coil to a relay. Any inductive load will cause back EMF when the coil is de-energized. A 24VDC output to a 1000 ohm coil will generate probably 50 to 250 VDC NEGATIVE voltage back to the output. FET drivers will self destruct.

Then yes, it is recommended. Do a search here for 'free wheeling diodes', 'surge suppressor', ETC

Wait for RSDORAN to give you the REAL info.

Rod (The CNC dude)

Surge Supressors

Shady
Q:What is the surge Supressors?
A:Surge Supressors, dampen unwanted electrical impulses that can damage sensitive digital circuits. However surge supressors cannot affect or modify long term (longer than a minute fraction of a second) voltage abnormalities.

Q:What is the most widley used Surge Supressors?
A:Metal Oxiade Variastor(MOV)is the widley used surge suppressors which is commonly used in protecting the electrical devices from voltage Spikes.Usually connected in the input stage of any device.it should be selected according to the input voltage level.

Q:What is the theory of operation of MOV?
A:A varistor is a type of resistor with a significantly non-ohmic current-voltage characteristic. The name is a drived from variable resistor, which is misleading it is a voltage dependent,

Q:When To use surge suppresors with PLC outputs?
A:Use surge supressors with PLC output when you want to protect you
PLC output from voltage spikes which may destroy the output during
on/off.

Q:what kind of output?
A:1-For triac outputs which drives 100/200 volt loads use MOV's
2-For Transistor outputs driving inductive loads use free
wheeling diodes.a lot of PLC manfactures include this diode in
thier transistor outout modules.
3-No need for surge supressors with relay output.

Q:what is its effect on the cost the cost of PLC systems?
A;Nothing.

I hope that this answers will help .
BR

<i>3-No need for surge supressors with relay output.</i><P><P>Why?

No need for surge supressors with relay output

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This is a very interesting point and one I have strong ideas about.

Most people don't seem to bother and get away with it most of the time. But the fact is that if a relay is being used to switch and inductive load the Back-EMF energy has to go somewhere. Usually it finishes up:

1. Creating a spike of electromagnetic interference that has the potential to disrupt the PLC or other electronic gear nearby. And bear in mind that sure it may not cause a problem today, but you MAY have stored one up for next year when some totally unanticipated new installation or modification occurs.

2. What energy isn't radiated electromagnatically goes into creating a small arc at the relay contact. Again most of the time you get away with it, but you HAVE reduced the life of the relay. Maybe now instead of millions of cycles it is only good for thousands. But hey most times that is a few years off and how much do most folks think that far into the future?

Clearification

No need for surge supressors with relay output.
Usually i use interface relays with my PLC output even it's a relay output.This is why i mentioned that no need for surge suppressors with relay output module.The interface relay coil (24V Dc) doesn't make a big threat on the plc output relay contact.
BR

Click to expand...

So I see you are not driving a load with the relay output, but another relay to handle the load. This 2nd relay may have the life of it's contacts extended with a surge supressor.

Experience has shown me that the AB 700-HLT1L1 despite it's ratings, needs all the help it can get.



Some issuues I have had with Outputs, Surges, and Relays.

Mov on contact outputs.

If the output is driving any type of solinoid, or contactor, we use MOVs on that output to protect the output contacts from arching. Smaller inductive loads such as KH or KUP style relays dont get the MOV..generally. Resistive loads..never.

What About Free Wheeling diods, can they do the job of MOVs with contactors and soloniod vaves??

Yes freewheeling diodes will do more or less the same thing as MOV's, but they also tend to extend the dropout time of the solenoid. This is because the backemf is is shorted by the diode and the resulting current circulates through the solenoid for some time after the main supply has been removed. In most applications the amount of time involved is small, but it pays to remember this effect.

Also diodes only conduct in one direction, so although they will dump/clamp most of the coil energy in the first halfcycle that they conduct, any subsequent halfcycles with the opposite polarity are not clamped. By contrast MOV's are bidirectional and acutally dissipate the most of the coil energy within the MOV, not simply recirculate it back through the solenoid coil.

Diodes are cheaper, MOV's do a better job; but for most jobs the diode is ok.

3-No need for surge supressors with relay output.

It depends; I did a 4-axis motion system that was exhibiting spurious behavior. What we found was a PLC relay output (24 vdc) controlling a 120 vac signal via an interposing relay. With an o-scope, we discovered that the 24 vdc relay coil would put 70 volts peak-to-peak noise spike superimposed on the 24 vdc control circuit for our motion controller, which would cause all four axes to enable (not good). Placing surege suppression on the interposing relay coil cured the problem.

mgvol:

Thanks for the above posting. It exactly illustrates the kind of unexpected thing that happens when people take short cuts with surge suppression