Inductive loads

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Thread Starter

Patrick Levinson

Hello, could somebody out there refresh my memory (or show me where to refresh it via web) about protections against inductive loads in PLC digital output modules?
I recall the diode for relay outputs to 24 VDC
solenoids, but don't remember/find recommendations for protection of relay outputs and/or triac outputs in 110 solenoids... I seem to remember MOVs for triacs?
for relays? Thanks in advance...Pat
 
When an ac coil breaks down ie. when the output turns off the coil kicks back a voltage spike (Inductive Kickback). This can kill relay outputs and triac outputs. Most people us a MOV(Metal Oxide Varistor) across the output. You can get them off the shelf from AB. Put the MOV as close to the load as possible.

gmanout
 
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Dean Kindrai

Check any of the major PLC manufacturer's websites. They'll give you recommended circuit diagrams...
 
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MOV's work well, I think. I also see people using snubbers with RC networks. I do not know which is best to apply to a given situation.

I'm sure someone can fill us in.

Bill Sturm
 
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Al Pawlowski

If I am not mistaken, Metal Oxide Varistors change characteristics over repeated operation and are therefore not a good choice for ac snubbing. They are better suited for short-term, gross overvoltage protection. I would recommend, and use myself, RC (resistor & capacitor in series) snubbers. They are available in two lead packages expressly for relay contact protection.
 
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Dan Lewandowski

I recommend automationdirect.com, their manuals are online and excellent, actually enjoyable to read.

> Check any of the major PLC manufacturer's websites. They'll give you recommended circuit diagrams...
 
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Hakan Ozevin

Always use RC+varistors instead of RC or varistor only. It will cost almost nothing extra and will prevent lots of problems.

++++++++++++
Hakan Ozevin
 
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