protecting plc outputs

dougl

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• Oct 20, 2003

• #1

I'm looking for recommendations of components to protect relay outputs from inductive kicks. What info is needed to size diodes, etc.

 

catserveng

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• Oct 20, 2003

• #2

DC outputs can use diodes, I usually size to device coil draw, you can also use a zenier diode, clamp at level 50% above operating voltage.

Ac outputs need an RC device, Allen Bradley makes them for their contactors, as do other manufacturers, have also found RC devices for relay protection from Newark Electronics.

 

Eric Nelson

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• Oct 20, 2003

• #3

Have a look at THIS THREAD

beerchug

-Eric

 

LadderLogic

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• Oct 21, 2003

• #4

A lot of newer relays and solenoid valves come with either the diodes (for DC) or the snubbers (for AC) built in. You may just as well consider using this kind of devices only (ask your vendors around, if not sure).
One thing, any suppressing device is effective the most when wired immediately across the coil. The other - in case of a DC coil you would avoid a lot of trouble that happens if someone wires a diode incorrectly...

 

Prince

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• Oct 21, 2003

• #5

Added to this I normally use a propper fuse when I am working with valve coils

 

Eric Nelson

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• Oct 21, 2003

• #6

Just Wondering...

I put diodes across my DC relay coils, but since I use relays with LED indicators on them, is a separate diode really necessary? A LED is, after all, a light emitting DIODE, and it IS connected across the coil, so does that count for anything?...

Of course, someone can always replace the relay with a non-indicating type, so I'll still add the diodes regardless. The resistor in series with the LED probably reduces/removes it's clamping effect anyway. Just wondering...

beerchug

-Eric

 

Prince

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• Oct 21, 2003

• #7

Sorry Eric

But can a LED be fast enough for the operation ?
I am not sure ....

In most of the devicecs I have seen they have used a fast diod even if they had an LED mounted for signal condition monitoring ...

 

Vic

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• Oct 21, 2003

• #8

LED vs Diode

Eric

The LED will not replace the diode since they have to be wired the opposite polarity. The LED is forward biased when the relay is energized and the free wheeling diode must be installed so it is reverse biased when the relay is energized.

I always buy DC relays with both a LED and diode built in. Omron relays are available with these options. Just be sure to wire the power to the socket with the correct polarity.

 

Eric Nelson

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• Oct 21, 2003

• #9

Re: LED vs Diode

Vic said:

The LED will not replace the diode since they have to be wired the opposite polarity. The LED is forward biased when the relay is energized and the free wheeling diode must be installed so it is reverse biased when the relay is energized.

Click to expand...

Well, yeah, duh, that's why they light up Eric...

What was I thinking?...

Thanks for clearing that up Vic!...

beerchug

-Eric

 

PhilipW

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• Oct 21, 2003

• #10

Just as food for a little thought...diodes do the job just fine, but they really aren't DESIGNED as surge absorbing devices.

I prefer to use MOV's (Metal Oxide Varistors) that are intended as energy absorbing devices. In fact they are rated by Breakdown Voltage, and Energy Absorbtion.

Also their breakdown is VERY fast and bi-directional which results in somewhat better elimination of the back-EMF pulse. Whats more they are polarity insensitive, so no problemo wiring the little suckers in backwards.

On the other hand they cost a little more than diodes..oh well.

 

tlvaun

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• Oct 21, 2003

• #11

I was just wondering how you guys size your diodes. I put a oscilloscope on a SMC NVFS2000 series solenoid without surge protection and would get spikes around -150V from the best I could tell so I've been using 1N4003 (200V) diodes for them.
I'm not familiar with using RC filters or snubbers on ac coils. If someone could give a little more information on these it would be appreciated.