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# MOV for PLC

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#### Kal_B

##### Member
Hello everyone,

I calculated and MOV for this PLC ,which I will supply with 208VAC and it produces 24VDC output which I will use for sensors inputs.

Voltage:
Maximum allowed voltage 264VAC / 0.707 = 373VAC (Rounded)

Current:
In the data sheet there's Current consumption, max. 150 mA at 240 V AC and in the manual there's
Input current (max. load) 40 mA at 240 V AC and Output current rating (max.) 300 mA (short-circuit protected).

Which I find confusing. If the CPU consumes 40mA at VAC and the maximum output current rating is 300 mA then I should rate my MOV at twice the total of 300+40 = 680 mA.
or does the Current Consumption, max of 150 mA means that the CPU may at times need 150 mA and that I should calculate my rating at twice 150 + 300 = 900 mA?
Either way I couldn't find MOV with that little current rating but I would like to know what these ratings mean,

Thanks

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• PDFsam_s71200_system_manual_en-US_en-US.pdf
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A MOV rating is not really related to the load, it's to suit the likely spike energy capability of the supply at that point.

Think of it more a "safety valve" fitted across the supply to release (or actually absorb) voltage spikes on the supply and prevent them damaging connected equipment.

The size depends on how big a spike could reach that part of the electrical supply system.

If it's working through normal size wiring for feeding such as a PLC and with proper fusing, I'd use something like this, rated at 60 joules.

That can stand 50 amps for a fraction of a second to dissipate a spike, which is likely more than the PLC supply wiring can provide. It should be effective.

If I was trying to protect something on a 100A supply, then I'd use a MUCH larger one..
(We've actually fitted lighting arrestor grade units [Furse] to large three phase distribution boards in some factories, to prevent arcing on overhead crane tracks causing damage or erratic operation with electronic control systems on some machines).

I've just been working on a project with a S7 1212, I'm fairly sure the power supply in the Plc has a Mov.
Is there a special reason why you were thinking of an extra one?

I've just been working on a project with a S7 1212, I'm fairly sure the power supply in the Plc has a Mov.
Is there a special reason why you were thinking of an extra one?

Thanks dr pepper. I'm not aware of having MOV protection for input supply. I will do my homework and see if I can find out.

Thanks

A MOV rating is not really related to the load, it's to suit the likely spike energy capability of the supply at that point.

Think of it more a "safety valve" fitted across the supply to release (or actually absorb) voltage spikes on the supply and prevent them damaging connected equipment.

The size depends on how big a spike could reach that part of the electrical supply system.

If it's working through normal size wiring for feeding such as a PLC and with proper fusing, I'd use something like this, rated at 60 joules.

That can stand 50 amps for a fraction of a second to dissipate a spike, which is likely more than the PLC supply wiring can provide. It should be effective.

If I was trying to protect something on a 100A supply, then I'd use a MUCH larger one..
(We've actually fitted lighting arrestor grade units [Furse] to large three phase distribution boards in some factories, to prevent arcing on overhead crane tracks causing damage or erratic operation with electronic control systems on some machines).

Thanks rjenkinsgb.
I followed the example in the bottom of this page which included the following quote:
The current rating of the MOV could be twice that of the SMPS rating, meaning if the SMPS wattage is rated at 24 watts at the secondary, then the primary could be calculated as 24/285 = 0.084 amps, therefore the MOV current could be anywhere above 0.084 x 2 = 0.168 amps or 200mA.

'And what you said explains why I could not find MOV with that little current rating.

I followed the example in the bottom of this page which included the following quote:

To be blunt, the MOV selection info in that page is total nonsense!

One thing to bear in mind with the industrial scene, and I assume this is the case with something like a Plc, Ze tends to be low, esp for a machine that uses lots of power, this can mean that a transient might have a fair bit of energy behind it.
If a machine has issues with its power to the point where the electronics are affected, then a power line conditioner is often used.

Oops I meant Zs not Ze, but you get the picture.

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