protecting output PLC

smsebnaa · Aug 29, 2015

I want to protect digital output of plc from reverse current. my plc controls motors with AC coil contactors.
i use selector switch to select manual or automatic mode. when transition between automatic to manual, may be reverse current is occurred.
i should use AC diode?

shooter · Aug 29, 2015

what is the type of the outputs?
if relais a snubber will do the job.
if transistor a simple diode over the coil should be the solution.

smsebnaa · Aug 29, 2015

i use relay output contact.
when i change selector switch from automatic to manual, if plc contact is close yet, so the plc contact maybe injured in manual mode. i should use snubber?

Mickey · Aug 29, 2015

If controlling an inductive load surge protection is recommended, see pdf's

smsebnaa · Aug 29, 2015

surge suppression is used for high current, but my current is about mA only. is there any protection for digital output, when another current path comes to output if plc do not work rapidly (plc do not close contact at a moment)?

Mickey · Aug 29, 2015

surge suppression is used for high current, but my current is about mA only

It does not matter, its more about the voltage spike created by the collapse of the inductive load. This spike can cause arcing across relay contacts and may be felt by sensitive electronics powered by the same power source.

Use surge protection for inductive loads. Cheap insurance.

Bit_Bucket_07 · Aug 29, 2015

smsebnaa said:
I want to protect digital output of plc from reverse current. my plc controls motors with AC coil contactors.
i use selector switch to select manual or automatic mode. when transition between automatic to manual, may be reverse current is occurred.
i should use AC diode?

It sounds to me like you are concerned that your selector switch could switch a different power source to the output via the motor starter coil wire. That is a different issue from mere surge suppression. You must wire the selector switch so that the PLC output is isolated from the motor starter coil when you switch to manual mode if you want to assure that this does not happen.

Mickey · Aug 29, 2015

I was thinking that he thinks the load is going to momentary turn off when he makes the change.

Bit_Bucket_07 · Aug 29, 2015

Mickey said:
I was thinking that he thinks the load is going to momentary turn off when he makes the change.

Well, the load will switch on and off during normal operation without a selector switch. That is a surge suppression issue. However, if the selector switch circuit that he plans to use connects a different power source to the output, then that's a wiring issue that needs to be resolved. Surge suppression alone wouldn't correct that problem.

Bit_Bucket_07 · Aug 29, 2015

smsebnaa said:
surge suppression is used for high current, but my current is about mA only. is there any protection for digital output, when another current path comes to output if plc do not work rapidly (plc do not close contact at a moment)?

That's an incomplete schematic.

Mickey · Aug 29, 2015

Bit_Bucket_07 said:
However, if the selector switch circuit that he plans to use connects a different power source to the output, then that's a wiring issue that needs to be resolved. Surge suppression alone wouldn't correct that problem.

I guess that's possible, but I am not reading that into his post. I just think he is showing the same switch in 2 different states.

I guess we will have to wait for the OP to come back.

smsebnaa · Aug 30, 2015

i edited and modified my last image. please see my new attachment.
my problem is stage 2. when transition occurred from auto to manual mode and plc do not act rapidly.

Mickey · Aug 30, 2015

smsebnaa said:
i edited and modified my last image. please see my new attachment.
my problem is stage 2. when transition occurred from auto to manual mode and plc do not act rapidly.

Sorry smsebnaa, the doc you posted only has three blank columns with a title above each. At least thats all I see

plc do not act rapidly

Can you give a little more detail?

ivo.maenen · Aug 30, 2015

Mickey said:
Sorry smsebnaa, the doc you posted only has three blank columns with a title above each. At least thats all I see

Can you give a little more detail?

see pdf

Mickey · Aug 30, 2015

Thanks ivo.maenen.

Maybe some sort of debounce logic would help.If I understand your problem correctly. See curlyandshemp's post at link below.

https://www.google.com/url?sa=t&rct...6oQeNBybaWNEPrNXw&sig2=vA0PUkWhTqtQSZssgA-R1g

protecting output PLC

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