PLC Sinking Output Wiring

Thread Starter

pandian

Joined Sep 27, 2009
33
Hello all,

I'm doing wiring on a PLC sinking output module and I have concern on that which needs clarification.
Thanks in advance for the help.

The Set-up:

Connect a buzzer (24VDC) to a Sinking Output module. eg.: Omron C200H-OD219, rated 24VDc, 0.5A. Buzzer + terminal to 24V supply. Buzzer - terminal to PLC output point.

My concern:

If any of the buzzer terminal shorted to chassis, infinite current in the loop might cause damage.

My question:

Does the output module have overcurrent protection?
If no, will it damage the whole output module or just the point?
Is it necessary to install an isolation relay (even the buzzer consumes only 20mA)?
 

GetDeviceInfo

Joined Jun 7, 2009
1,737
I couldn't find any info on your device, so I can only speak generally.

Because of potential for problems, you would only sink indicators, and source actuators. You seem to be Ok there.

typically, you would fuse your supply for that branch, more for the branch protection than PLC protection.

Depends on how extreme the fault to the possibilities to which the depth of injury to the input, rack, or whole enclosure.

If your sunk current is within the rating of the port, you should be fine without isolation. The input itself will be isolated from the rack.
 

GetDeviceInfo

Joined Jun 7, 2009
1,737
output devices
What type of devices are we talking?

Safety is your concern. Say a device in the field is powered, and actuated by sinking. The line becomes shorted and unintentional actuation occurs. If the device was sourced, and the line shorted, it would not actuate.

Indicators are not that critical, and are often sunk in immediate panels, but usually not out to the field.

The convention is 'safety first'.

I'll just throw in that different industries have different governing agencies, all adhering to different standards. Within those standards one can find directives in applying to any/all aspects of physical and process. I worked in the printing industry for awhile, and the further we investigated, the more rules we found.
 
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Thread Starter

pandian

Joined Sep 27, 2009
33
My device is a 24VDC operated buzzer (as indicator in this case), to be connected to PLC, CS1G-CPU44H, Output module, C200H-OD219 (a Sinking transistor type).

In my case,
if the negative terminal of buzzer shorted to ground, it will be activated thus buzzer will sound. The current in the loop will be whatever drawn by the buzzer, in our case its 20mA.

if positive is shorted to ground, then the loop current will be infinity right? So it will damage the output point or the module itself. So is there any common practice to prevent this?
 

GetDeviceInfo

Joined Jun 7, 2009
1,737
if the negative terminal of buzzer shorted to ground, it will be activated thus buzzer will sound.
Your sinking output will switch this line to common. The buzzer positive is connected to 24vdc source, and will limit the current.
 

thatoneguy

Joined Feb 19, 2009
6,359
A piezo buzzer "looks" like a capacitor and oscillator to a DC circuit, essentially a very small load.

You should be OK designating it as an indicator.
 

BillB3857

Joined Feb 28, 2009
2,536
If it is a "coil and contact" buzzer, you may want to put diode protection on your PLC output if it doesn't already have it..
 

Jaguarjoe

Joined Apr 7, 2010
767
If the (-) terminal of the buzzer shorts to ground it would be shorting the PLC's sinking pin to ground. It would not harm the module and the buzzer would sound.
If the (+) terminal of the buzzer shorted to ground it would be shorting the (+) terminal of the power supply to ground. Smoke would escape from the power supply but the sinking pin on the PLC would not be affected and the buzzer would not sound.
 

Thread Starter

pandian

Joined Sep 27, 2009
33
Your sinking output will switch this line to common. The buzzer positive is connected to 24vdc source, and will limit the current.
How if the positive and negative terminals of buzzer (or any device) is shorted (not to ground), which will feed infinite current to the loop (from 24V power supply to 0V through output device's particular point? It will obviously damage the point or the module right?
Any prevention for this?

Thanks in advance.
 
Last edited:

Thread Starter

pandian

Joined Sep 27, 2009
33
If the (-) terminal of the buzzer shorts to ground it would be shorting the PLC's sinking pin to ground. It would not harm the module and the buzzer would sound.
If the (+) terminal of the buzzer shorted to ground it would be shorting the (+) terminal of the power supply to ground. Smoke would escape from the power supply but the sinking pin on the PLC would not be affected and the buzzer would not sound.
OK, i understand now. Its very true.
Thanks
 
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