TVS Diodes - choosing appropriate mounting location.

[PaulKraemer]

Hi,

The attached electrical schematic shows a proximity switch wired into a PLC input module. It shows a 1.5K ohm resistor and two of what I believe to be TVS (transient voltage suppression) diodes labeled 26 vdc. I am not certain of this because I was not familiar with this symbol prior to today, but I did find it referencing a TVS diode in an online chart that shows common electrical symbols.

In the modification kit I received this drawing with, there were two identical components with a round, red body and two leads. I have pasted a picture of one of these onto the schematic. Please correct me if I am wrong, but I assume these must be the TVS diodes. (I am not 100% confident of this because when I searched for images of TVS diodes, the images I found did not look like this. Generally, they were cylindrical in shape, with one lead entering one end of the cylinder and the other lead exiting the other end of the cylinder.

Anyway, if these round, red components are in fact my TVS diodes, I am trying to figure out the best way to connect them as shown in the schematic. The voltage source that will power the prox switch comes from the PLC input module. The easiest place for me to place (and connect) the TVS diodes (and the resistor for that matter) will be as close as possible to the terminals of the PLC input module.

I'm just wondering if anyone one here might be able to comment as to whether this sounds like a good strategy, or if there is a reason I should come up with a way to mount these components closer to the location of the prox switch.

Any advice will be greatly appreciate.

Thanks in advance,
Paul

 

[Comcokid]

The picture you attached shows what appears to be a Metal Oxide Varistor or MOV. They perform a similar function to TVS diodes. They do not have a polarity (they are bi-directional). Schematic shows these connected from 24VDC to ground and proximity switch output to ground. Typical connection to protect from transients that may be induced directly on the wires as they run between the proximity switch and the PLC module.

The 1.5K resistor is likely a pull-up resistor. Indicates the proximity switch is open-collector or open-drain output (typical for the output on many non-analog sensors).

For all of these the best place to mount them is close to, or right-at the PLC inputs.

 

[itsmoked]

Well.. That's pretty funny. The schematic shows TVSes while they sent you MOVs.

MOVs are much slower than TVSes but can handle far larger events, depending on physical size, up to truly massive.

TVSes would better serve your purpose here because there would not be an energy event there, only noise events, but since 98% of the time no one would add either, MOVs will probably suffice.

Put them close to the PLC.

BTW Most PLCs have this type protection built in.

Keith Cress
kcress -

http://www.flaminsystems.com