I am using a 224XP (6ES7 214-2AD23-0XB0) with a 16I/16O expansion module (6ES7 223-1BL22-0XA0). I am connecting 3 solenoid valves (~12W), 3 ice cube relays, 2 indicator lights and a contactor (13.3W). I am looking for information about any external protection I need to add to the outputs (fusing, surge suppression, etc.). I am planning on using a 5A switching power supply with current limit. I seem to remember that at least some of the Siemens stuff I have used in the past (s5 and s7-300) had self-protecting circuits on each output. I cannot find anything in the S7-200 manual that says the s7-200 outputs do or do not have this feature. I found some information about using a diode and/or a zener diode for surge suppression, but not whether the outputs are self-protected. It seems to me that if they are protected, I do not need individual fuses for each point; if they are not protected, a normal fuse will only protect the output if there is only a partial short of a coil (a direct short would still blow the output circuit). So is there any point in individual fuses for each point?

What do you guys connect externally to S7-200 outputs?

BTW, I know I only have 9 outputs - I have a different source for one group.

Thanks for any insight.
Ian

Fuses will protect only from overcurrent conditions and shoudl not be relied on for surge or for collpasing field in relays or solenoids.
I would have fuses on the outputs for sure.

The collapsing field from solenoids when they turn off - especially so in DC - are another issue. I would use MOV to protect the PLC outputs in this case. I suppose it can be argued that in teh case of icecube relays they are not needed but for a few dollars of insurance versus cost of a module I would do it.

Dan Bentler

provide

The S7-200 manual says: "The DC outputs have internal protection that is adequate for most applications." A little more detail might be nice - define "most applications" at the least. I am planning on using external surge suppression on the coils; there is no good argument against it in this case. The one solenoid that is critically timed needs consistency more than speed of repsonse anyway. Many ice cube relays come with an internal diode across the coil these days.

I am questioning whether individual fusing for the outputs is necessary or useful. If the fuse doesn't actually protect the output, there really is no reason for it to be there. If the short circuit current of the power supply (before foldover) is sufficient to destroy the junction of the output transistor or damage the lands on the PCB faster than a fuse can act, there is no point in installing a fuse. Likewise, if the output self-protects on overcurrent, a fuse will never get the chance to blow, so there is no point installing a fuse. Installing a fuse that cannot protect a circuit just because you want to protect that circuit does not make sense. It just complicates the system without adding any real value.

Ian