How to size up Siemens S7-300 relay output module?

Hi guys,

I'm working on a proposal using Siemens S7-300 PLC, I'm new to Siemens. I wanted to put in a relay out put module, but referring to the manual, it has a few types like:

1) SM 322; DO 16 x Relay AC 120V. The AC 120V means what? Then at the rated load voltage it mentioned 24VDC to 120VDC & 48VAC to 230VAC, means if we supply 24VDC to 120VDC or 48VAC to 230VAC, it'll output the respective voltage?

Anyone can clarify me this? Appreciate it.

AC120 is what they use on ships, and in US and Taiwan.
I prefer to connect PLC output to a relay, and let the relay take the load.

Singapore_Mats said:

AC120 is what they use on ships, and in US and Taiwan.
I prefer to connect PLC output to a relay, and let the relay take the load.

Click to expand...

1) 120V AC is refer to what? It's not their output load.

2) For AB PLC, their relay output (1746-OW16), can have output load of AV or AC, depends on the wiring. The OW16 did not mention anything on the 120V AC or 240V AC

edit: double click on the mouse fpr this post I think

AC 120V is the voltage to drive the relay coil.

The 24VDC to 120VDC is the allowed DC voltage range allowed across the contacts.

The 48VAC to 230VAC is the AC Voltage range allowed across the contacts.

Peter,

I don't think I'd like to see a PLC module that needed 120VAC to energise the relay coils! In Siemens S7-300 range all the relay modules require a 24VDC supply to energise the coils.

Flyers,

Peter is correct about the range of voltages which the relay contacts are rated to switch. I'm not quite sure why the -1HH01- module is described as "AC 120 V" because that is not a limit. It can certainly handle higher loads. In other literature I've got it's described as "120/230V" indicating, I guess, 120VDC max and 230VAC max.

Mats,

What do you see as the advantages and disadvantages of using external relays connected to a solid state output module, compared to using a module with built in relays?

Regards

Ken

A look at the circuit diagram for the relevant module always helps as you will eventually need to know what to connect to the relevant terminals. Dissapointingly, I was unable to find a link to the circuit diagram using the Siemens website (it normally comes up trumps), so if anyone can post a link it would be useful.

Chapter 3 http://support.automation.siemens.com/WW/skm/pdfviewer.asp?HitsPerSite=10&ehbid=html_76%2Fehb%2F8859629%2Ehtm&lang=en&nodeid=10072094&query=SM 322&page=1&view=new

I was looking at a manual called "S7-300_RHB_e.pdf" (Hey, where do they get these snappy names?)

Ken

Hi guys,

I'm referring to:

To answer your original question(s):
- The 'AC 120V' in the specs appears to be a typo. In other docs the 1HH01 module is refered to as 'AC 120/230V'. Even this notification is not exactly correct: It should have been something like 'DC 120V/AC 230V', but maybe this is too long for their catalogs? ;-)
- Yes, the relay output module will only output the load voltage you have supplied. But something you can't figure out from table 3-5 is that you also have to feed it an (extra) 24Vdc.

If you look at the diagram, the internal circuitry of a SM322 output module seems to get its power supply from the backplane bus. Like Ken already mentioned, it's the optocouplers & relay coils that need to be 24Vdc powered externally.

The main reason why the maximum DC load voltage is limited to 120Vdc, is the higher risk of contact damage, even at low currents. When switching AC the risk is MUCH lower: Chances are very slim that the contact always closes or opens near maximum voltage level. If you look a little further down the specs, you'll notice a spectacular drop in operating cycles when using DC voltages, compared to AC. The number of operating cycles would become unacceptably low if higher DC voltages were allowed.

But I guess you already figured this one out...