Increase the output current rating of an S7-200

Preeya

Member
Join Date
Nov 2003
Posts
112
Hi Guys,

I need to connect a 35W 24V soleniod valve to an S7-200 (CPU 222 DCDCDC) output. I use just this one output of the PLC. The rated current per point (max) for the DC output is 0.75A. As you can see this is not enough for the load of that size.

I cannot interpose a relay too as this is a high speed output (resolution needed to milliseconds) that operates at several hundred times a minute.

When I called Siemens Tech support, they told me to parallel 2 outputs and connect to the valve. This way, they said the output rated current per point would increase to 1.5A. Only condition being the outputs have to synchronize in the program.

Is it this easy to increase the rated current of an output when I have more spare outputs?!

Thanks for your time
 
Thanks for your reply. As long as I use FET outputs I would be safer with the speed. With relay there is always a delay. So these are normal outputs.

So I should just connect a wire between the outputs & take one wire from one of the outputs to the load? Is that how you connected?
 
Yuppers, I made sure the last line in the program set the sedond output to the state of the first with a large obnoxious comment not to change the line and why.

nOrM
 
Preeya,

you could also use an opto-relais. Phoenix Contact has some which could do. As examples
  • OV-24 DC/60 DC/2: input 24VDC output 5-60VDC 2A ton 20µs toff 200µs
  • EMG 17-OV- 24 DC/24 DC/2: input 24VDC output 24VDC 2A switch-over frequency 1000Hz
Kind regards,
 
I am amazed that Siemens tech support told you to paralell two outputs. That is mathematically correct, of course, but not generally considered good practice. First of all, the load sharing between the two outputs can't be gauranteed. Second, the inductive kickback on a solenoid will generally fry an output in short order even if the output is rated for the nominal current.

Using a solid state relay as jvdcande suggested is the smart solution.
 
I'm with Tom in wondering why Siemens told them to double up on adjacent outputs - probably so they can sell another PLC or repair the contacts when the smoke comes out of the silicon. I certainly would not attempt this if it could cause any sort of a safety concern.

I'm trying to nurse a few more years out of 30+ 214s, many purchased when they were first released in the spring of 1995 with revision 0.9 software on them. Siemens no longer supports them and my facilities are not scheduled for replacement until at least 2007. I'm re-evaluating the loading on the contacts and adding additional TVS to try to minimize the 120VAC inductive surges on them.
 
I know that it's a different manufacturer but Moeller actually states in the literature for some PS4 series PLC's that up to 4 outputs can be combined to incerase output capability.

Of course, Your Milage May Vary.

nOrM
 
I'm curious as to a solenoid valve that can handle so many switches. Most solenoid valves I've worked with are good for about 100K operations. If, as you state, you're going to be switching it "several hundred times a minute," you're going to hit 100K in short order. (1000/60 ~ 17 hours)

As others have said, I'd use an IGBT switch to do the job (assuming it is 24VDC), but I'm really interested in a solenoid valve that can handle this duty.

Can you shed more light on the application?
 
Yes I was surprised when Siemens support first suggested this to me. But you know what, its on the technical specifications of the S7-200 PLC manual too.. Connecting two outputs in parallel - YES for DC outputs!

Don, the application is dosing liquid nitrogen to containers (normally cans or bottles). Its a very simple application where a sensor detects a container on the line & gives the signal to the PLC. The PLC program waits for a specific delay and then opens the "dosing valve" for specific dose time. Its just an one input, one output application but the main issue here is all the operations are in milliseconds & needs ms resolution. We are using a Ledex Tubular Solenoid that has a life time of 25 million operations.

Jean Pierre, I will look at the phoenix contacts. Thanks for the link. I think I better put a relay in between.

Thanks for all your replies!
 
Last edited:
Is 3A the maximum I can expect from a DC solid state relay? Do you know of any make that has a higher rating for the output?
 
Nope. I have used solid-state relays rated up to 40A in the past. The make was Opto22. But I strongly :D doubt these were as fast switching as te ones I've pointed out earlier in this thread.


I've quickly checked the RS-Electronics catalog and guess what: I've found these gems back! But now they're marked Craydom. These range from 10A up to 125A. They now even have three phase versions up to 50A per phase.

Kind regards,
 
jvd, I did a quick search for these earlier and the ones I found that were above 3 amps were for AC current. I believe he needs to switch DC the highest I found were 3 amps.
 
Yes.. I need a DC relay with over 3A rating.. It has to be a solid state or opto-relay also to maintain high speed and relay life.

The phoenix contact jvd mentioned is perfect but I was wondering if there is one with a higher current rating.
 

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