Relay Boards vs. PLCs? Bioengineer seeks switching electrode help

Singh02

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Join Date
May 2006
Location
Sacramento
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Hi all

Our team is debating whether to use relay boards or PLCs. We have 3 weeks to complete. Anyone know any general pros or cons of using either switching device? Also, any good sites for buying information fo essentially are beginners? Out generator will pump out 550 KHz, 200 Vac, and 1.5 amps.

Brief project description:
To jump right into it. We have 4 electrodes ( A,B,C, D) which we want to switch between active and ground every 5 seconds (so 1 will be active with 3 inactive).

Hence, A should be on for the first 5 seconds, off the next 15 seconds.

B should be off the first 5 seconds, on the next 5, off the next 15.

Etc.
 
You will definitely get the most flexibility and expandability with a PLC.

The only drawback is the initial cost up front. This may include the time to learn PLC programming and purchasing any software to actually program the PLC.
 
Get yourself one of those mini PLCs like the Omron Zen. Every manufacturer seems to have one these days. You do not need software or an external programmer to get going. For about $200.00 you are ready to control your system.

Jim Knee
 
even if you need somthing very simple (no timers counters or relays with dozen contacts) you will quicly find out why you love plcs. it only takes one to wire 5-6 relays, change mind and rewire them... after that you will know, it's plc's all the way...
 
A few options:



  1. An AB micrologix 1000 1761-L10BWB (6 inputs 4 outputs) will cost you less than a hundred dollars.

    http://www.ab.com/programmablecontrol/plc/micrologix1000/controller.html

    Logix500 programming software for this PLC (the 10 IO model only) can be downloaded for free from ABs website. However if you don't have any experience with PLCs then this has a steeper learning curve.
  2. A PICO plc such as the the AB Pico which is really just a relabled Moeller pico plc. The unit is more money, but it is easy to learn.

    http://www.ab.com/programmablecontrol/plc/pico/controller.html

    You can download the software for free from the AB web site.

    http://www.ab.com/programmablecontrol/plc/pico/picosoft.html



    However, you will have to buy a programming cable or be content to enter the program by hand using the simple function keys on the unit - which is not too difficult. You can also get these units under the Moeller brand from the mail order electronics house Newark Electronics www.newark.com
  3. You can check out the PLCs on automationdirect.com They are some of the cheapest PLCs on the market but its going to be hard to beat the ML1K with free software or a PICO.
 
Personally I do not care much for the smart relays (PICO etc) but they can be programmed without software which can offer benefits.

Adding to what Alaric mentioned if you can use the AB Micrologix 1000 then it has a LIM instruction, this is a limit instruction that allows you to activate an output when certain values are within the limit.

EXAMPLE: With the ML 1000 you could use 1 timer set for 20 seconds that resets itself at 20 seconds. Using the LIM instruction you could activate #1 between 1 and 5 seonds, #2 between 6 and 10 seconds, #3 between 11 and 15, and #4 between 16 and 20. The LIM instruction would use the values from the Timer accumulator.

FYI; the above appears to suggest that would only be 4 seconds for each electrode but you have to understand some things. A PLC actually uses ZERO as a number (which it is) so the timer will start at zero, electrode #1 would activate when the timer turns 1 i.e. it will run from the start of 1, thru 2, 3, 4, and to the end of 5 seconds.

I have a simple example of a LIM instruction being used to create a flasher on this page; http://www.patchn.com/ab_plc_flasher.htm

In general this could be done on a plc with about 6 rungs of code depending on what other aspects may need to be included and/or what brand plc is used. The above example was proprietary to AB.
 
You can't switch 550KHz with standard relays. You need a Vacuum Relay.
Coils are rated at 12VDC, 26.5 VDC, and 120VAC. The 26.5 VDC works fine at 24DC. I think ITT Jennnings ( [size=-1]http://www.jenningstech.com ) [/size]started the 26.5 V spec to work with unregulated 24DC supplies, and now everyone rates their 24DC coils at 26.5V.
Nice paper on Vacuum Relays:
[size=-1]http://www.tycoelectronics.com/documentation/whitepapers/pdf/eDigest-High_Voltage_Relays.pdf[/size]

Look at the K45C here:
http://relays.tycoelectronics.com/kilovac/

After you have selected 4 Vacuum relays, use a PLC to drive them. My preference is Automation Direct. The DL05 would work fine. The software is free for 100 words, $395.00 for the full version.
If your sequence won't fit in 100 words, then other solutions above would be cheaper. The Micrologix is a good choice. The learning curve is steeper as Alaric said, but you will get help from many on this forum.
4 Time Delay Relays with a continuous cycle option will cost about $300.00, so PLC will be cheaper.
 
Last edited:
Hi

why not try using a Omron ZEn PLC... or some other similar fashioned compact PLC,S. It is very simple and also has some potential free contacts . thus it will not affect the PLC as it does not draw the current from the PLC'S.

Also in compactness and aesthetic PLC'S look better than the Relay, since a seperate control has to be given ( Timers, latches, drivers etc.) when using a relay.

Also Relay consumes around 500 mA of current for switching while PLC's consume less current. and as your appln needs frequent switching , PLC'S are better option than relay..
 
Singh02 said:
Out generator will pump out 550 KHz, 200 Vac, and 1.5 amps.

I think this RF signal might require a special switching apperatus. I wouldn't try switching it with standard relays, or PLC relay output cards.
 
Last edited:

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