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Joined: Dec 2001
Posts: 50
Does anyone have any experiance with 24vdc power supplies? I have a controller for a machine that is powered from a 24vdc power supply. The software keeps becoming corrupted and we think perhaps we have it narrowed down to a problem with the power. The power supply is a siemans SITOP power supply. The line coming to the machine is a 480v 3Ø. It is reduced to 240v through a transformer before it enters the machine. after entering the machine it is reduced further to 115v for the power supply. this 115v also supplys power for all the other controls on the machine. (which are plentiful, mostly contactor coils). Any Ideas?

Joined: Apr 2002
Posts: 53
Finally! A question that is in my field! [Linked Image]
Deal with these questions daily.
1. Most common issue is grounding. Industrial environments are notorious for ground loops.
2. Make sure PLC and IO have separate power supplies. Field I/O usually can be "dirty" from control lines running near high voltage supply lines, VFD's and other devices that have high EMF properties. Rule of thumb to follow is 14 inches per kilovolt and cross these lines perpendicular. Remember that the PLC cpu's operate on milliamps and very low voltages. It does not take much to corrupt memory in these units.
3. See if you can correalate a real world event to trigger the problem. Things to watch for are starting motors, VFD's, miking a radio within a couple of feet of the unit. (that last one can be real nasty to track down)
4. Replace the power supply and see if problem goes away.
5. Put a UPS on the 115v supply for the 24vdc power supply.

If all this does not work then there may be faulty memory in the CPU. Replace the CPU.

Lemme know what turns up.

Joined: Dec 2000
Posts: 218
Joe, do the coils of the contactors all have suppressors on them? We have a crane control that requires them. If one breaks I know it immediately as it shuts the CPU in the crane off. Might be a good idea to check for them. By the way what brand of controller are you using?

Joined: Jul 2002
Posts: 2
Junior Member
Does the power supply have any pass transistors. You can usally see them sticking out on the sides of the power supply. I had a customer with the same problem once and the power supply worked ok untill the machine went into a certain cycle, and thats where it would over heat and blow fuses. This power supply had several power pass transistor and only one of them was bad, problems only showed up when called to put out heavy current.
You can touch the power supply and see how hot it is, if its HOT you know that its running at least 100 degrees +. Just a sugestion...

Joined: May 2001
Posts: 717
OK, the answers you got are probably more experienced in this area than mine.

Currently covered all my answers perfectly and his suggestion of replacing supply is a good one, BUT I did run into a wierd one lately that may be of some assistance.

I replaced a computer about a year ago, and used the old UPS unit on it. It kept crashing, hey, it's Microsoft, that's what it does [Linked Image]
I asked an IT guy who said "Is the UPS an adequate supply?" Floored me, didn't even think about it, let's face it most computers are 2-300 watt supplies. Unplugged it from the old UPS, plugged it in the wall, no more problem.

Could be of some use.

Joined: Apr 2002
Posts: 53

Power supplies in computers are getting bigger. The AMD Tbird chip requires a minimum of a 300 watt power supply. My computer has a 500 watt power supply. I use a lot of scsi components in my box.

A great resource for anyone using and working with PLC's is the website

Joe, you may want to post a question there. The folks there are pretty competent with controls and automation (but this website is much friendlier! [Linked Image] )

[This message has been edited by Currently (edited 08-06-2002).]

Joined: Dec 2001
Posts: 50
I shoulda clarified. This controller is actually an old numeric controller with mechanical relays.
What Currently said about radios made me think. This controller is in a box suspended by an arm which connects to the main electrical cabinet. All the comnnections go thru this arm. The operator has a bid a** radio sitting on top of this arm. He doesnt have a mike or anything like that connected, but could that be a problem?
As far as seperate power supplies, The inputs have the same power source as the controller (an external power supply) the power going to the power supply however is shared with the rest of the controls and this includes the power going to the NO/NC relays on the controller.
spkjpr, yes the coils have suppressors installed. they were factory installed because previously they had that very problem you described.
The controller had been replaced only to have the same problem reoccur 3 weeks later. The most recent incident was during an electrical storm when the power went off for a breif moment.
Siemans has a 24vdc UPS that would fit in nicely with the power supply. However I was hoping to nail down the problem before I throw to much money at it.

Joined: Apr 2002
Posts: 53
Radio you are talking about is a reciever.
No problems there. Handheld (walkie-talkies) are problems. Security personell are notorious using these next to PLC's.
Dealt with a case once that drove us nuts for almost a month. Turns out the watchman was doing a radio check on his rounds right next to a PLC at 2am every morning. This kicked the PLC out of RUN mode and they had to wake the tech up and have him reload the program into the unit. When he keyed the Microphone button, that would send enough RF to give the PLC an identity crisis <grin>

Joined: Aug 2001
Posts: 31

I would suspect it would be more of a power problem than a radio problem. The vintage systems in the field rarely have interference problems from radios...has this always been a problem...or is this new? If it was an interference problem, it would have reared it's ugly head upon installation and commisioning.

It's more than likely a dirty or bad coil in your relay system. If the coil (even with suppression) is damaged or failing...the current draw would make the power supply exceed it's speced rating. This will make everything else flake out.

I've seen this a lot with the older TI/Seimens/GE 305 plc systems that have been in the field for over 10 years.

I see two posible solutions. 1.) check and double check the coils on the contactors that the NC system controls. 2.) Split out the power supply. Which is add one to control your system and one to control the coils. This seems odd.....but it has helped many people who did not want spend a lot of money investing in a complete new system.

Just some thoughts,

Carl Lee Tolbert
Technical Support
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PH: (770)844-4200
Joined: Aug 2002
Posts: 42
Currently, check out very informative site, plc's, motion control etc..

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