Power Supplies

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Thread Starter

Mike

Some engineers at my company insist that a separate 24VDC supply should be used for analog circuits. In my case the analog signals are 4 to 20mA PLC cards, both input and output. From my past exp. I have never needed to have separate power supplies for analog and digital signals. Can anyone provide comments either for or against separate power supplies for analog and digital signals?
 
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Bob Gillooly

Depending on the type and make of your analog I/O
and the collective load of your outputs you may
need and want an external power supply. typically
if an input card is used and it derives it's power from a backplane you may not need an external PS. (check install recommendations)
If the control work is sensitive and
accuraccy is important I would recommend a regulated PS. for your outputs particularlly if you have alot of them. Some Allen-Bradley I/O require an external PS for inputs and outputs and they will remind you to make sure the voltages match the transmitters voltage. The application of external equipment is a good CYA, as most I/O cards and rack mounts have wimpie power supplies.
 
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Michael Griffin

This would normally be for noise purposes. If you use the same power supply, you may end up with noise from valves and other devices on your analogue signal. You wouldn't see this happen unless you put a scope on it.



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Michael Griffin
London, Ont. Canada
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Gerald Moore

I agree with the above comments. I have had an occassion or two where other 24VDC products have affected my analog signals. One most recently with a Safety Mat Controller, that was added to an existing machine. When the Mat was reset my pressure transmitter went bonkers. I had to separate the 24VDC supplies.

Good Luck!
Gerry

 
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David Norris

PLC cards can usually be setup either having a +24 volt power supply to the card or a separate power supply to the field devices and the return coming to the cards. Both methods work fine with a few caveats. All signals need to be grounded to the same point and all field signals should be shielded and going to the same ground. I tend to use one power supply for multiple analog signals (properly sized, of course).

I am, of course, assuming that all signals have some current limiting circuits within them (which almost all PLCs do today). That will prevent a field short from knocking down the power supply and taking all the i/o down.

One other point, I am not addressing redundancy since that was not an issue in your request.
 
Separate power supply is better as it provides safety barrier for the I/O cards.If there is any problem at transmitter side / cable fault the card as well as the system will continue its operation except the input will not be sensed. It is my experience that field problems create the sytem to hang or go to stop mode.
 
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Gerald Moore

I agree with Bob Gillooly's and Michael Griffin's comments. I have had an occassion or two where other 24VDC products have affected my analog
signals. One most recently with a Safety Mat Controller, that was added to an existing machine. When the Mat was reset my pressure transmitter went bonkers. I had to separate the 24VDC supplies.

Good Luck!
Gerry
 
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David Norris

PLC cards can usually be setup either having a +24 volt power supply to the card or a separate power supply to the field devices and the return
coming to the cards. Both methods work fine with a few caveats. All signals need to be grounded to the same point and all field signals should be shielded and going to the same ground. I tend to use one power supply for multiple analog signals (properly sized, of course).

I am, of course, assuming that all signals have some current limiting circuits within them (which almost all PLCs do today). That will prevent a field short from knocking down the power supply and taking all the i/o down.

One other point, I am not addressing redundancy since that was not an issue in your request.
 
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Anthony Kerstens

In addition to isolation from discrete I/O loads, it would be desireable to use a separate power supply with better voltage regulation. That is, use a cheaper p/s for the discrete loads that can tolerate some noise, and the more expensive p/s where it counts.

Anthony Kerstens P.Eng.
 
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I guess it "depends". I have frequently used the "user" supply terminals on AB SLC power supplies to power a few analog transmitters or switches, but I will NOT use them for powering inductive outputs such as relays or solenoids.
I figure there is some chance of getting noise back from inductive output devices, so choose to use a seperate power supply if outputs are involved (although I did run an LED display off the built in power supply once).

Use your best judgement, but remember that cheap is not always best, and you can get in a world of hurt trying to save a few bucks. Besides, power
supplies are relatively inexpensive these days, so best to error on the side of caution.

Bob Peterson
 
The reason for separating analog and digital signals is multifold:
Consider various scenes,
1. Combining analog and digital at times creates noise problems.
2. Surge and lull due to large digital loads being switched on and off can at times create voltage level fluctuations (maybe momentarily),
which could disturb some of the analog circuits.
3. A short circuit in digital load of such problems in poorly designed supplies can cause entire analog signals to be lost.
Anand
 
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