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Old July 26th, 2002, 04:33 PM   #1
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PhilRey is offline
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Switching vs. linear power supply for PLC I/O

What are your opinions regarding the use of switching vs. linear regulated power supplies?

The power supply will be used to power PLC I/O, solenoids, LED indicators, and 4-20mA transmitters. It will be mounted in the same enclosure as the PLC.

I will be using a 15 amp supply, so the size and heat dissipation are considerably less with a switching supply. However, I am concerned that the noise from a switching supply may effect the PLC or analog signals. I have used both types of supply without noticing any negative effects.

Is there an industry standard regarding the power supply type selection, or am I just worrying about nothing?

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Old July 26th, 2002, 06:05 PM   #2
Terry Woods
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If you don't pick-up scissors, let alone run with them, then you won't put your eye out! At least, not with scissors.

Some systems can tolerate a lot of noise... some can't.

Switching supplies are a source of noise. Sure, the manufacturers put filters on them and they can get the obvious noise down pretty damned good. But when things go wrong, isn't it usually the case that noise in the power supply is considered much later in the troubleshooting process?

When it works, it works! When it doesn't, sometimes you don't know it's the power supply that's not working!

I think an analog supply is more reliable (if it has the right power). I also think it's easier to troubleshoot an analog supply. Not to mention that you can often repair it yourself!

How many times have you replaced switching supplies in a PC? I have analog supplies that have been running for 10 years.

If you limit the number of troublemakers in the game, you have fewer troublemakers in the game.

In the final analysis, it's almost a toss-up. But, I get more peace-of-mind with analog supplies.
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Old July 26th, 2002, 06:05 PM   #3
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The concept/idea behind switching power supplies was to create an "efficient" coverter. The concept overall works, they are much more effiecient (hence less power/heat). They have their issues though, stability being the major one.

Overall its your choice, I doubt you can go wrong with either as long as you match the ratings to fit your application.
__________________ Industrial Electrical & Maintenance @ "Tell me and I forget, show me and I remember, involve me and I understand."
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