New guy confused about power supplies

[Blue92]

Hello all, first post for me here. Im currently learning PLC programming at my local community college, and while its been great, there are some holes in the program. Mainly its that we do a lot of programming, but not a lot of the wiring, choosing components etc.

Anyways, Im putting together a trainer to use at home. The system will utilize an AB Micro820 controller, a few led's, limit switches, solenoids/air cylinders, push buttons etc. No more than 10 ins and 7 outs. My main question is that I dont know what size power supply to use. AB's recomended supply for the Mixro800 series is pushing 24v at 1.6amps. Being a college kid, I dont want to pay AB prices when I can get an Omron off ebay for $25 shipped. However I havent been able to find another 1.6 amp supply for sale anywhere. Would I be OK to run a 2.1 or 2.5 amp PS? Is there such thing as too many amps for a PLC? I get not having enough, but dunno about having too much.

 

[the_msp]

It will pull what amps it needs.

A 5A power supply doesn't push out 5A unless the load requires it.

I'd go for 5A for future expansion, and add plenty of small fuses, fuse the inputs and outputs etc separately... if you're learning it's better to blow a small fuse than damage any equipment (or yourself)

 

[Firejo]

You would not only be OK but I'd recommend going higher than needed now for possible future expansion (adding output devices that need 24VDC for example). One thing you probably want to study up on is power requirements. If you're going to work in the Industrial Automation industry you will need to know how to determine what is needed. More than once I've seen someone have problems with an application because they haven't supplied enough power and/or they've got grounding problems. It's pretty easy to find info on the web about it and that's where I'd start but if they offer a course at your school you might want to look into that as well.

 

[Geospark]

Hi & welcome to the Forum, & the Automation World!

The controller will only consume whatever current it requires to operate. If it draws 1.6A on average then that is all it will draw whether it is connected to a 1.6A, 2.5A, 5A or 10A power supply.

Do not forget that the "real world" devices you connect up external to the controller, and in particular to the outputs, will also consume a certain amount of current (solenoids, cylinders, etc.). If you connect a sensor to an input the sensor will also require and consume power.

Ideally you should always try to allow a little "headroom" and also allow for the future. You might get a bit more adventurous in time? If you cannot stretch to 5A power supply unit then I would at least recommend you to use a 2.5A. But really you should be toting up the total load of both the controller and all the current consuming devices you intend to connect up and then add on at least 0.5A to 1A.

It does not have to be an AB PSU but make sure it's not China's "finest" either.

Edit: Apologies Firejo for double posting similar info but I think we're on the same page.

Regards,
George

 

[rupej]

As a side note, I've found that many automation devices such as PLCs actually draw far below their "rated" current in general use. I assume that is often a worst-case number.

 

[Blue92]

Thanks all! Exactly what I wanted to hear.

 

[James Mcquade]

When doing analog work, we always use a linear power supply.
Less noise.

james

 

[Geospark]

As a side note, I've found that many automation devices such as PLCs actually draw far below their "rated" current in general use. I assume that is often a worst-case number.

In this case, most definitely yes.

With no expansion modules this controller family consumes just 5.62 W (234mA). With the max. 2 x expansion modules it draws 8.5 W (354mA). So not even 0.5A.

G.

 

[Geospark]

Another important thing to consider is protecting the controller's outputs from inductive load switching. Have they mentioned anything to you about protecting the controller outputs using supression?

This can be important too. Find out the current rating for the outputs from the specification for the controller and make sure your output devices are not of a higher current rating. I can tell you the output rating (or roughly as I'd need a specific model number) but it's best if you try to look it up yourself.

A good practice is to add suppression to the output devices if they are "heavier" or "dirty" inductive loads. A solenoid valve or contactor coil are typical examples of output loads which are often supressed. But ideally you could use "interposing" relays to best protect the outputs going forward. But that's another topic again.

Any questions though, there are lots of good folks here with a wealth of knowledge on such things.

Regards,
George

 

[Firejo]

To add just a little more...
When researching power requirements don't forget to look at the inrush current (it gets called a lot of different things). Any electronic device is going to pull more current at start up than it uses when in normal operation. That needs to be accounted for when sizing a power supply.

 

[BobB]

I use Omron 2.5 amp power supplies for a lot more than that LOL.

 

[icky812]

As a side note, I've found that many automation devices such as PLCs actually draw far below their "rated" current in general use. I assume that is often a worst-case number.

It usually is worst case. However, you always need to take into consideration inrush. PLC's usually have a fair amount of inrush current during startup due to the number of electrolytic caps that have to be charged up. I can't count the number of folks that I have pointed out to that the wall wort, while it technically has the rated capacity, won't actually power the PLC. Up sizing the power supply is rarely a bad idea.

 

[geniusintraining]

We use these on our 820's https://www.plccable.com/24vdc-switching-power-supply-1-9-amps-120-240-to-24-volt-dc/

 

[alive15]

My recommendation to you is find an internship / co-op that will place you directly into the company's work-force. Then you will experience everything together. Most maintenance departments in industrial facilities are always looking for someone to hire and grow; even if it's unpaid internship, just working 20 hours a week even will get you the best experience to place onto your resume.