Sizing DC Power Supply for a Machine

Okay, everyone, I know this is going to sound like an extremely noobish question. I've spend about a half an hour searching for the answer on the boards already. Anyway...

I am working on designing a machine to dispense pallets. I am using 24V DC control and had been using a 5 Amp power supply. My boss wants to lower hardware costs and has asked me if we can downsize this power supply so now I am actually trying to calculate the load on the power supply.

This machine is controlled by a MicroLogix 1200 (1762-L40BWA), 24 DC inputs, 16 relay outputs. I've already added up my outputs (pilot lights, relays, motor starters) but what I am curious about is if I only need to add the power used by my input devices (photo eyes, proximity sensors) or if I need to also add power used by the input points themselves?

Any help is greatly appreciated.

Geez, times must be tough

Your inputs will draw very little power, probably not enough to worry about. However if you want to figure it out the sensors should have power requirements that you can use. Also figure out what the input impedance is for your PLC cards, use ohms law to figure out the current draw and add that to the rating of the sensor.

You also need to remember that not all inputs and outputs will be on at the same time.

After all of this you will have spent more money calculating than you can proably save by using a smaller supply.

I wouldn't worry too much about the input points. They will most likely be in the range of several hundred microamps to a couple of milliamps. On the flip side these numbers are easy enough to add up so you can probably include them pretty easily.

Be a little conscious of items with capacitors or DC fans in them. These may draw short term currents that can put your power supply into current limit.
Keith

Thanks for the quick replies. You don't know my boss (also the owner of the company, lol )

I usually try to figure things out with all devices powered, although I know not everything will be on at the same time, but I like to leave myself some breathing room.

I can also pick one from a different vendor as I know the Puls model I had is from their Dimension line and rather pricey. I picked it because it had a small footprint and my panel space was tight. We've redesigned our panel and space isn't such a premium now.

Are you planning on building lots of these? Is so then I can see the concern over cost.

If so the best way is to put an ammeter in series with your power supply and actually measure what you are drawing over a period of time while the machine is running. Your power supply needs to be able to handle whatever the peak is.

May want to check out www.automationdirect.com pricing on their slim line power supplies.

Also check www.solaheviduty.com

www.idec.com

My company just invested in a Mitsubishi laser cutting machine and nifty looking bender that is being set up as we speak, so our goal is to sell many of these machines. We used to buy these machines without the controls and provided our own because the controls they sold us were pretty sad and we are picky...

jad5150 said:

My company just invested in a Mitsubishi laser cutting machine and nifty looking bender that is being set up as we speak, so our goal is to sell many of these machines. We used to buy these machines without the controls and provided our own because the controls they sold us were pretty sad and we are picky...

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Gosh, I just love these conversations. "We thought we'd pinch a little money out of the project....that power supply is too big (i.e. expensive)....." What next? "Can't you use old wire over, or solder splices together from the left over wire?"

Phoenix Contact make some good power supplies (we use them in our DCS cabinets).

Sola are not as good as they used to be when they were manufactured by Puls as they swapped over to a chinese manufacturer.

I would recommend not cheating yourself. Your power supply should have some head room for possible future capacity. And I don't recomend using a power supply that is maxed out.

Don't forget about the in-rush on the DC coil side of the motor starter/contactor.

I did not realize there was that much of a difference when it came to power supplies...in general i.e. AD has one listed for $135 that provide 7.5A.

I have no idea how Sola is today but back a few years there were not many I thought any better and I would pay about $150 for a 5A, mainly because a 3A of the same model was around $10 cheaper, did not see a benefit.

If he really wants to be cheap you could go buy some old PC's at a thrift store and use the power supplies out of them, even new you can get them for $30 or so. They offer 12, -12, 10, 5, -5, 3, 17, -17, and 24, most are over 150w but you to be careful when using the 3 and 5v.

Tell the truth it has kind of surprised me some have not used PC supplies inside a cabinet, they have a fan and if necessary it could be ported outside the enclosure...just depends.

If you do not care how clean the power is you could just get a step down transformer and mount a bridge, but probably cost about the same as the PC units.

This may just be me but I have an affinity to have at least a 5A power supply, larger when needed or use multiple 5A if the occasion arises. This usually works out because then you only need to stock one type and if using MANY of them you usually can get a price break on quantity.

It has always amazed me that one of the first things you learn in electronics is that you can parallel matching supply voltage i.e 2 - 5A 24vdc supplies in parallel will offer 24vdc @ 10A BUT the industry considers it a NO NO.

rsdoran said:

Tell the truth it has kind of surprised me some have not used PC supplies inside a cabinet, they have a fan and if necessary it could be ported outside the enclosure...just depends.

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I have, speed controller died on tiny 12V DC motor. The motor didn't really need speed control. I couldn't find a power supply in stock (2AM).

Ripped an old PC apart hay wired a couple of relays and we were back in business.

Worked so well I went back a prettied everything up and left it, probably still running.

PULS are very nice but very pricey
Meanwell is the cheapest I have seen out there
Lambda are nicely packaged, cheaper then PULS, not cheaper than Meanwell
I work for the founder who is a mechE and thinks the electronics should be 1% of the machine cost

On a multiple output SMPS, the first voltage is the one that's regulated. All others slave off of it. In the case of a PC power supply, the 5 volt (or 3.3 volt) supply needs a load. About 500mA will do, I prefer 1 amp. It won't run at all without a load.
Maybe some of them will, but this is true for the majority.

Keith somewhere I have a thread (or post) about using a PC power supply, was just generalizing at this time so did not go into detail.

http://www.plctalk.net/qanda/showthread.php?t=22613