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Old September 18th, 2018, 12:09 PM   #1
redstar7t
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Beginner's Question on Current Output

Hello,

I'm using a Micrologix 1100 (1763-L16BBB) and have a question on what current I would be getting out of one of the transistor outputs (they're rated at 1.5A). I'm using a 700mA power supply to power both the PLC and the outputs. The PLC itself is using about ~500mA. So with my power supply these outputs are going to supply 200mA total among the loads correct?

So say as an example I only wanted to use one output, and that output is to a very small 24VDC CPU fan that has a max current rating of 100mA, and I don't know the resistance of this fan, should I avoid using it? Since that output terminal could potentially supply 200mA?

Or do the outputs only supply as much current that's needed? So if the fan only needs 100mA, the output would only supply 100mA?

Seems like a simple question but I'm having trouble finding the answer
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Old September 18th, 2018, 12:16 PM   #2
Ih8Siemens
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"Or do the outputs only supply as much current that's needed? So if the fan only needs 100mA, the output would only supply 100mA?"

One thing to always remember, a supply doesn't "supply" current in the sense that it will always push a current at a static rate. A device will draw the current it needs to run.
So when doing load calculations or making sure you dont surpass the maxium wattage for your power supply, you always check the input ratings for your devices and add them all up.

Does that help or am I not understanding your question?
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Old September 18th, 2018, 12:21 PM   #3
Steve Bailey
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Quote:
Or do the outputs only supply as much current that's needed?
The outputs don't supply current. That's what the power supply does. The outputs are just switches that allow current to flow when they are turned on. The voltage level in the circuit and the resistance of the load determine the current flow in accordance with Ohm's law.
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So if the fan only needs 100mA, the output would only supply 100mA?
More accurately, if the fan only needs 100mA the power supply will supply 100 mA.
If the load requires more current than the rating of the output, the output will fail. Some outputs will fail in the off (non-conducting) state. Some will fail in the on (conducting ) state.
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Old September 20th, 2018, 08:22 AM   #4
redstar7t
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These answers are what I was looking for. Cleared things up, thanks
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Old September 20th, 2018, 08:49 AM   #5
Jeff23spl
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keep in mind a fan is an inductive load. If 100mA is the nominal, at time of switching on, it will take more to get up to speed. You may have a rating for inductive load that would be lower than resistive ones....
On the other hand, your 700mA power supply will have an inrush rating for this and would probably take it.
And last, if you are switching often a inductive load, a free wheeling diode may increase the contact lifespan.
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