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Old June 13th, 2018, 09:00 AM   #1
Pete_S
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Increace current consumption in S7 1215C

Hi,

I am programming an S7 1215C to function in a sudo SCADA demo. I am running the 120V supply for the 1215 through a voltage and current transducer. These 2 transducers are in turn feeding the 2 onboard analog inputs. I am trying to determine the easiest way to periodically run logic that would increase the current draw of the 1215C so I can make less static dashboards. Any suggestions would be greatly appreciated.
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Old June 13th, 2018, 09:37 AM   #2
cardosocea
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Run a motor? Start a Heater? Both?
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Old June 13th, 2018, 09:54 AM   #3
Pete_S
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Quote:
Originally Posted by cardosocea View Post
Run a motor? Start a Heater? Both?
Sorry I should have clarified. I am trying to increase the current consumption of the S7 1215C with logic. So the logic that would put the highest load on the CPU is what I am thinking but not really something they spec in the manual.
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Old June 13th, 2018, 10:18 AM   #4
janner_10
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You're better off just using logic within the PLC to simulate an analogue input go higher and lower.

I doubt the current draw of a empty plc and a PLC full of logic will be so negligible, it would almost be immeasurable.

Unless I mis-understand your question.
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Old June 13th, 2018, 10:29 AM   #5
Pete_S
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Quote:
Originally Posted by janner_10 View Post
You're better off just using logic within the PLC to simulate an analogue input go higher and lower.

I doubt the current draw of a empty plc and a PLC full of logic will be so negligible, it would almost be immeasurable.

Unless I mis-understand your question.
No, you understand my question. So current draw is not a function of logic processing? I understand it it is probably not a linear relationship, but I would think there would be some increase in current draw. I was thinking simulating large recipe edit by moving a boat load of registers. OB1 with a few networks running random number generator FB's and my signal scaling is a negligible current draw. Guess I would have to load outputs to really see an increase in current? I don't have any peripherals to drive with outputs but I could in theory just run resistors of appropriate value from output to ground, or even from output to input, which would increase the load on the power supply. Thoughts?

Last edited by Pete_S; June 13th, 2018 at 10:35 AM.
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Old June 13th, 2018, 10:44 AM   #6
BryanG
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It depends on the sensor you are using to measure current. If it will see very small changes then just switching outputs on and off will increase/decrease the current drawn, more so if they are relay outputs. If it won't see small changes then resistors at a suitable wattage gives you more options and control. Watch out for heat dissipation and having live connections to resistors in places that might get touched.
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Old June 13th, 2018, 12:14 PM   #7
Pete_S
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Originally Posted by BryanG View Post
It depends on the sensor you are using to measure current. If it will see very small changes then just switching outputs on and off will increase/decrease the current drawn, more so if they are relay outputs. If it won't see small changes then resistors at a suitable wattage gives you more options and control. Watch out for heat dissipation and having live connections to resistors in places that might get touched.
Could I just use the output to drive the corresponding input, Ie Q1=I1? Then increasing current draw in theory as output and input circuits would be consuming power.
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Old June 13th, 2018, 12:37 PM   #8
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Obviously you must NOT connect 110V to any DC output or inputs, or you will briefly pull a lot of current and let the smoke out.

From the manual a light loaded 1215 with an AC supply will take 100mA, a fully loaded one would take 300mA.

Connecting outs to ins will increase current consumption, but you are talking very low mA.

Inputs take about 5mA each but that is 24V d.c.. So 5mA at 24V is 0.12 watts, 0.12 watts at 110V is 0.0011mA plus inefficiency in the supply. Very low mA change to sense.

Something to be aware of is that there will be up to a 20Amp inrush as you power up the PLC, make sure your sensor can cope with that.
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Old June 13th, 2018, 02:03 PM   #9
mk42
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Quote:
Originally Posted by Pete_S View Post
No, you understand my question. So current draw is not a function of logic processing? I understand it it is probably not a linear relationship, but I would think there would be some increase in current draw. I was thinking simulating large recipe edit by moving a boat load of registers. OB1 with a few networks running random number generator FB's and my signal scaling is a negligible current draw.
Realistically, the PLC is always running the CPU at full blast all the time, and the code you run doesn't change how hard it is working. It's hypothetically possible that there might be some minuscule difference in power draw between different operations, but I doubt it would be meaningful. These aren't like an i7 that have technology built in to run slower at lower demand. Every PLC I've ever worked with was single threaded, at least for the main logic.

Slightly related, I read an interesting paper last fall about someone who was pointing a big antenna at a 1200, and could detect some frequency differences, based on copying a big or small DB.

Realistically, it's like the other posters have said. You're going to need some real load (even if it is just a resistor or a pot) that you can turn off and on to change the detected values. Or you can just fake it in code, which is what i've seen in almost every demo system ever.
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Old June 13th, 2018, 11:29 PM   #10
Rob...
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Easier to simulate it.

The AI inputs will not have the resolution needed to make any decent measurements.

Increasing the workload of the PLC will not make it work any harder. Just increase the scan time.
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