Siemens SIMATIC S7-1200 System Manual page 1557

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The CPU in this example provides sufficient 5 V DC current for the SMs, but does not

provide enough 24 V DC current from the sensor supply for all of the inputs and expansion

relay coils. The I/O requires 456 mA and the CPU provides only 400 mA. This installation

requires an additional source of at least 56 mA at 24 V DC power to operate all the included

24 V DC inputs and outputs.

Sample power budget

CPU power budget

CPU 1214C AC/DC/Relay

System requirements

CPU 1214C, 14 inputs

1 SB 1223 2 x 24 V DC Input/ 2 x 24 V

1 CM 1241 RS422/485, 5 V power

3 SM 1223, 5 V power

1 SM 1221, 5 V power

3 SM 1223, 8 inputs each

3 SM 1223, 8 relay coils each

1 SM 1221, 8 inputs each

Total requirements

Current balance

Current balance total

S7-1200 Programmable controller

System Manual, V4.2, 09/2016, A5E02486680-AK

3 * 145 mA = 435 mA

1 * 105 mA = 105 mA

Calculating a power budget

14 * 4 mA = 56 mA

2 * 4 mA = 8 mA

3 * 8 * 4 mA = 96 mA

3 * 8 * 11 mA = 264 mA

8 * 4 mA = 32 mA