Siemens SIMATIC S7-200 System Manual page 488

S7-200 Programmable Controller System Manual
Calculating a Sample Power Requirement
Table B-1 shows a sample calculation of the power requirements for an S7-200 that includes the
following:
S7-200 CPU 224 AC/DC/Relay
-
3 each EM 223 8 DC In/8 Relay Out
-
1 each EM 221 8 DC In
-
This installation has a total of 46 inputs and 34 outputs.
Tip
The CPU has already allocated the power required to drive the internal relay coils. You do not
need to include the internal relay coil power requirements in a power budget calculation.
The S7-200 CPU in this example provides sufficient 5 VDC current for the expansion modules, but
does not provide enough 24 VDC current from the sensor supply for all of the inputs and
expansion relay coils. The I/O requires 400 mA and the S7-200 CPU provides only 280 mA. This
installation requires an additional source of at least 120 mA at 24 VDC power to operate all the
included 24 VDC inputs and outputs.
Table B-1 Power Budget Calculations for a Sample Configuration
CPU Power Budget
CPU 224 AC/DC/Relay
System Requirements
CPU 224, 14 inputs
3 EM 223, 5 V power required
1 EM 221, 5V power required
3 EM 223, 8 inputs each
3 EM 223, 8 relay coils each
1 EM 221, 8 inputs each
Total Requirements
Current Balance
Current Balance Total
474
5 VDC
660 mA
minus
5 VDC
3 * 80 mA = 240 mA
1 * 30 mA = 30 mA
270 mA
equals
5 VDC
390 mA
24 VDC
280 mA
24 VDC
14 * 4 mA = 56 mA
3 * 8 * 4 mA = 96 mA
3 * 8 * 9 mA = 216 mA
8 * 4 mA = 32 mA
400 mA
24 VDC
[120 mA]