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Automating Manufacturing Systems - Process Control and ...

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plc analog - 22.15<br />

Given,<br />

Calculate,<br />

N = 8, R min<br />

= 0<br />

V max<br />

= 10V<br />

V min<br />

= 0V<br />

V desired<br />

= 6.234V<br />

R = R max<br />

= 2 N = 256<br />

V max<br />

–<br />

⎛<br />

V min ⎞ V ERROR<br />

= ---------------------------- = 0.020V<br />

2R<br />

⎝<br />

⎠<br />

V I<br />

V in<br />

⎛ ⎞<br />

⎠ R 1<br />

= INT<br />

– V<br />

---------------------------- min<br />

– ( – ) + 0 = 159<br />

V max<br />

V min<br />

⎝<br />

V I<br />

– 0<br />

V ⎞ ⎛<br />

C<br />

= ------------- ( Vmax – V<br />

R – 1<br />

min<br />

) + V min<br />

= 6.235V<br />

⎝<br />

⎠<br />

The current output from a D/A converter is normally limited to a small value, typically<br />

less than 20mA. This is enough for instrumentation, but for high current loads, such<br />

as motors, a current amplifier is needed. This type of interface will be discussed later. If<br />

the current limit is exceeded for 5V output, the voltage will decrease (so don’t exceed the<br />

rated voltage). If the current limit is exceeded for long periods of time the D/A output may<br />

be damaged.

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