PLC input interface light emitting diode display circuit a

For PLC circuits whose input power is DC, the input contact is the switching element S. In order to display the working status of the switch,
light-emitting diodes are often used as display elements. The light-emitting diode VL and the switching element s can be connected in parallel or in series.
Figure 13-55 (a) shows the parallel connection method, and Figure 13-55 (b) shows the series connection method. In the picture,,. is the equivalent resistance at the contact (approximately
lkfl); Ri is the input resistance of the PLC (approximately 3kfl); R is the resistance set to avoid malfunction of the PLC (approximately 1_5kfl).