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Pushbutton / Indicator Lamp: Calculate Control Resistance

FAQ No. FAQ01906

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How do you calculate the control resistance for A3D/M2D Push Buttons LED lamps?


Set the control resistance within the range for the LED lamp characteristics.

(Set the forward current for the LED lamp to 8 mA min.)

Use the following formula to find the external resistance.

R = (E - VF)/IF (Ω)

E: Operating voltage (V)
VF: LED forward voltage (V)
IF: LED forward current (A)

Control Resistance (Recommended Values)

VoltageRedYellow (White)Green
5 VDC165 Ω140 Ω145 Ω
12 VDC515 Ω490 Ω495 Ω
24 VDC1,100 Ω1,090 Ω1,095 Ω

Calculation Example: A3D

LED lamp illuminating color: Red

E = 24V
IF = 20mA
Ta = 25℃

Based on the VF-IF characteristics in the diagram below (red):
VF = 1.7 V when IF = 20 mA.

Therefore, inserting the values into the formula above (R = E - VF/IF(Ω)):
R = (24 (V) - 1.7 (V))/0.02 (A) 1100 (Ω).

The recommended resistance is 1.1 kΩ at 1 W (2 x IF2R).

Note:Approximately twice this value is appropriate to provide a margin in the capacity of the resistor.

LED Characteristics (VF-IF Characteristics)

A3D Series

A3A Series

Forward Current Reduction Curve

A3D Series

Applicable models: A3D Push Buttons, A3A Push Buttons, etc.