Circuit measures optocoupler’s response time

With this circuit, you measure the attack and release times of photoresistor-type optocouplers.

Peter Demchenko, Vilnius, Lithuania -- Test & Measurement World, 1/20/2012 2:19:53 PM

The design uses an oscillating Schmitt trigger with the optocoupler DUT (device under test) in the feedback loop. The photoresistor and resistor R1 form a voltage divider that controls the input of the Schmitt trigger. The optocoupler’s LED connects to the trigger output. You can measure the duration of the output pulses with an oscilloscope or a digital meter. The duration of the negative output pulses is equal to the switching on-time, or attack time. The duration of the positive pulses is equal to the switching off-time, or release time.

You can determine an optocoupler’s rise and fall times by incorporating a photoresistor in the feedback loop of an oscillator circuit.

The attack and release times depend on the value of R₁; you can observe both by varying the value of R₁. With the component values in the figure, the durations of the output pulses are a 0.15-ms attack time and a 2.7-ms release time.

During oscillation, the resistance of the photoresistor sweeps in from R_P1 to R_P2. The circuit sweeps these photoresistor values according to R1, the power-supply voltage, and the Schmitt-trigger thresholds, as the following equations show: R_P1 = R₁ × V_T2/(V_CC – V_T2) and R_P2 = R₁ × V_T1/(V_CC − V_T1), where V_T1 is the positive-going threshold voltage and V_T2 is the negative-going threshold voltage of the Schmitt trigger.

In the case of the 74HC14 logic family, you can determine the thresholds from the data sheet and your power-supply voltage, according to the following equations, which yield typical values: V_T1 = 0.53 × V_CC and V_T2 = 0.31 × V_CC.

Using 5 V as a power-supply voltage and solving the following equations, you can determine the photoresistor range: R_P1 = 0.45 × V_R1 and R^_P2 = 1.13 × V_R1.

This approach lets you pick a value for R₁ so that the photoresistor range is suitable for your device. You can also vary the value of resistor R₂ to observe the LED-current-to-attack-time characteristic of the DUT but not affect the release time. Note that R₂ limits the current through the LED; if its value is too large, oscillation will not occur.

Using this circuit allows you to match custom optocouplers comprising green, superbright LEDs and an MPY7P photoresistor. T&MW

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This article first appeared as a “Design Idea” in the September 8, 2011, issue of EDN.