How to Determine Optocoupler Operation Saturation or Linear

There are several ways on how to determine Optocoupler operation. The old school method is to build an actual circuit and measure the collector-emitter voltage. If the reading is low enough (equal to the saturation voltage of the device) or ideally zero, the Optocoupler is operating at saturation. If the reading is higher than the specified saturation voltage but lower than the collector supply level, the device is operating at linear mode.

Another method on how to determine Optocoupler operation is through simulation. Simulation is easier than the first mentioned method. However you need to have a simulation software to do so. Another thing is that you need also to understand how the simulation works and how to model circuits. For some designers and engineers, these task are tedious. In addition simulation cannot directly give worst case result. You can get the worst case if you apply in your model. I have also an article that teaching how to do monte carlo analysis (worst case) using LTSpice. You may check it here.

If we are going to rely on actual measurement on how to determine Optocoupler operation, we’re in trouble. It’s not completely a good idea! Imagine if you have several opto circuits in your design, are going to build a sample circuit for all of them and do the test? It’s okay if there are already prototypes, but if you are still in the design stage, actual measurement is a no no approach. Even though you have already the prototypes still it is not a good practice to measure the VCE of all the opto circuits for this consume your time.

As a designer or engineer, you have to found ways to make life easy and speed up the design lead time. In this article I will teach you how to determine the Optocoupler operation using computations.

Ways on How to Determine Optocoupler Operation

Method 1. Comparing the circuit CTR and the Device CTR

Circuit CTR is the ratio of the circuit collector to forward current. On the other hand device CTR is the current transfer ratio defined in the datasheet.

For the circuit to operate in saturation, below criteria must be true. If otherwise, the circuit operates at linear mode.

CTR circuit < CTR device minimum

In this approach on how to determine Optocoupler operation, the Optocoupler circuit is assumed to be saturated. At saturation the collector current is maximum which is only opposed by the collector resistor.

Example 1

To make this clear, let us consider below circuit as an example. Consider Vcc and Vdd are equal to 5V. The device CTR is 120% while the Optocoupler forward voltage drop is 1V.

Solve for the forward current

Solve for the collector current

As mentioned, we are assuming that the circuit is operating at saturation. The collector current is

Ic = Vcc / R2 = 5V / 2k = 2.5 mA

Solve for the circuit CTR

CTR circuit = Ic / If = 2.5 mA / 8 mA = 31.25%

The device CTR is 120% as given above. Therefore the above criteria is true and this means that the circuit operates at saturation.

CTR circuit = 31.25% < CTR device minimum = 31.25% < 120%

Method 2: Solve for the Circuit Collector-Emitter Voltage VCE and compare to the Device Saturation Voltage

In this technique, the opto circuit is assumed to be operating in the linear region. For it to be true, below criteria must be true. If otherwise, the circuit is operating at saturation.

VCE circuit > VCEsat device maximum

VCEcircuit is the computed collector-emitter voltage of the Optocoupler while VCEsat device maximum is the specified maximum saturation voltage in the datasheet.

Example 2

Using the same circuit above, let us check the operation using Method 2.

We will not solve the forward current anymore, it’s the same value as the previous example.

Solve for the collector current

Ic = CTR x If = 120% x 8 mA = 9.6 mA

Solving for the circuit VCE

VCE circuit = Vcc – Ic x R2 = 5V – 9.6 mA x 2k = -14.2 V

The computed VCE is minus 14.2V. How come it is negative that there is no negative level on the supply voltages?…This is indicating that the circuit is saturation. Let us go back to the criteria above

VCE circuit > VCEsat device maximum

The computed VCE is very much below zero. The usual saturation voltage of Optocoupler is from 0.7V to below levels so no doubt that that circuit is operating at saturation.

Example 3

Let us replace R2 with 300 ohms in the above circuit and re-check the operation of the circuit.

Using Method 1

Ic = Vcc / R2 = 5V / 300 = 16.67 mA

CTR circuit = Ic / If = 16.67 mA / 8 mA = 208.33 %

The computed circuit CTR is higher than the device CTR. The circuit is operating at linear mode.

Using Method 2

Ic = CTR x If = 120% x 8 mA = 9.6 mA

VCE = Vcc – Ic x R2 = 5V – 9.6 mA x 300 = 2.12 V

The computed VCE is higher than the usual saturation voltage. The device is operating in the linear region.

With the two computation methods demonstrated above,  how to determine optocoupler operation is no longer a problem. By following these methods you can ensure that the circuit will operate as designed. If you want a complete optocoupler biasing course, click this. For designing optocoupler circuit from scratch, read this.