Opto-isolators

V2.04 5-Aug-2004

1. Introduction

Opto-isolators, or opto-couplers, are made up of a light emitting device, and a light sensitive device, all wrapped up in one package, but with no electrical connection between the two, just a beam of light. The light emitter is nearly always an LED. The light sensitive device may be a photodiode, phototransistor, or more esoteric devices such as thyristors, triacs etc.

The cheapest kind have phototransistors. Below is a basic circuit diagram using one of these types (4N25):

The output of this circuit simply follows the input:

Note the slight curving of the square wave input. All opto-isolators will only work up to a certain frequency. Some are much faster than others. Make sure that the opto-isolator you use is fast enough for the signals you are putting through it - more details in section 4. The reason the rise time is slower than the fall time of the output waveform is that the rising edge is due to the 4k7 pull-up resistor, which has to discharge the capacitance in the opto transistor. If this needs to be speeded up, the 4k7 resistor value can be reduced, at the expense of using more current when the output is low.

When the LED is driven with a current of 10mA or so, it shines onto the phototransistor, which then starts to conduct (turn on). This takes the output voltage low. However much electrical noise is on one side, it can never be transmitted over to the other side. We may use an opto-isolator to send PWM signals from the low-power electronics side to the MOSFET drivers on the high-power side, and we may use them to transmit information from the high- power side back to the low-power side.

To complete the isolation of the low and high power sides, each must be powered by a completely separate battery. The high power side will be powered by the main 12v or 24v battery. The low-power side can be powered by a much smaller battery, maybe 6v.

2. Opto-isolator parameters

If you open a datasheet for an opto-isolator in a separate browser window, we can go through some of the parameters and describe what they mean. Click here to open a datasheet for the Sharp PC123 in another window, because we will be referring to it.

Collector-emitter voltage

This is the maximum voltage that can be present from the collector to the emitter of the receiving phototransistor (when it is turned off - no light) before it may break-down.

Creepage distance

This is physically how far a spark would have to travel around the outside of the package to get from one side to the other. If the package has contaminants on it, solder flux, or dampness, then a lower-resistance path can be created for noise signals to travel along.

Forward current

This is the current passing through the sending LED. Typically, an opto-isolator will require about 5mA to turn the output transistor on.

Forward voltage

This is the voltage that is dropped across the LED when it is turned on. Most normal diodes drop about 0.7v, but with LEDs it is typically 1 - 2 volts.

Collector dark current

This is the current that can flow through the output phototransistor when it is turned off.

Collector-emitter saturation voltage

When the output transistor is fully turned on (saturated), this is the voltage there will be between the collector and emitter.

Isolation resistance

This is the resistance from a pin in the input side to a pin on the output side. It should be very high.

Response time

Thee rise and fall times are the times that the output voltage takes to get from zero to maximum. The rise time is very much dependant on the load resistor, since it is this that is pulling the output up. Therefore this value is always quoted with a fixed load resistance. Note however that the value, 100 Ohms, is much less than you are likely to use in practice. This is another of the manufacturer's attempts to make the product look better than it is!

Cutoff frequency

This is effectively the highest frequency of square wave that can be sent through the opto-isolator. It is actually the frequency at which the output voltage is only swinging half the amplitude than at DC levels (-3dB = half). It is therefore linked with the rise and fall times.

Current Transfer Ratio (CTR)

This is the ratio of how much collector current in the output transistor that you get given a certain amount of forward current in the input side LED. It is affected by how close the LED and phototransistor are inside the device, how efficient they both are, and many other factors. In fact it is not a constant but varies wildly with LED forward current as we will see.

2.1. The graphs

The graphs are essential to see how the device actually performs, rather than how the manufacturer wants you to think it performs on initial reading of the front page parameters! We'll take an example and work it through the graphs. This has shown how a design is developed using the given parameters from a datasheet.

3. Types of opto-isolator

The opto-isolator in the example above was a simple photo-transistor output type. These are the cheapest types, although they're not always the most useful. For a start, the LED required 10mA of forward current. If you are driving it from a microcontroller, it may not be capable of sinking that much current, so you would then need a transistor to boost it. In this case, you may better off using a logic to logic opto-isolator. These require a 5 volt supply and will accept a logic level input. The output may also be logic-level, or it may still be open-collector like the example above. Logic level opto- isolators generally can be run at much higher frequencies. 10Mbits per second is typical.

4. Frequency and Rise & Fall times

We are likely to be driving at least some of the opto-isolators with PWM signals for speed control of the motors. What frequency are these PWM signals, and will the opto-isolators be able to cope with them?

The PWM signals are generated elsewhere in your circuit of course, so you may already have decided what frequency they will be. This frequency, together with the number of discrete speed steps that you have, determines how fast the opto-isolator must be. The diagram below show a PWM signal with 12 discrete speed steps, with a 1/12 level signal (signal A). This means the signal is high for one twelfth of the time:

The number of speed steps may be greater. Most microcontrollers with PWM outputs use an 8-bit register, giving 256 discrete speed steps.

The frequency of this PWM signal is the reciprocal of its total time period, tp. Fpwm = 1/tp. However, any circuit that transmits this must be able to respond to the single pulse. To respond to this, it must be able to respond to a frequency rather higher than Fpwm, that shown in orange in signal B. The frequency of this signal is

The 16 in the equation comes from the number of discrete steps, so in general, we can say that if we want n discrete steps, the opto-isolaator must be able to handle a frequency of

Most opto-isolator datasheets quote the rise and fall tmes of the opto-isolator outputs rather than a maxmum frequency. How can we use these values? The diagram below shows a more realistic signal from an opto-isolator.

 In this diagram, the rise and fall times are shown equal, but they are often not - especially with open-collector or open-drain type optos where an external pull-up resistor controls the rise time. It can be seen from this and the previous equations that

Therefore, if we are given an opto with defined rise and fall times, we can work out:

The maximum frequency for n discrete speed steps:

The maximum number of speed steps for a PWM frequency of FPWM:

Example

Question

The Hewlett Packard (Agilent) HCPL-3120 optocoupler has the following parameters:

tr = tf = 0.1μs

Given that the PWM frequency has been set at 25kHz to be above audible range, what is the greatest number of discrete speed steps that can be attained.

Answer

Using the equation

n = 400. Therefore n 8-bit PWM register with 256 discrete steps would work fine.

Note that even if we drove this optocoupler with more than 400 steps, all that would happen is that there would be no measurable difference between a setting of, say, 650, and 651, and if a 1024 discrete step PWM signal was set to a level 1/1024 (signal high for one 1024th of the time), then the opto would not respond, and the effective level would be zero.

 

More complicated PWM signals

This next diagram shows a PWM signal with 12 discrete steps, set at 3/12 level:

The three segments during which the signal is high are not necessarily next to each other. This signal will generate a smoother resultant speed in the motor than if all three pulses were next to each other. Some PWM generators may generate a signal like this, some may have all three pulses next to each other, in which case the required response time of the optocoupler need not be so high.

5. Some links to opto-isolator manufacturer's pages

Hewlett Packard (Agilent)
Sharp
Toshiba
Siemens (Infineon)



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