Fig. 1

Photodiode Circuits Operation and Uses

by Lewis Loflin

Photodiodes have many varied uses today both as light sensors and used for driving power MOSFETs when used in photovoltaic opto-couplers. This series of webpages will explore all of this.

Here we start with basic operation of a photodiode, its construction, and improving switching speed. In other pages we will explore both commercial devices and how to build your own circuits. What follows is a listing of related pages.

This is oriented towards the hobbyist or junior engineer in a community college. We are interested in practical uses more than just theory. Driving AC-DC motors or light level detection are just a few applications.

Fig. 1 illustrates the current and voltage relationship of a photodiode used as a light sensor. A photodiode is simply a PN silicon diode where light will generate a current proportional to light intensity on the PN junction depletion region. The photodiode is reversed biased where the Cathode goes to a positive voltage and th Anode goes to the negative side of the supply.

The graph shows the current to light relationship. Even in complete darkness a small current called dark current will flow. This property of often used to measure light intensity.

For more on this see Photodiode Op-Amp Circuits Tutorial.

Fig. 2

Fig. 2 illustrates using an ohm meter on a diode check setting. A photodiode will check much the same as any other silicon diode with a forward biased voltage drop of about ~0.5V.

Fig. 3

Fig. 3 shows how a photodiode can act as a mini solar cell that outputs a voltage in bright light. This property is used in photovoltaic opto-couplers to switch ON-OFF power MOSFETs which are voltage operated devices. This involves connecting several PN junctions in series to generate several volts when a LED emitter is switched on.

For more on this see MOSFET DC Relays Using Photovoltaic drivers

Fig. 4

Fig. 4 shows the physical construction on the silicon wafer. We have a thin anode (P-type) that allows light into the depletion region while the bulk of the material is N-type.

Fig. 5

Refer to Fig. 5. When a PN junction is reversed biased it forms a small capacitor. The depletion region becomes an insulator and acts as a capacitor dielectric between two conductors. The higher the reverse bias the wider the depletion region and less capacitance - the region varies based on the reverse bias voltage. Note: don't exceed the breakdown voltage!

Capacitance limits the switching frequency and distorts high speed waveforms so reducing capacitance is vital to increase high frequency response.

PIN photodiode intrinsic region reduces capacitance.

Fig. 6

Fig. 6 shows the construction of a PIN photodiode. The big difference is the introduction of an intrinsic region where the silicon is lightly doped or undoped totally - just a piece of silicon. This helps create a larger spacing between the outside conductors reducing capacitance.

Fig. 7

Fig. 7 shows two example photodiodes. The large surface area increases sensitivity, but will introduce higher capacitance sacrificing switching speed. Choosing between frequency response and sensitivity is a trade off depending on application.

Fig. 8

Fig. 8 shows a photodiode test circuit I constructed. D1 was a MRD105 type that is reversed biased. When the UV LED is turned on a small current Id is amplified by Q1 a NPN transistor to turn on a LED. What I found out from using several LEDs of differing colors was D1 was color sensitive in particular to blue and UV. This property is useful in digital cameras, etc.

Fig. 9

Fig. 9 illustrates the internal construction of 6N135 and 6N136 opto-couplers. The circuit is identical to my test circuit in using a reverse bias on the diode and a NPN transistor to boost current - it's likely using a PIN photodiode, but wasn't specific in the specification sheet.

Fig. 10

Fig. 10 shows a typical test connection for the 6N135 and 6N136. The diode can be connected to say 12-volts while RL would be connected to a 5-volts supply. Doing that will improve switching speed and still enable one to connect the output to a microcontroller. Leave out CL when using this for a real application.

Fig. 11

Fig. 11 illustrated the effect of capacitance on a waveform and why it is undesirable. These type opto-couplers would be used for interfacing sensors such as in fuel injections system, high speed robotics, etc.

Fig. 12

Fig. 12 AVAGO AFCL-5211T internal diagram with two complete emitter-detector pairs.

The reason we use opto-couplers in general is to interface differing voltage levels, voltage isolation between high voltage sensors and low-voltage micro-controllers, and noise immunity.

I hope this introduction was useful. Feel free to do more research into these devices.

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