Current and Voltage Measurements in a UV Photodiode

Introduction

UV Photodiodes are constructed and operate in similar way to other photodiodes and solar cells. When light falls onto them, electrons are mobilised producing either a current, a voltage or both depending on the operating mode.

This application note will look at each mode giving the advantages and disadvantages of each with reference to UV photodiodes.

Voltage and Current Mode

A UV photodiode can be used as a solar cell as shown below:

When light falls on the photodiode, a photodiode current is generated which flows from Cathode to Anode of the photodiode. This current can be used to power a load as shown by resistance R in the above circuit. The ammeter and voltmeter are used to monitor the current and voltage in the circuit. To get the maximum power output from the photodiode requires the correct value of R which depends on the characteristics of the photodiode and may change under different light levels.

Using one of our UV photodiodes in the mode is not very useful as the amount of power that can be generated is very small (of the order of nano watts). The output current and voltage are also non-linear with respect to the input light levels so this circuit makes it difficult to make UV measurements.

Voltage Mode

By removing the resistor in the above circuit, the current is reduced to zero and just a voltage is produced.

The output voltage produced in this way from our UV sensors is of the order of 50mV under fluorescent lights and of the order of 1V under direct bright sunlight. This makes it easy to make a simple measurement by simply connecting the UV sensor directly to a digital voltmeter. Unfortunately, the output is completly non linear and the relationship is not specified in any of the data sheets. It is therefore not possible to make anything more than crude UV measurements using this method. It is useful though to make a quick check of the operation of the devices.

Current Mode

Current mode is the standard method for making measurements with our UV sensors. In this mode, the photodiode is shorted so there is no voltage across the part. The current then produced by the sensor is directly proportional to the light levels falling on the sensor and is linear over many orders of magnitude.

The typical current levels produced are in the nA range which is far below what most digitial multimeters will measure. It is therefore necessary to amplify the current produced. The standard way to achieve this is with a transimpedence amplifier which converts the current produced into a voltage.

The above circuit requires a rail to rail input and output op-amp which can operate from 5V and has a high input impedence. Note that the UV photodiode has been inverted so as to produce a positive output with rising light levels. A resistor R value of 10Mohms will give an overall gain of 10⁷ V/A. A capacitor of around 100pF can also be added to reduce the noise seen at the output.

The output from the op-amp can be amplified further if required and then output to a digital voltmeter or fed into the A to D input of a microcontroller.