Current and Voltage Measurements in a UV Photodiode
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
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
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
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 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
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