# Design and Construction of an LED-Based Excitation Source for Lock-In Thermography
## 2 Hardware Design
### Circuit Topology
The shunt voltage is referenced to ground by the averaging differential amplifier formed by the right TLV272 operational amplifier. As this amplifier operates below unity gain, capacitors are added in the feedback network to avoid oscillations. The TL431 adjustable voltage reference IC on the right offsets the output of the differential amplifier by 2.5 V and thus avoids saturation of the amplifier. The 168 k\(\Omega\) resistors are composed of a 100 k\(\Omega\) and a 68 k\(\Omega\) resistor each. The number of LED strings can be varied to accommodate a different number of LEDs. The value of the 168 k\(\Omega\) resistors then has to be modified to maintain an equivalent parallel resistance of 56 k\(\Omega\) of the resistors connected to the LED strings. For example, the value of these resistors has to be changed to 112 k\(\Omega\) when two strings are in use. The left TLV272 operational amplifier forms a PI current controller, which controls the IRFP244N output transistor via a discrete gate driver. When a larger number of LEDs is used, the current of all strings can be raised up to 8 A. This maximum permissive current is limited by the power dissipation of the output transistor. The forward voltage of an LED string can be as high as 55 V, which is in turn limited by the maximum rating of the components in the 15 V supply. This means that LED arrays with an electrical power of up to 440 W can be driven by this circuit. With the two 2N7000 MOSFETs on the top left, either an internal or an external reference value can be fed to the current controller. To minimize the effects of charge injection in these MOSFETs, relatively high values are used for the gate resistors. They are controlled by an Arduino Nano microcontroller (not shown in Figure 4). The two upper trimming resistors allow to adjust the maximum LED current. They are designed so that the LED current through one string can be adjusted in the range between 0.7 A and 1 A with an input voltage of either 3.3 V or 5 V at the external input or 10 V at the internal input. The current range of 0.7 A to 1 A allows the use of alternative LEDs with a maximum forward current of down to 0.7 A. To allow a fast switching of the output, the lower 2N7000 can short the reference voltage to ground level. This switching cannot be slowed down by large value gate resistors, therefore the gate charge injection has to be compensated by an inverted charge injection. This is done by the cascode amplifier formed by the two BC547 transistors on the bottom left. A common emitter inverter circuit was also investigated, but it has been proven to be too slow to effectively compensate the injected pulse when switching. The Arduino Nano microcontroller does not provide a direct analogue output. Instead, it provides a pulse-width modulation (PWM) functionality, which is used to modulate the LEDs. By directly manipulating the corresponding registers of the ATMEGA328P microcontroller, the PWM frequency can be set to a value of 7.8 kHz, which is an order of magnitude above the frame rate of common infrared cameras. To obtain an analogue value from the PWM signal, a second order Bessel lowpass filter with a cutoff frequency of 100 Hz is applied. This functionality is provided by a third operational amplifier (not shown in Figure 4). The filter has a DC amplification factor of two, resulting in a higher voltage range of up to 10 V. The goal of this measure is to minimize the relative influence of the residual output voltage when the output is pulled to ground level. As the excitation source has a high optical output power, an interlock is added for connection to a safety switch. If the switch is opened, the PHP79 NQ08 LT MOSFET switches off the output. By feeding the interlock state to the controller, the excitation source can be programmed in a way that the output does not turn on when the switch closes again, but has to be enabled again by a remote command.