How Does the ICL8038 Function Generator Work? How Can Triangle, Sine, and Square Waveforms Be Produced?

In this Instructables article, I aim to explain how the ICL 8038 function generator works. This article is specifically designed for physiotherapy students in electrotherapy lectures and may also be useful for medical students and scientists studying electrophysiology.

Symmetrical Power Supply Layer

Transformer: 220V in, 12V out (two output terminals)

7809 Positive Voltage Regulator IC

7909 Negative Voltage Regulator IC

1. 330 µF Electrolytic Capacitor (2 pieces)
2. 100 µF Electrolytic Capacitor (2 pieces)

KP 304 Bridge Rectifier

1N4007 Diode

Heat shrink tubing

M3 screw and M3 nut

ICL8038 Layer

ICL8038

1. 100 kΩ (3 pieces)
2. 10 kΩ (1 piece)
3. 1 µF (1 piece)

3 Terminal Connector

2 Terminal Connector

The schematics of the power supply layer is shown in the figure.

First, securely mount the transformer onto the Perinax board to ensure stability during operation.

Carefully solder the cables to the input port, making sure to achieve a solid and reliable connection.

Afterward, use heat shrink tubing to isolate the input ports, providing additional protection against short circuits and environmental factors.

This step is crucial for maintaining the integrity and safety of the electrical connections

Transformer Outputs:

Our transformer has three output terminals. You will find a center tap along with two outer terminals. (These terminals are usually referred to as the primary or secondary winding outputs.) The center tap will serve as a reference point (split power supply amd providing a ground reference).

KP304 Rectifier:

The KP304 is a compact bridge rectifier consisting of four diodes arranged in a bridge configuration. This design allows for the conversion of alternating current (AC) to direct current (DC) efficiently. The rectifier features four terminals:

1. AC1 and AC2: These are the alternating current (AC) input terminals where you will connect the transformer outputs.
2. +: This terminal provides the positive DC output.
3. -: This terminal serves as the negative DC output.

Make the Connections:

1. Connect the Outer Outputs:
2. Take one of the outer output wires from the transformer and solder it to the AC1 terminal of the KP304 rectifier.
3. Connect the other outer output wire from the transformer to the AC2 terminal of the rectifier. This configuration allows the rectifier to utilize the full AC waveform for effective conversion.
4. Center Tap Connection:
5. The center tap of the transformer, can be left unconnected at this point.

The shorter leg of the electrolytic capacitors indicates the negative terminal. Additionally, a negative (-) symbol is marked on the side of the capacitor to denote the negative polarity.

Remember that the transformer has three outputs, with the center tap used as ground. The other two outputs will be used as +12V and -12V.

The electrolytic capacitor between +12V and ground should have its negative (-) leg connected to ground and the positive (+) leg connected to +12V.

Similarly, the capacitor between -12V and ground should have its negative (-) leg connected to -12V and the positive (+) leg connected to ground.

Carefully solder the transformer's center tap and the 330uF electrolytic capacitors onto the board, following the layout shown in the schematic. Ensure all connections are secure and properly aligned to avoid any electrical issues.

Role of 330 Microfarad Capacitor Between Bridge Rectifier Output and Regulators (7809 and 7909)

Filtering: This capacitor helps to minimize ripple in the smoothed DC voltage generated by the bridge rectifier, resulting in a more stable and consistent output voltage with reduced fluctuations.

Energy Storage: It temporarily stores energy to quickly respond to sudden changes in load. When there’s a sudden increase in load, the capacitor provides the necessary energy, helping to maintain a steady output voltage.

Noise Reduction: By filtering out high-frequency noise, the capacitor ensures that the input voltage supplied to the regulators is cleaner and more stable.

The 7809 voltage regulator features three pinouts. The first pin (Vin) is for the input voltage, the second pin is Ground, and the third pin (Vout) provides a stable 9V regulated output.

To ensure proper operation, connect the input voltage (typically +12V) to the Vin pin, following the placement of an electrolytic capacitor to filter any input noise. The second pin should be connected to the ground, serving as the common reference for both input and output voltages

The pin configuration of the 7909 negative voltage regulator differs from that of the 7809 positive voltage regulator.

The first pin is Ground, the second pin (Vin) is for the input voltage, and the third pin (Vout) provides a stable -9V regulated output.

Zhe first pin should be connected to ground.

Connect the input voltage (typically -12V) to the Vin pin.

Remember that the shorter leg of electrolytic capacitors indicates the negative terminal. Additionally, a negative (-) symbol is marked on the side of the capacitor to denote its negative polarity.

The electrolytic capacitor connected between the 7809 output (third pin, +9V) and ground should have its negative (-) leg connected to ground and the positive (+) leg connected to the +9V output.

The capacitor between the 7909 output (third pin, -9V) and ground should have its negative (-) leg connected to the -9V output and the positive (+) leg connected to ground.

Carefully solder the output pins of the 7809 and 7909 regulators, along with the 100µF electrolytic capacitors, onto the board, following the layout shown in the schematic. Ensure that all connections are secure and properly aligned to avoid any electrical issues

Function of Capacitors at the Output of Regulators

Output Stabilization: Capacitors at the regulator outputs minimize fluctuations in voltage, ensuring a stable output, especially during sudden load changes.

Temporary Energy Storage: They store energy temporarily to quickly respond to changes in load, such as when a circuit component activates.

Improved Frequency Response: These capacitors enhance the regulator's ability to handle high-frequency signals, allowing for effective operation across a wider frequency range.

Noise Filtration: They reduce high-frequency noise in the output, providing a cleaner voltage and ensuring stable operation of connected components

A diode is a semiconductor device that acts as a one-way switch for electrical current. It allows current to flow in one direction while blocking it in the opposite direction.

Diodes have specific markings to indicate their orientation, which is crucial for their proper functioning:

The anode is the positive terminal, and the cathode is the negative terminal of the diode. Current flows from the anode to the cathode.

The cathode is usually marked with a line or a stripe on the diode's body. This stripe indicates the direction in which current cannot flow.

When connecting a diode in a circuit, ensure that the anode is connected to the positive side of the circuit and the cathode is connected to the negative side or load. Reversing the connections will prevent the diode from functioning correctly.

Diode Orientation:

When connecting diodes at the output, ensure that they are oriented correctly.

The cathode side of the diode is marked with a line.

The anode (positive side) of the diode is unmarked.

Connection Points:

For the +9 V output, connect the anode of the diode to the +9 V terminal.

For the -9 V output, connect the anode of the diode to the -9 V terminal.

Function of Diodes Between +9 V and -9 V After the Regulators

Prevention of Voltage Drop:

The diode prevents reverse current flow between +9 V and -9 V, thereby avoiding unwanted voltage drops in the circuit. This enhances the safety and proper functioning of the regulators.

Protection Function:

In the event of a fault or incorrect connection (e.g., excessive current drawn by a component), the diode blocks reverse current, protecting the regulators from potential damage. This helps prevent harm to the components.

Feedback Prevention:

The diode prevents the mixing of output voltages. For instance, if a load circuit on the -9 V output tries to send current back to the +9 V output, the diode blocks this reverse flow, preventing feedback.

Load Distribution:

Incorporating a diode helps manage load distribution more effectively. This configuration allows for a more balanced current flow, especially when using symmetrical power supplies.

Connecting Output Cables of a Symmetrical Power Supply After Diodes

In a symmetrical (or dual) power supply, you typically have three main output terminals: positive voltage (+V), negative voltage (-V), and ground (GND). These outputs are used to provide both positive and negative voltages relative to a common ground. Here's how to correctly connect the output cables after the diodes:

The catode of the diode in the positive supply line will be connected to the +9V output cable. A red cable is used for the positive voltage.

The anode of the diode in the negative supply line will be connected to the -9V output canble. A black cable is used for the negative voltage.

The ground cable connects to the common ground point in the power supply. Ground serves as the reference point for both the positive and negative voltages. This ground is shared across both the +V and -V outputs, ensuring a balanced voltage supply. The ground is represented by a green cable.

The schematics of the ICL8038 layer is shown in the figure.

In electrotherapy lectures, fundamental waveforms are initially introduced, followed by a detailed discussion of the properties of therapeutic waveforms. Basic waveforms such as square, triangular, and sine waves serve as the foundation for understanding more complex therapeutic signals.

Function or waveform generators are designed to produce these essential waveforms. Monolithic function generators, in particular, offer the advantage of generating these basic waveforms with a minimal number of external components, which not only simplifies the circuit but also enhances its reliability.

In ICL9038 function generator, a voltage-controlled oscillator (VCO) generates triangular and square waves. The triangular wave is shaped by an on-chip wave shaper to produce a sine wave.

Sawtooth and pulse waveforms are created by adjusting the oscillator for an asymmetric duty cycle.

The ICL8038 precision waveform generator operates using both positive and negative power supplies.

Pin 6 (+V): This is the positive power supply pin, where the positive voltage (typically +5V to +15V) is applied. The exact value depends on the desired output amplitude and application.

I prefered +9V.

Pin 11 (-V): This is the negative power supply pin. If a dual power supply is used, a negative voltage (typically -5V to -15V) is connected here. In single-supply configurations, this pin is often connected to ground (0V). We prefered -9V.

Proper supply voltage is crucial for the stable operation of the ICL8038, as it directly affects the amplitude and shape of the generated waveforms.

Pin 10 : Timing capacitor

This pin is where an external timing capacitor (C) is connected to the ICL8038. The value of this capacitor plays a crucial role in determining the frequency of the generated waveforms. By adjusting the capacitance, the timing of the internal oscillator changes, which directly affects the frequency of the output signals.

I prefered 1uF timing capacitor.

The capacitor, in combination with external resistors, helps set the operating frequency range for the function generator.

Note: Timing Capacitor Frequency Range

1Hz – 100Hz: 1uF

100Hz-1KHz: 0.1uF

1KHz-10KHz: 0.01uF

10KHz-100KHz: 0.001uF

Pins 4 and 5: Duty Cycle/Frequency Adjustment

The frequency of the output waveform generated by the ICL8038 is directly proportional to the charging and discharging currents of the timing capacitor. To achieve the desired duty cycle, external resistors RA and RB can be selected appropriately. These resistors are connected to pins 4 and 5, respectively, allowing for precise control over both the frequency and duty cycle of the output signal. By adjusting the values of RA and RB, users can finely tune the waveform characteristics to meet specific application requirements.

I prefered 100 K resistors

In this article I do not focus on function of these pins in details. I give a brief description.

FM Bias: Sets the baseline frequency using a fixed voltage.

FM Sweep Input: Changes the frequency dynamically with an external voltage.

These terms describe how you can set and adjust the output frequency in an FM system, either by fixing it (FM Bias) or by varying it (FM Sweep Input).

Just connect pin 7 (FM Bias) to Pin 8 (FM Sweep Input)

Sine adjust pins are used to produce more accurate sine wave.

To keep the sine wave distortion below 1%, you need to connect a 100 kΩ resistor between Pin 12 (sine wave output) and ground (or the negative voltage, -VEE). I prefered this one.

Note: To achieve even better accuracy and reduce distortion to less than 0.5%, you need two 100 kΩ potentiometer.

The first potentiometer is connected between VCC (positive supply) and ground, and its middle pin (wiper) is connected to Pin 1. The second potentiometer is connected the same way, but its wiper goes to Pin 12. This setup improves the sine wave quality even further, reducing distortion to below 0.5%.

Pin 9: Square Wave Output

Pin 9 is where the square wave signal is output.

This pin is an open collector output, meaning it needs an external resistor to produce a signal.

To get the square wave, you need to connect a resistor between VCC (positive supply) and Pin 9.

Pin 2 (Triangle Wave) and Pin 3 (Sine Wave): These outputs are buffered outputs, meaning they are not directly connected through a transistor, and the output signal can be taken directly without needing an external resistor.

In the ICL8038 datasheet, you will find a test circuit that includes pulldown resistors connected to the outputs of the sine and triangle waveforms. A pulldown resistor serves to pull a pin down to ground level (0V), ensuring that the pin remains at a defined low state when not connected to any load. This connection significantly reduces unwanted noise signals that can affect the integrity of the output.

Utilizing a pulldown resistor on the sine and square wave outputs can help stabilize the output signal at a specific level when it is idle or in a high-impedance state. This is crucial for maintaining signal clarity and reliability, especially in applications where consistent performance is required.

While the sine and square wave outputs are not configured as open collector outputs, adding a pulldown resistor is advisable but may not be mandatory.

We prefer not to add pull down resistors.

When mounted on a pertinax board and the components and wires are soldered, the final state of the 8038 layer should look like the images

Output Frequency of ICL8038

fout= 0.3/(RC)

100Kohm=10^5 ohms

1uF=10^-6 F

RxC=10^5x10^-6=0.1

fout=0.3/(RC)

= 0.3/01

fout=3 Hz

The amplitude of square wave is VCC = +9V

The amplitude of triangular wave is 0.33 VCC= 0.33x9V=2.97V

The amplitude of sine waves is 0.22 VCC=1.98V

Unfortunately, I currently do not have access to an oscilloscope, but I have been able to record waveforms by using voltage dividers in conjunction with an Arduino. This method allows me to capture and analyze signal data with reasonable accuracy, despite the limitations of not having a dedicated oscilloscope.

I generated basic signals using fixed resistors with the ICL8038 and have now transferred the prototype I built on pertinax to a PCB. You can download the artwork file and create your own signal generator

In this article, I described the fundamental setup of the ICL8038 with fixed resistors. In the second article, you can find a detailed description of how to adjust the frequency, sine wave distortion, and duty cycle.

Link to second article