How to Use ST DB3 DIAC (DO-35): Examples, Pinouts, and Specs

The ST DB3 DIAC is a bidirectional semiconductor device manufactured by STMicroelectronics. It is primarily used for switching and controlling AC power. The DIAC (Diode for Alternating Current) operates by remaining non-conductive until a specific breakdown voltage is reached, at which point it conducts current in both directions. This characteristic makes it ideal for triggering TRIACs in phase control applications.

• Light dimmers
• Motor speed controllers
• AC power control circuits
• Triggering circuits for TRIACs
• Overvoltage protection circuits

The DB3 DIAC, housed in a compact DO-35 package, is widely used in low-power AC control applications due to its reliability and ease of integration.

The DB3 DIAC has a simple two-terminal configuration, as shown below:

The device is bidirectional, meaning there is no polarity requirement for its terminals.

1. Basic Triggering Circuit:
   The DB3 DIAC is commonly used to trigger a TRIAC in AC phase control circuits. Connect the DIAC in series with a resistor and capacitor to form an RC timing circuit. This configuration determines the phase angle at which the DIAC triggers the TRIAC.

2. Connection Guidelines:

   • Connect the DB3 DIAC between the gate of the TRIAC and the RC timing circuit.
   • Ensure the breakdown voltage of the DIAC matches the desired triggering voltage for the TRIAC.

3. Example Circuit:
   Below is a basic example of a light dimmer circuit using the DB3 DIAC:

   AC Mains
     ~
     |         R1
     |----/\/\/\/\----+----+
     |                |    |
     |               ---   |
     |               --- C1|
     |                |    |
     |                +----+
     |                |
     |               DB3
     |                |
     |                |
     |               TRIAC
     |                |
     |                |
     |                |
     |                |
     +----------------+
   

   • R1: Resistor to limit current and set the timing.
   • C1: Capacitor to control the phase angle.
   • DB3: DIAC to trigger the TRIAC.

• Voltage Ratings: Ensure the AC voltage in your circuit does not exceed the DIAC's breakdown voltage range (28V to 36V).
• Current Ratings: Do not exceed the maximum repetitive peak current (2A) to avoid damaging the device.
• Thermal Management: Operate the DIAC within its specified temperature range (-40°C to +125°C) to ensure reliability.
• Polarity: The DB3 DIAC is bidirectional, so there is no need to worry about terminal polarity during installation.

While the DB3 DIAC is not directly compatible with low-voltage DC systems like Arduino, it can be used in conjunction with an optocoupler or TRIAC to control AC loads. Below is an example Arduino sketch for controlling an AC load using a TRIAC triggered by the DB3 DIAC:

/*
  Arduino AC Load Control Example
  This sketch demonstrates how to control an AC load using a TRIAC triggered
  by the DB3 DIAC. The TRIAC is optically isolated from the Arduino using
  an optocoupler.

  Note: This code assumes the use of an optocoupler to interface the Arduino
  with the AC circuit. Directly connecting the Arduino to the AC circuit is
  dangerous and not recommended.
*/

const int triggerPin = 9; // Pin connected to the optocoupler input

void setup() {
  pinMode(triggerPin, OUTPUT); // Set the trigger pin as an output
}

void loop() {
  digitalWrite(triggerPin, HIGH); // Trigger the optocoupler
  delay(10);                      // Delay to allow TRIAC conduction
  digitalWrite(triggerPin, LOW);  // Turn off the trigger
  delay(990);                     // Wait for the next AC cycle
}

1. DIAC Not Triggering:

   • Cause: The RC timing circuit may not be configured correctly.
   • Solution: Verify the resistor and capacitor values to ensure the correct phase angle.

2. Overheating:

   • Cause: Excessive current or operation outside the specified temperature range.
   • Solution: Check the current and voltage ratings of the circuit. Add heat sinks or improve ventilation if necessary.

3. No Output from TRIAC:

   • Cause: The DIAC may not be reaching its breakdown voltage.
   • Solution: Ensure the AC voltage is sufficient to trigger the DIAC.

Q1: Can the DB3 DIAC be used with DC circuits?
A1: No, the DB3 DIAC is designed for AC circuits and requires alternating voltage to function properly.

Q2: What is the difference between a DIAC and a TRIAC?
A2: A DIAC is a bidirectional trigger device, while a TRIAC is a bidirectional power switch. The DIAC is often used to trigger the TRIAC in AC control applications.

Q3: How do I calculate the RC timing values for my circuit?
A3: The RC timing values depend on the desired phase angle. Use the formula:
[ t = R \cdot C \cdot \ln\left(\frac{V_{supply}}{V_{BO}}\right) ]
where ( V_{BO} ) is the DIAC's breakdown voltage.

By following this documentation, you can effectively integrate the ST DB3 DIAC into your AC control circuits.