Simple Triac Flasher Circuit

This article shows you how to build a circuit that makes a normal light bulb blink on and off.

You can use a knob to control how fast the light blinks, from about 2 times per second to 10 times per second.

What is a Triac Flasher Circuit:

A Triac flasher circuit is an electronic circuit designed to produce periodic or flashing illumination in a connected load, typically an incandescent lamp.

The key component in such a circuit is the Triac, a semiconductor device that can control the flow of AC current.

Triac flasher circuits are commonly used for decorative lighting, signage or any application where a periodic flashing effect is desired.

Circuit Working:

Parts List:

The 1N4004 diode rectifies the mains input AC, and the resulting signal is directed towards a variable RC network stage.

This stage includes an electrolytic capacitor that, upon reaching full charge triggers the breakdown voltage of the diac DB3.

As the capacitor discharges through the diac it initiates the firing of the triac.

This firing sequence leads to the illumination, of the connected lamp producing a bright flash. Subsequently, the lamp shuts off.

A delay, predetermined by the 100k potentiometer, follows the illumination phase.

During this delay, the capacitor begins recharging towards the breakdown limit of the diac causing the lamp to pulse and then shut down.

This cyclic process of discharge, triggering, illumination, delay and recharge continues enabling the lamp to flash at the user adjusted frequency.

The 1k resistor in the circuit plays a crucial role in determining the current threshold at which the triac is supposed to fire.

This element helps in fine tuning the operational characteristics of the circuit.

Connection Details:

The mains AC phase is connected to the MT1 terminal of triac BT136 forming a series connection with a 40 watt bulb.

The neutral is linked to the MT2 terminal of the triac.

Additionally, the phase is connected to a 100k potentiometer through a 1N4004 diode and a 6.8k resistor.

The other end of the potentiometer is connected to the neutral through a 220uF capacitor.

This potentiometer capacitor junction is then linked to a diac.

The other end of the diac connects to the gate of the triac through a 1k preset.

Adjustability:

The 100k potentiometer serves as the primary control for adjusting the flashing rate of the bulb providing a wide range of frequencies.

Meanwhile, the 1k preset allows fine tuning of the flashing rate for precise adjustments.

Formulas and Calculations:

The above circuit relies on the charging and discharging time of capacitor C1 to control the switching of the triac BT136.

The time constant (τ) is a crucial factor that influences the flash rate, the formula is mentioned below:

Time Constant (τ):

The product of resistance R and capacitance C yields the time constant (τ), which is given in seconds (s).

It shows how long it takes for the voltage across the capacitor to reach a particular percentage roughly 63% of its ultimate value before charging or discharging.

The time constant formula is:

τ = RC

From the circuit, relevant components for calculating the time constant are:

• Resistor R = R1 100k
• Capacitor C = C1 220µF

Calculating the Approximate Flash Rate:

1. Calculate the time constant (τ):

τ = R1 * C1 = 100,000 Ω * 220 x 10^-6 F = 22 seconds

Note:

This is the time constant for a single cycle of the capacitors charging or discharging.

The cycles of charging and discharging are part of the actual flash rate.

2: Flash Rate Estimation:

A fundamental approximation for the flash rate (f) is about the reciprocal of twice the time constant (2τ), assuming equal charging and discharging periods.

This is so that the capacitor can only undergo one flash cycle of full charging and discharging.

Flash Rate (f) Estimation:

f = 1 / (2τ) = 1 / (2 * 22 s) = 0.022 Hz

Frequency to Flash Period Conversion:

The number of cycles per second, or frequency, is represented by the flash rate 0.022 Hz.

You may compute the flash period T, which is the reciprocal of the frequency, to provide a sense of the flash timing in seconds between flashes.

Flash Period (T):

T = 1 / f = 1 / 0.022Hz = 45.5 seconds

How to Build:

Building simple triac flasher circuit involves assembling the components according to the provided details.

Circuit Connection:

Triac Connections:

• Connect the MT1 terminal of the triac BT136 to the mains AC phase.
• Connect the MT2 terminal of the triac to the neutral.

Diode and Resistor Network:

• Connect the mains AC phase to a 100k potentiometer through a 1N4004 diode and a 6.8k resistor.
• Connect the other end of the potentiometer to the neutral through a 220uF capacitor.

Diac Connection:

• Connect the junction of the potentiometer and capacitor to one end of the diac.
• Connect the other end of the diac to the gate of the triac through a 1k preset resistor.

Lamp Connection:

• Connect the MT1 terminal of the triac in series with a 40 watt bulb.
• Connect the other end of the bulb to the neutral.

Power Supply:

• Connect the circuit to the mains power supply ensuring proper polarity.

Adjustments:

Set the 100k potentiometer to control the flashing rate of the bulb.

Fine tune the flashing rate using the 1k preset resistor.

Safety Precautions:

• Work on the circuit with the power off.
• Double check connections to ensure they match the provided instructions.
• Use insulated tools.
• If you are uncertain about any step or component seek guidance from an experienced individual.

Conclusion:

Always prioritize safety when working with electronic circuits, especially those involving mains voltage.

If you are unsure consider consulting with an experienced electronics enthusiast or a professional.

References:

Datasheet BT136