This circuit is AC Lamp Flasher using SCR which works directly on 230V AC mains.
It is simple and is with low cost project so it is useful for decoration light, warning lamp and festival lighting.
Basically, the circuit uses SCR for switching and it also uses RC timing network, because of that the lamp turns ON and OFF automatically.
Therefore, the flashing effect is created and the speed of flashing depends on resistor and capacitor values.
Circuit Working:
Parts List:
| Components | Values | Quantity |
|---|---|---|
| Resistors (All resistors are 1/4 watt) | 2.7k, 22k | 1 each |
| 1k | 2 | |
| Potentiometer 100k | 1 | |
| Capacitor | Electrolytic 100uF 25V | 1 |
| Diode 1N4007 | 3 | |
| SCR BT151 | 1 | |
| AC Load 230V AC Bulb 40W to 200W Incandescent Lamp | 1 | |
| AC Input 110V or 230V AC | 1 |
First, the above circuit works directly on 230V AC mains, so one should be take a proper safety for giving it a try.
when AC supply is given the capacitor C1 starts charging and the charging path is through R1, R2 and diode D2.
Meanwhile, SCR is in OFF condition and so the AC load (bulb) is OFF and as time passes the capacitor voltage increases slowly.
When capacitor voltage reaches gate trigger voltage of SCR then current flows through R3 into the gate terminal and therefore, SCR turns ON.
Now once SCR turns ON it allows current from AC input to pass through load and as a result the bulb glows.
However, during negative half cycle of AC, SCR turns OFF because SCR works only in forward direction and then again capacitor starts charging in next cycle.
Thus, the process repeats continuously and so the bulb keeps flashing.
Diode D1 and D2 help in proper charging and discharging direction and diode D3 protects SCR from reverse voltage and spikes.
Therefore, controlled flashing is achieved using simple components.
Formula with Calculation:
Time period of flashing mainly depends on R and C values.
Basic RC time formula: T = R × C
where,
- T is the time in seconds
- R is the resistance in ohms
- C is the capacitance in farads
Calculation as per circuit:
- R (total timing resistance VR1) is 100k ohm
- C1 is 100 microfarad = 100 × 10^-6 farad
- T = 100000 × 0.0001 T = 10 seconds
So lamp will approximately flash every 10 seconds.
If we reduce R or C value the flashing becomes faster and if we increase R or C value then flashing becomes slower.
SCR Gate Current Formula:
Gate current formula:
Ig = (Vc – Vg) / R3
Lets take:
- Vc is 10V
- Vg is 0.7V
- R3 is 1k
Ig = (10 – 0.7) / 1000
Ig = 9.3 mA
BT151 typical gate trigger current is from 5 to 15 mA, so 9 mA current is safe for our circuit.
How to Build:
To build a AC Lamp Flasher Circuit using SCR follow the below connection steps:
- Start the circuit by collecting all the circuit parts.
- SCR anode pin connect to to the junction of resistor R2 and diode D1 and D2 network.
- Cathode pin connect to one end of AC input supply.
- Gate pin connect to junction of R3 and R4 timing network.
- Diode D3 Connect across SCR for protection.
- Capacitor C1 positive terminal connects to resistor network side.
- Negative terminal connects to AC input supply.
- VR1 center pin is connected to one end of resistor R1 and R2, VR1 bottom pin is connected to one end of resistor R3.
- AC Load one side connects to AC input phase.
- Other side connects to SCR anode path.
Safety Tips:
- This circuit works on high AC voltage of 230V or 110V, so it is dangerous.
- Always use proper insulation.
- Do not touch when power is ON.
- Use fuse for protection.
Conclusion:
To conclude, this AC Lamp Flasher Circuit using SCR is simple and effective.
It uses few components and it works directly from mains supply.
Therefore, it is suitable for home decoration and signal lamps.
Also by changing resistor or capacitor value the flashing speed can be adjusted easily.
So beginners can experiment and learn about SCR working.
Finally, this project helps to understand RC timing and thyristor switching clearly in practical way.
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