Do you notice a pop sound on the speaker when you turn on the amplifier? This is happening due to the sudden current rush from the amplifier transformer to the amplifier board. To solve this problem, we are using soft start circuits. In this article, I’m sharing a simple soft start circuit for an amplifier transformer without any relay.
Use of Soft Start Circuit
- ✅ No speaker thump or pop up sound.
- ✅ Increase components Lifespan.
- ✅ Prevent sudden high current rush to Components.
What is Soft Start Circuit for Amplifier Transformer?
From the name it self clear that the device turns ON in Soft mode. This circuit provide low voltage output when power On condition and after few seconds full voltage current is passed through it. In amplifier transformer case, when the power is ON the voltage will be less and the Amplifier board get small voltage to Turn On, after few seconds the 230V reaches to transformer and give full power to output.
Amplifier Transformer Soft Start Circuit Diagram
Components Required
| Designator | Component Name / Part Number | Description / Specifications | Quantity |
|---|---|---|---|
| U1 | MOC3041M | optoisolator | 1 |
| TR1 | MAC224 | Triac (Typically 400V to 800V, 40A) | 1 |
| D1 | ZY15V | Zener Diode, 15V | 1 |
| D2, D3 | 1N4007W | Standard Recovery Rectifier Diode (1000V, 1A) | 2 |
| R1 | 47R/20W | High power Inrush Limiting Resistor, 20W | 1 |
| R2 | 10k | Power Dropper Resistor, 5W | 1 |
| R3 | 47R | Standard Resistor | 1 |
| R4 | 270R | Triac Gate Resistor | 1 |
| R5 | 15k | Standard Resistor | 1 |
| C1 | 220uF | Electrolytic Capacitor, 35V | 1 |
| F1 | FUSE | Safety Fuse, 3A | 1 |
| DIP1 | Switch | Main Power SPST Switch | 1 |
| J3, J4 | AC IN | Mains AC Power Input Terminals | 2 |
| J1, J2 | AC OUT | Soft started AC Power Output Terminals (to Transformer) | 2 |
Working Principle of the AC Soft Start Circuit
The Soft start circuit protect the electronic components of amplifier by limiting the sudden current spike due to power On. The 47 Ohms 20 watt resistor initially limits the high voltage inrush current passing to the transformer.
The initial state the Triac is not active and the current is passing through the 47 ohms resistor only. When the connected 220uf capacitor gets fully charged through the Resistor of 10K then the optocoupler IC get activated and turn On the Triac.
this delay in current rush protect the electronic components from sudden voltage rush. So this way the pop up sound of amplifier get removed.
How the MOC3041 opt isolator Controls the Triac
- When switch On current passing through the 20 Watt resistor and 10K 5 watt resistor to capacitor.
- The capacitor get charged in few seconds, and power On the internal LED of the MOC3041 IC.
- When the Internal LED turns On the current passing trough the IC and the Triac get voltage to conduct.
- When the Triac get enough current, it coducts the Ac voltage.
Why High Power Resistors Are Used in the Design
The high-power resistor is 47 ohms, 20 watts, connected across the Triac. This Resistor has a major role in limiting the initial current when the power is ON. After a few seconds, a large current flows through the Triac, not through the Resistor. This way soft start function successfully executed.
Safety Considerations When Working With Mains Voltage
This circuit uses direct AC current, so be careful and don’t touch any part of the circuit or components while it is in operation. Provide a proper shield on the circuit to protect it from accidental contact with the cabin.
Also Check – Other Soft start circuit
Frequently Asked Questions
The Large current push across the transformer while the power ON time. This is major problem in Audio amplifier circuits to creating pop up sound.
The relay is designed to work on fixed voltage range, if the voltage is low or relay contact damage will sop working the circuit. So the optoisolator IC is good choice.
The MOC3041 is not directly driving the high power transformer. This IC is used for Controlling the Triac. The Triac control the Current flow through the transformer. The MOC3041 can be suitable only when paired with an appropriately rated external triac
