Voltage-divider triggering is a technique used to control the gate of a Silicon Controlled Rectifier (SCR) in a circuit. To understand how this works, let’s break it down into several key components and concepts:
### 1. **Basic SCR Operation**
An SCR is a type of semiconductor device used for switching and controlling power. It has three terminals:
- **Anode**: The positive terminal.
- **Cathode**: The negative terminal.
- **Gate**: The terminal used to control the SCR.
The SCR conducts (allows current to flow between the anode and cathode) only when a sufficient voltage is applied to the gate terminal relative to the cathode, and the anode-to-cathode voltage is greater than a certain threshold (called the breakover voltage). Once triggered and conducting, the SCR remains on even if the gate current is removed, until the current through the device falls below a certain level (holding current).
### 2. **Voltage-Divider Network**
A voltage divider is a simple circuit configuration that divides an input voltage into smaller output voltages using two resistors connected in series. The output voltage is taken from the junction of these two resistors.
The formula for the output voltage (\( V_{out} \)) of a voltage divider is:
\[ V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2} \]
where:
- \( V_{in} \) is the input voltage.
- \( R_1 \) and \( R_2 \) are the resistors in the divider network.
### 3. **Voltage-Divider Triggering in SCR Circuits**
In the context of SCR circuits, voltage-divider triggering is used to control the gate voltage and thus turn the SCR on or off. Here’s how it works:
1. **Designing the Voltage Divider**: You design a voltage divider network with resistors \( R_1 \) and \( R_2 \) such that the voltage at the junction of these resistors (the output voltage) is appropriate to trigger the gate of the SCR.
2. **Triggering the SCR**: The voltage at the gate (determined by the voltage divider) needs to be sufficient to turn the SCR on. When the input voltage (or some other controlling voltage) is applied to the voltage divider, it produces a gate voltage that triggers the SCR.
3. **Maintaining Conduction**: Once the SCR is triggered and starts conducting, it will stay in the conducting state as long as the anode-to-cathode current is above the holding current. The gate voltage is not needed to maintain conduction, but it must be properly controlled to ensure the SCR turns on when needed.
### 4. **Practical Example**
Imagine you have an SCR in a circuit that you want to control using a 5V supply. You can use a voltage divider to produce a suitable gate voltage for the SCR. Suppose the SCR requires 2V at the gate to trigger. You can choose \( R_1 \) and \( R_2 \) such that:
\[ V_{gate} = 5V \times \frac{R_2}{R_1 + R_2} = 2V \]
From this, you can solve for \( R_1 \) and \( R_2 \) to achieve the desired gate voltage. Once this setup is in place, when the 5V supply is applied, the voltage divider will provide 2V to the gate of the SCR, triggering it into conduction.
### 5. **Applications and Considerations**
- **Applications**: Voltage-divider triggering is often used in light dimmers, motor speed controls, and other applications where precise control of the SCR is necessary.
- **Considerations**: The resistors in the voltage divider must be chosen carefully to ensure the gate voltage is within the correct range for reliable SCR operation. Additionally, the impact of varying input voltages and resistor tolerances should be considered in design.
In summary, voltage-divider triggering in SCR circuits involves using a voltage divider to set the gate voltage of the SCR, ensuring that it turns on when the gate voltage reaches the necessary threshold. This method provides a simple and effective way to control SCR operation in various electronic circuits.