How to select the appropriate input resistor for a Photo Coupled SSR?

Selecting the appropriate input resistor for a Photo Coupled Solid State Relay (SSR) is a critical task that directly impacts the performance and reliability of your electrical circuits. As a Photo Coupled SSR supplier, I've encountered numerous customers facing challenges in this area. In this blog, I'll share some insights on how to make this crucial selection.

Understanding Photo Coupled SSRs

Before delving into the selection of input resistors, it's essential to understand the basic working principle of Photo Coupled SSRs. These relays use an opto - coupler to isolate the input and output circuits. When an input signal is applied to the LED in the opto - coupler, it emits light, which is then detected by a photosensitive device on the output side, triggering the relay to switch.

The input circuit of a Photo Coupled SSR typically consists of an LED. To control the current flowing through this LED, an input resistor is required. If the current is too high, it can damage the LED; if it's too low, the relay may not switch properly.

Factors Affecting Input Resistor Selection

Forward Current Rating of the LED

The forward current rating ($I_F$) of the LED in the Photo Coupled SSR is a key parameter. This value is specified in the datasheet of the SSR. For example, the CPC1017NTR has a recommended forward current for proper operation. The input resistor is chosen to limit the current through the LED to a value within its safe operating range.

The formula to calculate the input resistor ($R_{in}$) based on the forward current is:

$R_{in}=\frac{V_{in}-V_F}{I_F}$

where $V_{in}$ is the input voltage, $V_F$ is the forward voltage drop across the LED, and $I_F$ is the desired forward current.

Input Voltage

The input voltage ($V_{in}$) is another crucial factor. It can vary depending on the control signal source. Common input voltages include 3.3V, 5V, 12V, and 24V. If the input voltage is too high and the input resistor is not properly selected, excessive current will flow through the LED, potentially causing damage.

Forward Voltage Drop of the LED

The forward voltage drop ($V_F$) across the LED also varies from one SSR to another. It depends on the type of semiconductor material used in the LED. For instance, the TLP176G has its own specific forward voltage drop value, which can be found in its datasheet. This value is used in the resistor calculation formula mentioned above.

Step - by - Step Selection Process

Step 1: Determine the Input Voltage

First, identify the voltage of the control signal source. This could be a microcontroller output, a sensor output, or any other control signal. Make sure to measure the actual voltage accurately, as there may be some variations in the power supply.

Step 2: Find the Forward Current and Forward Voltage Drop from the Datasheet

Refer to the datasheet of the Photo Coupled SSR. Look for the recommended forward current ($I_F$) and the forward voltage drop ($V_F$) of the LED. These values are usually provided in the electrical characteristics section of the datasheet.

Step 3: Calculate the Input Resistor

Use the formula $R_{in}=\frac{V_{in}-V_F}{I_F}$ to calculate the value of the input resistor. For example, if the input voltage $V_{in} = 5V$, the forward voltage drop $V_F = 1.5V$, and the desired forward current $I_F = 10mA$, then:

$R_{in}=\frac{5 - 1.5}{0.01}=350\Omega$

Step 4: Consider Tolerance and Power Rating

When selecting the input resistor, consider the tolerance of the resistor. A resistor with a lower tolerance will provide more accurate current control. Also, calculate the power dissipated by the resistor using the formula $P = I^2R$. Make sure to choose a resistor with a power rating higher than the calculated power to avoid overheating.

Special Considerations

Temperature Effects

The forward voltage drop of the LED and the resistance of the input resistor can be affected by temperature. As the temperature increases, the forward voltage drop of the LED may decrease, and the resistance of the resistor may change. It's important to consider these effects, especially in applications where the temperature varies significantly.

Noise and Interference

In some electrical environments, there may be noise and interference on the input signal. To mitigate these issues, you can add a bypass capacitor in parallel with the input resistor. This capacitor helps to filter out high - frequency noise and provides a more stable input signal to the SSR.

Application - Specific Examples

Industrial Automation

In industrial automation systems, Photo Coupled SSRs are widely used to control high - power loads. For example, in a conveyor belt system, the SSR may be used to control the motor. When selecting the input resistor, you need to ensure that the control signal from the Programmable Logic Controller (PLC) can properly drive the SSR. If the PLC outputs a 24V signal, and the SSR has a forward voltage drop of 2V and a recommended forward current of 15mA, then the input resistor can be calculated as:

$R_{in}=\frac{24 - 2}{0.015}\approx1467\Omega$

Home Automation

In home automation applications, such as controlling lighting or appliances, Photo Coupled SSRs are often used. The control signals may come from a smart home hub, which usually outputs a 5V or 3.3V signal. When using a DPST SSR in such an application, the input resistor selection should be based on the specific characteristics of the SSR and the control signal voltage.

Conclusion

Selecting the appropriate input resistor for a Photo Coupled SSR is a multi - faceted process that requires a good understanding of the SSR's electrical characteristics, the input signal source, and the application environment. By following the steps outlined in this blog and considering the special factors, you can ensure that your SSR operates reliably and efficiently.

If you're in the process of selecting Photo Coupled SSRs or need assistance with input resistor selection, we're here to help. Our team of experts has extensive experience in this field and can provide you with tailored solutions. Feel free to get in touch with us for procurement and further technical discussions.

References

• Datasheets of CPC1017NTR, TLP176G, and DPST SSR.
• "Solid State Relay Handbook" by various industry experts.
• Electrical engineering textbooks on circuit design and power electronics.