How to calculate SCR current-limiting resistor

The current-limiting resistor (Rs), is a resistor placed in series with the anode of the SCR in an overvoltage protection (OVP) circuit. The crowbar overvoltage protection (OVP) circuits can be used in a variety of power supply circuits, including series voltage regulator designed with op-amp, shunt voltage regulator, linear voltage regulator, buck converter, boost converter, flyback converters, and others power supply circuits. Crowbar circuits are typically used to protect sensitive components from overvoltage conditions by rapidly short-circuiting the output when an overvoltage is detected, triggering a protective action like blowing a fuse or activating a shutdown mechanism. As such the design of overvoltage protection circuit involves careful selection of current limiting resistor $R_{\mathrm{s}}$ which is explained with calculation of the value with worked out example. 

The primary function of current limiting resistor $R_{\mathrm{s}}$ in the overvoltage protection circuit shown below is to limit the current flowing into the SCR when it is triggered. This helps to prevent excessive current from damaging the SCR or other components in the circuit. By controlling the amount of current, the resistor ensures the SCR operates within its safe current handling capacity, allowing the protection circuit to function effectively without overloading or causing damage to the components. 

To calculate the resistor $R_5$ in an SCR-based overvoltage protection circuit, where the cathode is grounded, the gate is connected to the over voltage protection RC delay circuit (comprising $R_{}$ and $C$), and $R_5$ is connected between the anode and the output voltage rail, we need to understand the function of $R_{\mathrm{s}}$ in the context of the SCR's operation.

Function of $R_{\mathrm{s}}$:

The resistor $R_5$ plays an important role in limiting the current flowing through the SCR once it is triggered. This resistor helps in controlling the on-state current through the SCR and ensures the proper behavior of the circuit. It also contributes to protecting the SCR from excessive current once it is conducting.

Calculating $R_{\mathrm{s}}$:

1. Understand the SCR Triggering Process:

   • When the overvoltage condition is detected, the Zener diode (connected at the gate) starts to conduct, charging the capacitor $C$ in the RC delay network.
   • Once the voltage across the capacitor reaches the gate trigger voltage (V_GT), it triggers the SCR into conduction.
   • The SCR will now conduct, allowing current to flow from the anode to cathode.

2. Calculate the Current Through the SCR (I_SCR):

   • The current flowing through the SCR once it is triggered is governed by the output voltage (or load voltage) and the load impedance.
   • Assume that the load connected to the SCR has a resistance $R_{\text{load}}$. The current through the SCR, $I_{\text{SCR}}$, can be approximated as:
   $$I_{\text{SCR}} = \frac{V_{\text{out}}}{R_{\text{load}}}$$

   where $V_{\text{out}}$ is the output voltage of the circuit (typically 5V in your case).

3. Design the Current-Limiting Resistor $R_5$:

   • The resistor $R_{\mathrm{s}}$ should limit the maximum current that flows through the SCR once it is triggered. It helps to prevent excessive current from flowing through the SCR, which could damage the component.
   • To design $R_5$, we need to consider the maximum current that you want to allow through the SCR (usually limited by the SCR’s current rating or the desired maximum load current).
   • Using Ohm's Law, $R_{\mathrm{s}}$ can be calculated as:
   $$R_5 = \frac{V_{\text{out}}}{I_{\text{max}}}$$

   where $I_{\text{max}}$ is the maximum current you wish to allow through the SCR.

4. Example Calculation:

   • Let’s assume that the output voltage $V_{\text{out}}$ is 5V and the maximum allowable current $I_{\text{max}}$ is 2A (as per your requirement).
   • Using Ohm’s Law:
   $$R_5 = \frac{5V}{2A} = 2.5 \, \Omega$$

   So, the value of Rs would be 2.5Ω to limit the current to 2A.

Considerations:

• Power Dissipation in $R_{\mathrm{s}}$: Ensure that $R_{\mathrm{s}}$ can handle the power dissipation, which can be calculated as:

  $$P = I_{\text{max}}^2 \times R_5$$
  • For example, with $I_{\text{max}} = 2A$ and $R_{\mathrm{s}}=2.5\mathrm{\Omega }$, the power dissipated in $R_5$ would be:
  $$P = 2^2 \times 2.5 = 10W$$

  So, the resistor must be rated for the appropriate power dissipation, in this case, at least 10W.

• Selecting $R_{\mathrm{s}}$: Choose a resistor with a power rating that exceeds the calculated power dissipation, and a resistance value that ensures the desired current limiting.

Summary:

• To calculate Rs, use the formula $R_{\mathrm{s}}=\frac{V_{\text{out}}}{I_{\text{max}}}$, where $V_{\text{out}}$ is the output voltage (5V) and $I_{\text{max}}$ is the maximum current (2A).
• In the example, $R_5$ would be 2.5Ω.
• Ensure that Rs is rated for the power it will dissipate, and choose a resistor with an appropriate power rating.