RC Delay circuit design in Crowbar Overvoltage Protection Circuit

Here it is explained the role of RC Delay circuit in Crowbar Overvoltage Protection Circuit and how to calculate their values. The circuit diagram along with the RC delay in Crowbar Over Voltage Protection circuit is shown below.

The resistor (R) and capacitor (C) are part of the RC delay circuit used to create the time delay before the SCR (Silicon Controlled Rectifier) is triggered in a crowbar overvoltage protection circuit. This over voltage protection circuit is placed at the output of voltage regulator like Series Voltage regulator using transistor or Series Voltage Regulator with Op-Amp.

Here's RC delay works?

1. Resistor Calculation for Gate Triggering:

• The R resistor is calculated using Ohm's Law to limit the current flowing into the gate of the SCR. This ensures that the gate trigger current (IGT) is not exceeded, as well as to ensure that the Zener diode (if used) is providing a stable voltage for triggering.
• The formula you referred to:

• Where:
  • $V_{\text{Zener}}$is the Zener voltage (which provides a stable voltage to trigger the gate).
  • $V_{\text{GT}}$ is the gate trigger voltage of the SCR.
  • $I_{\text{GT}}$ is the gate trigger current, which is the current needed to trigger the SCR.
• This resistor ensures that the SCR gate gets the proper current to turn on, and it protects the gate from excess current that could damage the SCR.

2. RC Delay Circuit:

• The RC circuit (with the resistor $R$ and capacitor $C$) is used to introduce a time delay before the SCR gate is triggered.
• How it works:
  • When an overvoltage occurs, the capacitor begins to charge through the resistor.
  • If the overvoltage persists for long enough (as determined by the RC time constant), the voltage across the capacitor rises and triggers the gate of the SCR.
  • The RC time constant is given by: $$\tau = R \times C$$ 
  • The time constant determines how long it will take for the capacitor to charge and trigger the SCR gate.
• If the voltage spike is short or transient, the capacitor won't charge enough to trigger the SCR, thus preventing false triggering.

3. Gate Triggering and Delay:

• The R resistor ensures the proper current is delivered to the gate of the SCR.
• The RC circuit ensures a time delay to avoid triggering the SCR due to brief transients.

How to calculate R and C values?

To calculate the Rg and C values for a 20μs delay in the SCR gate triggering circuit, we need to use the RC time constant formula:

where,

• $\tau$ is the time constant (20μs).
• $R$ is the resistor value.
• $C$ is the capacitor value for the delay.

Steps for Calculating R and C:

1. Time Constant for Delay: Since we need a delay of 20μs, the time constant $\tau$ should be 20μs.

2. Choosing Suitable Zener Diode: For a 5V output, we need to choose a Zener diode with a voltage slightly higher than 5V (to trigger the SCR at the appropriate time). A Zener diode with a value of 5.1V would be a good choice, as it provides a stable voltage for triggering the SCR. We’ll use this value to calculate the voltage across the capacitor at the time of triggering.

3. Gate Trigger Voltage (VGT) of SCR: We need to find the gate trigger voltage ($V_{\text{GT}}$) for the SCR. For the S8006L (which is used here), this value is typically 1.2V (gate trigger voltage).

4. Gate Trigger Current (IGT): The gate trigger current ($I_{\text{GT}}$) is also needed to calculate $R$. For the S8006L, the typical $I_{\text{GT}}$ is 20mA.

Step 1: Calculate $R$

The resistor value $R$ ensures that the gate gets the appropriate trigger current. Using the formula from before:

where:

• $V_{\text{Zener}}$ = 5.1V (chosen Zener voltage).
• $V_{\text{GT}}$ = 1.2V (gate trigger voltage of SCR).
• $I_{\text{GT}}$ = 20mA (gate trigger current).

Substituting the values:

So, the R resistor value is 195Ω.

Step 2: Calculate the Capacitor (C) Value for 20μs Delay

Now we calculate the capacitor value $C$ using the time constant formula $\tau = R \times C$:

where:

• $\tau$ = 20μs (time constant for 20μs delay).
• $R$ = 195Ω .

Substituting the values:

So, the C value is approximately 100nF (rounded to a standard capacitor value).

Summary:

• R (resistor): 195Ω.
• C (capacitor): 100nF.

These component values will provide you with a 20μs delay in the SCR gate triggering circuit.

If you want to fine-tune the delay, you can adjust either the R resistor or the C capacitor to meet your exact timing requirements.

Following are related tutorials which might be useful to this circuit which teaches circuit to boost or limit current in power supply circuits.

• How to design Series Voltage Regulator with Current Limiting
• How Current Boosting in Voltage Regulator Works