Power supply decoupler circuit diagram for protecting UUT

LC03-3.3 SOP8 TVS Static Protection 3.3V

Photocoupler

A desktop accessory as shown in Figure 1 can be installed between the adjustable voltage test power supply and the breadboard or UUT (device under test) to protect the device from accidental overvoltage and reverse polarity. It draws energy from the power supply, pulses a 5V dual-coil latching relay, and cuts off the power supply to the load under abnormal conditions. The latching relay uses a permanent magnet to attract the DPDT (double pole double throw) contact, and the latest pulse is passed to the associated coil set. Its coil is rated at 5V, but can operate down to as low as 3.5V.

Figure 1. The core of this protection circuit is a bistable latching relay that prevents the load from being damaged by overvoltage and incorrect polarity.

Under normal conditions, the relay will connect the energy of the power supply at the input voltage, pass the load to the output voltage through the inductor, and keep the emitter of Q3 below 0.6V through BR1. C2 is charged to a voltage 1.2V lower than the input voltage. C1 cannot be charged by reverse biased D1 and D2.

Q1 constitutes a variable Zener function that sets the overvoltage threshold of the base radiation voltage by adjusting the coarse adjustment and trimming potentiometers. If the voltage is greater than the threshold (eg, the user inadvertently hits the supply voltage knob), the base pulse voltage is increased to 0.6V required to turn Q1 on. This action then turns Q2 on, turning Q3 on through D1. Q3 draws current from C2 through the corresponding relay coil, opens the contacts of VOUT and BR1, and closes the contact of Q4 collector and BR2 with an ac input, lighting the wrong LED. The charge on C2 enables the relay to complete this latching action, even if it is disconnected from its own power supply.

The scale of the potentiometer should be calibrated once or adjusted when the power is applied but the UUT is not connected. This method helps to quickly and easily set the overvoltage threshold of the decoupler. Voltage follower Q4 charges its emitter capacitance to no more than 4.5V, which is determined by its base 5.1V Zener diode. When the overvoltage condition is removed, briefly press the RST (Reset) button switch to discharge the Q4 emitter capacitor to the other relay coils to return to the normal position.

If a reverse polarity voltage is applied between VIN and the ground terminal, Q3 is turned on by the D2 bias. BR1 adds the correct polarity to C2 and the relay coil, allowing Q3 to operate the relay for overvoltage conditions. Although the polarity is reversed, BR2 can also illuminate the LED.

The design includes a 47mH~500mH inductor and a 1000μF~4700μF capacitor in the output stage to delay the rise of output voltage and current. This step avoids damage to the receiving circuit during the delay of the relay's operating time. Select the inductor with sufficient current rating and minimum dc resistance to achieve the desired load current.

The following equation calculates the rise of current I through inductor L when there is a voltage V, which is a function of time T: I = (V / L)T.

The following equation calculates the rise of voltage when a charge Q is stored in capacitor C: V = Q / C. These equations can be used to calculate the L and C values ​​required for the relay to operate during time T. By integrating the first equation over the time limit of 0 to T, the charge can be obtained. Since the voltage on the output capacitor must be within the safe limits of the receiving circuit, the charge on the capacitor must be small enough that the voltage is almost unchanged, which means that the voltage and current are almost constant. At this time, the electric charges are (I × T) / 2 and I = (V / L) T.

The relay's data sheet usually specifies its operating time (Reference 1). Alternatively, you can measure with an oscilloscope or use a dual event timing circuit (Reference 2). This circuit uses only one of two sets of contacts. If your design requires a negative power supply (with NPN instead of PNP, diode reverse bias) using a second protection circuit, you can cross-couple the additional contacts to the opposite power supply so that any one power supply failure will not cause two The contacts are broken.

This example is about a general scenario. Various parameters that meet the requirements of the power receiving circuit can be verified and corrected accordingly.