Power decoupler to protect UUT

A desktop accessory shown in Figure 1 can be installed between an adjustable voltage experimental power supply and a breadboard or UUT (device under test) to protect the device from accidental overvoltage and reverse polarity damage. It obtains energy from the power supply to provide a pulse for a 5V dual-coil latching relay, which 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) contacts, and the latest pulse is passed to the relevant coil group. Its coil is rated for 5V, but it can work as low as 3.5V.

Figure 1. The heart of this protection circuit is a bistable latching relay that protects the load from damage due to overvoltage and incorrect polarity.

Under normal conditions, the relay will connect the energy of the power supply at the input voltage, transfer it to the load at 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 through the reverse biased D1 and D2.

Q1 forms a variable Zener function, with the base-emitter voltage overvoltage threshold set by the coarse and fine potentiometers. If the voltage is greater than the threshold (for example, the user accidentally touches the power supply voltage knob), the base-emitter voltage increases to the 0.6V required to turn on Q1. This action then turns on Q2, which in turn turns on Q3 through D1. Q3 draws current from C2 through the corresponding relay coil, opening the contact between VOUT and BR1, while closing the contact between the collector of Q4 and one of the ac input terminals of BR2, lighting the error LED. The charge on C2 enables the relay to complete this latching action even if it is disconnected from its power supply.

The potentiometer should be calibrated once, or adjusted with power on but no UUT connected. This method helps to quickly and easily set the overvoltage threshold of the decoupler. The voltage follower Q4 charges its emitter capacitance to no more than 4.5V, which is determined by the 5.1V Zener diode at its base. When the overvoltage condition is removed, short-pressing the RST (reset) push button switch discharges the Q4 emitter capacitance to the other relay coils, returning them to their normal position.

If a reverse polarity voltage is mistakenly applied between the VIN and ground terminals, Q3 is biased on through D2. BR1 applies the correct polarity to C2 and the relay coil, allowing Q3 to operate the relay for the overvoltage condition. BR2 can also light up the LED despite the reversed polarity.

The design includes a 47mH~500mH inductor and 1000μF~4700μF capacitor in the output stage to delay the rise of the output voltage and current. This step avoids damage to the receiving circuit during the relay delay time. The inductor should be selected with sufficient rated current and minimum dc resistance to obtain the desired load current.

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

The following equation calculates the voltage rise when a charge Q is stored in a capacitor C: V = Q/C. These equations can be used to calculate the values ​​of L and C required to operate the relay for a time T. Integrating the first equation over the time limit from 0 to T gives the charge. 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 hardly changes, which means that the voltage and current are almost constant. At this point, the charge is (I×T)/2 and I = (V/L)T.

The relay's data sheet will usually specify its operating time (Reference 1). Alternatively, it can be measured with an oscilloscope or with a two-event sequencing circuit (Reference 2). This circuit uses only one of the two sets of contacts. If your design requires a second protection circuit for the negative supply (NPN instead of PNP, reverse biased diode), you can cross-couple the extra contacts to the opposite supply so that a fault in either supply does not open both contacts.

This example describes a general solution. It is possible to verify the various parameters that meet the requirements of the power receiving circuit and make corresponding corrections.