Power-supply decoupler protects your UUT

Raju Baddi, Tata Institute of Fundamental Research, Pune, India - May 10, 2012

You can install the bench-top accessory in Figure 1 between a variable-voltage bench power supply and a breadboard or UUT (unit under test) to protect against accidental overvoltage and reverse polarity. It draws its power from the supply and pulses a 5V, two-coil latching relay to interrupt power to the load under abnormal conditions. The latching relay uses permanent magnets to hold the DPDT (double-pole/double-throw) contacts in the position, which the most recent pulse to the associated coil sets. It is nominally rated at 5V for the coils but operates at voltages as low as 3.5V.

Power-supply decoupler protects your UUT figure 1

In the normal state, the relay passes power from the attached supply at the input voltage to the load at the output voltage through the inductor and holds Q₃’s emitter at 0.6V less through BR₁. C₂ charges to 1.2V less than the input voltage. C₁ cannot charge through reverse-biased D₁ or D₂.

Q₁ forms a variable-zener function, which you adjust by using the coarse and fine potentiometers to set the overvoltage-threshold base to the emitter voltage. If the voltage should exceed the threshold—for example, if a user accidentally bumps the power-supply voltage knob—the base-to-emitter voltage increases to the 0.6V necessary 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 appropriate relay coil to open the contact to V_OUT and BR₁ and close the contact to the collector of Q₄ and an ac input terminal of BR₂, which lights the error LED. The charge on C₂ allows the relay to fully complete its latching action, even though it has disconnected its own voltage supply.

You should calibrate a dial for the potentiometers or adjust them with the power supply on but with the UUT load unconnected. This approach helps to quickly and easily set the decoupler’s overvoltage threshold.

Voltage follower Q₄ charges its emitter capacitor to no more than 4.5V, which the 5.1V zener diode at its base sets. After the overvoltage condition is cleared, momentarily pressing the RST (reset) pushbutton switch discharges Q₄’s emitter capacitor into the other relay coil, latching it back into its normal position.

If you accidentally apply reverse polarity to the V_IN and ground terminals, Q₃ biases on through D₂. BR₁ applies the normal polarity to C₂ and the relay coil to allow Q₃ to operate the relay for the overvoltage condition. BR₂ lights the LED despite the reversed polarity.
Talkback button

The design includes a 47- to 500-mH inductor and a 1000- to 4700-μF capacitor at the output stage to delay the rise of output voltage and current. This step avoids damage to the recipient circuit during the operating-time delay of the relay. Choose the inductor for sufficient current rating and minimum dc resistance for the load current you anticipate.

The following equation calculates the rise of current, I, through an inductor, L, as a function of time, T, when a voltage, V, exists across it: I=(V/L)T. The next equation calculates the rise in voltage of a capacitor, C, when a charge, Q, is deposited into it: V=Q/C. You can use these equations to calculate the required values of L and C if the relay were to operate in time T. You can obtain the charge by integrating the first equation with time limits zero to T. Because the voltage across the capacitor at the output must be within safe limits for the recipient circuit, the charge on the capacitor must be small enough that its voltage hardly changes, meaning that the voltage and the current are more or less constant. In this case, the charge is (I×T)/2 and I=(V/L)T. Click here to see the circuit layout.

The data sheet usually specifies the relay’s operating time (Reference 1). Alternatively, you can measure it with an oscilloscope or a dual-event timing circuit (Reference 2). The circuit uses only one of two sets of contacts. If your design requires a second protection circuit for a negative-supply voltage—with NPN swapped for PNP and the diodes reversed—you can cross-couple the extra contacts in series with the opposite supply so that a fault of either supply would disconnect both.

This Design Idea describes a general approach. You should verify that the various parameters meet the requirements of the power-recipient circuit and make appropriate modifications.

References