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Protect your UUT with power-supply decoupler

31 Oct 2012  | Raju Baddi

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The bench-top accessory in figure 1 can be installed 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.

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 VOUT 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 VIN 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.

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. Image below shows 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 article describes a general approach. You should verify that the various parameters meet the requirements of the power-recipient circuit and make appropriate modifications.

1. "G6A2," Omron Electronics.
2. Baddi, Raju, "Bi-Event Timer for Physics Lab," January 2012.

About the author
Raju Baddi is from Tata Institute of Fundamental Research in Pune, India.

To download the PDF version of this article, click here.

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