July 17, 1997

Protection circuit covers a wide range

Lee R Watkins, Motorola Inc, Phoenix, AZ

Even if you take all the appropriate precautions to prevent transient feedthrough from the main power source, voltage transients can sometimes bypass all the safeguards and damage products and equipment. Protecting all products under test in a lab is difficult because of the many types of products and the associated multitude of biasing requirements. For this application, a suitable overvoltage- and transient-protection circuit needs to operate from about 3 to 95V and from a few microamps to several amps, preferably without the aid of additional power supplies.

A circuit that provides overvoltage and transient protection for electrically biased products connects a detector in series with the power supply and the product (Figure 1). The detector circuit (Figure 2) is rea so nably small and connects easily to the product or circuit you need to protect. The detector derives its operating power from the product's power supply, except for a 15-µA load from a small battery. When the detector's status changes to a tripped condition, only a manual reset can change that status.

You typically set up the detector with the detection voltage just higher than the supplied voltage. The detection voltage is continuously variable so you can set it to within a few millivolts of the actual power-supply voltage if necessary. After normal setup and before any transients occur, the green LED is lit, and the power supply connects to the product through a relay contact, RLY₂, which provides a path of near-zero resistance between the power supply and the product under test. An overvoltage condition activates a latching relay, RLY₁, which disconnects the power supply from the product, turns off the green LED, and turns on the red LED. This condition does not change until you push the reset switch. Depressing the reset switch is the only way to restore the connection from the power supply. On/off cycling of the power-supply voltage, which can occur during an electrical storm, does not affect the status of the detector.

In Figure 2, the 3.5V, 1.2-Ahr lithium battery powers two CMOS ICs, IC₁ and IC₂. The battery also supplies the base-drive current to Q₁ when the circuit detects an overvoltage condition. When RLY₁ toggles, the circuit removes power to the relay, which remains in the latched condition. Under normal operation, which is the monitoring mode, the current drain on the battery is approximately 15 µA. This drain allows for continuous operation from the battery for approximately nine years. The circuit satisfies all other power requirements by using the product's power supply, and the circuit does not depend on the power supply's voltage, as long as this voltage is greater than 4V.

Figure 2 shows RLY₁ in the normal untripped position. In this position, the power supply provides power to the product under test through the contacts of the power relay, RLY₂, and to the green LED. D₂ and D₃ are constant-current diodes and supply approximately 9 mA total to D₁. This current is independent of the power-supply voltage. Zener diode D₄ holds the base of Q₂ at a constant voltage, and D₅ controls the current through D₄ at a constant level of approximately 4.5 mA. R₁, R₂, or R₃ in series with R₄ and R₅ divides the power-supply voltage, depending on the setting of S₁. This resistor network determines the detection circuit's trip-point voltage.

For example, consider a required bias voltage of 50V and a desired trip level of 50.1V. In this case, you set the voltage divider to the 30-to-75V range by connecting S₁ to R₂, set the power supply to 50.1V, and slowly adjust R₄ until trip occurs. Then, set the power supply to 50V and press S₂, thereby resetting the circuit to the normal state.

Now, when the voltage on the power supply exceeds 50.1V for any reason, the voltage level on Pin 1 of IC₂ exceeds the Schmitt-trigger level, and the output level of Pin 2 changes state. Note that IC₂ inverts the polarity of Pin 2's output so that the final output at Pin 4 is the proper polarity. This polarity inversion is necessary because IC₁ is a JK flip-flop that triggers only on a positive transition.

The output of IC₁ Pin 1 then toggles and provides a drive to the base of Q₁, which transfers the contacts of RLY₁. Once transferred, the contacts of RLY₁ latch and disconnect the power. This action conserves battery power and prevents the circuit from resetting upon the removal of the overvoltage condition. The upper contacts of RLY₁, which normally provide power to the base of Q₂ to energize RLY₂, open, thereby opening the connection between the power supply and the product. The contacts of RLY₁ also energize D₆, D₇ (which lights the red LED), and D₈.

The circuit detects an overvoltage condition in approximately a few nanoseconds. The time required to actually open the circuit depends on the response times of RLY₁ and RLY₂. These times range from 10 to 15 msec. (DI #2056)

| EDN Access | Feedback | Table of Contents |