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Simple circuits sort out the highest voltage

-November 09, 2006

In a water-cooled power converter, analog-output sensors measure the cooling water temperature at three locations. If any of the three temperatures rises above a preset threshold, an alarm sounds and attracts the attention of the system's operator. When the alarm activates, knowing which measurement site has reached the highest temperature saves troubleshooting time and prevents system damage. The circuit in Figure 1 delivers an analog-output voltage equal to the highest of three input voltages that drives a display for continuous temperature monitoring. LED indicators identify which of three sensors shows the highest temperature. An external adjustable-threshold comparator (not shown) monitors the analog-voltage output and activates an audible alarm.

Each of three analog input signals spans a range of 0 to 10V. Driven by the highest-voltage input, which you apply at IN1 in this example, operational amplifier IC1A functions as a voltage follower with diode D1 in its feedback path. The op amp's open-loop gain divides the diode's forward-voltage drop to a fraction of its nominal value, producing an "ideal diode" with a voltage drop of millivolts.

Op amps IC1B and IC1C function as high-input-impedance inverting comparators. Each "sees" the highest input voltage on its inverting input and one of two lower input voltages, IN2 and IN3, on its noninverting input and delivers an output voltage near that of the negative-supply-voltage rail. Thus, only IC1A delivers a positive-voltage output to MOSFET Q1's gate, and IC1B and IC1C deliver negative outputs to the gates of Q2 and Q3. Q1 turns on, lighting LED D4 and drawing approximately 5 mA to develop 11V across R3, which guarantees that Q2 and Q3 and their corresponding LEDs remain off. The voltage that develops across R1 represents the largest voltage of the three inputs, and resistor R4 and capacitor C1 form a lowpass filter that reduces high-frequency noise that the sensor cables pick up. Voltage follower IC1D buffers the filter's output voltage. Figure 2 shows the results of an LTSpice simulation featuring three sinusoidal inputs and the resultant analog output summed with a small dc-offset voltage for clarity.

The breadboarded circuit works as designed. Given its electrically noisy location near a 300-kHz, 30-kW switched-mode power converter, it uses slow-switching 1N4004 diodes to avoid malfunctions, which the rectification of stray high-frequency interference introduces. In less noisy environments, use any small-signal diode whose peak-inverse voltage exceeds at least 30V. Most varieties of operational amplifiers work well in the circuit, but for greater high-frequency immunity, use a JFET-input quad op amp, such as Texas Instruments' TL084.

Although the circuit's prototype uses red-LED indicators, LEDs of other colors work well. To change the LEDs' current to another value, change the values of R2 and R3, keeping approximately the same 3-to-2 ratio. For example, values of 1.8 kΩ for R2 and 1.2 kΩ for R3 drive the "on" LED with approximately 10 mA. If you increase the LED current, note that the resistors continuously dissipate power. For greatest reliability, choose resistors rated for twice the calculated power dissipation.

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