Constant voltage indicator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Indicators, detectors
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When it is required to measure a constant voltage without great accuracy, all kinds of light and sound indicators are used. The so-called "probe-screwdrivers" are popular for monitoring the constant voltage in a car (12 ... 24 V) with indicators based on LEDs.
I offer my probe circuit on non-deficient operational amplifiers K553UD2 (with field-effect transistors at the input and, accordingly, with a large input resistance). The scope of the device is automotive equipment, power supplies, a home laboratory of a radio amateur, etc., where it is necessary to control a constant voltage in the range of 5 ... 20 V.
The circuit implements two similar voltage comparison nodes. The reference level is the voltage across the zener diode VD1. The device is powered by a controlled voltage. Voltage levels are indicated by two LEDs (preferably different colors). But the number of control levels can be easily expanded by connecting several similar op amps in the same way. Instead of LEDs HL1 and HL2, capsules with built-in generators can be used as sound indicators.
If none of the LEDs HL1, HL2 is lit, then the controlled voltage is below 10 V. When only HL1 is lit, the voltage is within 12 ... 13 V, when only HL2 is lit, the voltage is in the range of 13.5 ... 15 8. If both LEDs light up or flicker, controlled voltage is more than 16,5 V.
All fixed resistors - type MPT-0,25, MF-25, tuning - multi-turn, SP5-1VB, SP5-2, SPZ-19 or similar with a linear characteristic of resistance change. Capacitors - KM6B. Zener diode VD1 is replaced by KS133A, KS133B. LEDs can be replaced with L-239EGW, KIPD18V-M or similar.
Establishing the node consists in setting the threshold for turning on each LED individually with trimming resistors R4 and R7. As a power source (and controlled voltage) during the adjustment, an adjustable stabilized source with a voltage of 5 ... 20 V is used.
Before the first turn on, the trimmer resistors sliders are brought to the Middle position. A voltage of 12 V is set on the power source. A sufficiently accurate voltmeter (multimeter) is connected in the DC voltage measurement mode. By adjusting the resistance R4, the LED HL1 is lit. The voltage is increased to 13,5 ... 14 V, HL7 is lit with a tuned resistor R2. Reduce the voltage to 10 V and make sure that the LEDs do not burn.
Author: A.Kashkarov, St. Petersburg
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