Pulse shaper for tachometer calibration. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automobile. Speedometers and tachometers
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A simple square waveformer operating from a 50 Hz AC household power supply has been described in [1]. There it is described in sufficient detail about the principle of its operation and the timing diagrams of the pulses obtained at the output are given. In the magazine "Boats and Yachts" it was recommended to use such a shaper instead of a standard signal generator and a frequency counter when checking and calibrating non-contact electronic tachometers for outboard boat engines, the schemes of which were once published in [2, 3]. Calibration in this case was possible only at one frequency of 50 Hz, which corresponds to the shaft speed of a single-cylinder two-stroke outboard engine of 3000 min.-1.
The scheme of the shaper improved by me is shown in the figure. It allows you to check and calibrate the electronic tachometer at two frequencies (50 Hz and 100 Hz), which improves the accuracy of the speed measurement. This is ensured by a guaranteed error in maintaining the frequency of alternating current in the city power supply network of ±0,4% (according to clause 5.6 [4]) and the use of a class 1,5 pointer measuring head in the tachometer. Commercially available electronic tachometers are only 10 percent accurate.
Shaper circuit
The improved shaper differs from the original one in that a pulsating voltage is supplied to the base of the transistor VT1 through resistors R2 and R4 from the rectifier on diodes VD1-VD4, which is powered by the winding of the transformer T1 with an average output. With open contacts of the SA1 switch, the rectifier is half-wave and the output pulses follow at a frequency of 50 Hz (3000 min-1), and when closed, it is full-wave, and the pulse frequency is 100 Hz (6000 min-1). The differentiating circuit C2R5 allows you to get not rectangular, but pointed pulses at the output, similar in shape to those removed from the high-voltage wire of the engine ignition system.
The device is easy to manufacture, does not contain any scarce radio elements and does not require adjustment. As T1, a unified armored power transformer TPP236-127 / 220-50 was used. It can be replaced by another with an overall power of at least 9 VA with two secondary windings for 15 ... 20 V or with one secondary winding for double the voltage, having a tap from the middle. Transistor VT1 can be any of the MP25, MP26 series or a modern low-power pnp transistor with a permissible collector-emitter voltage of at least 60 V. An npn structure transistor can also be used, but in this case, the polarity of switching on diodes and oxide capacitor C1 should be changed.
In addition to the above, I recommend using a pointer indicator in tachometers, which, unlike a digital one, adequately displays the dynamics of changes in the engine speed.
Literature
- Lezin A. Prefix for receiving rectangular pulses. - Radio, 1971, No. 1, p. 51.
- Egorov M. Electronic tachometer suitable for any engine. - Boats and yachts, 1973, No. 4, p. 66-68.
- Belyatsky Yu., Savin I. Electronic tachometer with extended ranges. - Boats and yachts, 1981, No. 1, p. 52-55.
- Interstate standard GOST 13109-97. Standards for the quality of electrical energy in general-purpose power supply systems. - URL: so-ups.ru/ fileadmin/files/company/rn-tpolitics/frequency/specdocs/sto_standard/GOST_13109-97.pdf.
Author: N. Elmanovich
See other articles Section Automobile. Speedometers and tachometers.
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