Unnoticeable errors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur
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Generations of radio amateurs grew up learning from literature, errors in which, when reprinted, remain to this day. If it did not deceive people today, such information would not be worthy of attention.
"Firefly" in the switch (Fig. 1) magazine MK 11/89 p.46 [1]. It is clear that the LED will inevitably be broken by the reverse voltage of the network, the value of which reaches 300 V, and only the forward voltage wave is used for lighting. By adding the diode (or LED) shown in the dotted line, we can save the situation. It should be noted that a neon lamp is preferable here: it selects more power to glow with a lower passing current and a larger voltage drop.
How can you effectively use an LED for signaling from the mains voltage? The answer is simple - be sure to use two different LEDs in pairs (and select the color of each as needed, depending on the semantic load of this alarm).
Figure 2 showsfive-signal-two-wire transmission scheme (tested at the boiler house of the Ladyzhinsky State Enterprise "ENZIM"). Due to the fact that light and sound signaling are needed in two places, it was necessary to use few cable cores - this simplified installation and commissioning. In addition, one of the signals (green “normal”) lights up even if there are no violations of the parameters at the facility. This way, a power loss will be noticed by operators. On the other hand, one of the two signal cores of the cable is continuously monitored.
Loud bells often contain a diode that passes only one half-wave of the supply voltage to the winding. It was this call that made it possible to combine emergency parameters from two cable cores and feed them to one point using diodes VD5, VD6 and VD7, VD8. Ballast resistors R1...R4 limit the current in the LED circuit. Pairs of reverse-connected LEDs not only save each other from the reverse half-wave of the mains voltage, but also signal their parameter very well together and separately (using a common ballast resistor).
In this scheme, it was possible, by saving cable cores and resistors, to reliably transmit an alarm about a violation of the operating parameters of the installation.
Purpose of switching contacts:
- relay K1 (normally energized) signals a pressure violation;
- relay K2 (closed in normal mode) signals a temperature violation; one of the contacts of switch S1 in the closed state signals the removal of protection (adjustment mode);
- toggle switches S2 and S3 allow the operator to turn off the sound signaling during the setup period.
In low-voltage circuits, using back-to-back pairs of LEDs is even easier. Figure 3 showsbattery terminal polarity indicator circuit, which can be attached to the “positive” “crocodile” connected to the charger. The glow of the red LED confirms the correctness; the inflorescence near the red wire signals an error. Poor contact causes the LED to flash intermittently when connected. It should be remembered that the LED consumes current from the battery, so even in recharging mode, you cannot leave the device unattended for more than 2 weeks. The conductor connecting the terminals through the ballast resistor R1 must be thin and have silk insulation, so that if the insulation is chafed during many years of use, this conductor can burn out harmlessly.
Figure 4 showscircuit of the output stage of the transistor ULF, reprinted in many reference books [2,3]. This circuit will begin to work without huge distortions only after connecting the resistor indicated by the dotted line, when the bases of the input transistors reach the potential of half the supply voltage.
Figure 5 showscompound emitter follower circuit from [4].
In order for the transistors to open, it is necessary to cut (remove) the base resistor and turn it on as shown in the dotted line. This scheme would not have attracted so much attention if it had not been used in the dialing unit of “stamped” telephone sets.compound cascade (Fig. 6). An analysis of the operation of such a cascade shows that more correct signal transmission will occur when connecting the transistors in Fig. 7, in which the first transistor “monitors” the relationship between the input and output signals with its BE transition, and the second, being a current amplifier, simply increases the gain of the entire cascade. Of course, both transistors must be rated for the circuit voltage. The first of them should be with a lower noise level and a higher gain.
The circuits in Figs. 5 and 6 modified in this way will work more correctly (the linearity of signal transmission will increase).
References:
- Krylov A. "Firefly in the switch//Model designer. -1989. No. 11. - P.46.
- Tereshchuk PM et al. Semiconductor receiving and amplifying devices. Ref. 4th ed. -K.: Science. thought, (p. 397, fig. VI.24, a).
- Berezovsky M.A., Pisarenko V.M. A short guide for radio amateurs. K.: Tekhnika, 1975 (p. 227, fig. 24, b, p. 225, fig. 21).
Author: N.P. Goreiko
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