Conversations on Electrical Engineering: Electricity - from simple to complex. Part 4.
Unlike a light bulb, the LED is a non-inertial device, so its radiation can be modulated, for example, in a human voice. Then the photo-recording device, located at a considerable distance, can highlight this speech. Consequently, it is possible to manufacture a relatively simple NEGOTIATING device using LED light.
By the way, why is the word "DIODE" clearly present in the name of this component? The fact is that if a light bulb with two leads is connected to the battery, so that it lights up, paying no attention to what output is connected to the "plus" and which is to the "minus", then For the LEDs everything is different! They have the polarity of voltage supply is of paramount importance. That's why when working with LEDs (their symbolic designation is shown in Fig. 8), it is necessary to strictly adhere to the following rules.
1. The voltage at which the LED works is at least 2 V and not more than 2.4 V respectively for "red" and "green"!
2. Connecting the LED (even for the shortest time) to a battery of the KBC type (4.5 V) directly - leads to a INSTANT INSTALLATION of the device!
3. If the correctness is observed in the connection, the LED is able to operate normally for many SOTTEN THOUSAND HOURS!
And what does it mean, the correct connection of the LED? It is shown in Fig. The dotted circuit is a functionally full "combat deuce," consisting of the actual LED and protective resistor Rs. The value of Rs. It is easy to calculate based on the following requirements.
The direct current through the LED, as a rule, should not exceed 5-10 mA. The voltage of the battery (most often it is the "Crohn") is 9 V. The voltage drop on that part of the concept circuit, which is surrounded by a dotted line, is taken equal to 4 V. Then, using the "combat deuce" obtained earlier and R included sequentially,
Well, what happens if we by mistake or absent-mindedly connect the "combat deuce" directly to the "Crohn" battery? The current through the LED, which naturally, will increase, but the voltage on the LED itself will practically not change! This is one of the interesting features of this device. But then the whole "excess" of tension will take over
The current in this case is I = 7000 mV / 200 Ω = 35 mA. The LED will then glow very brightly, but will remain operational for a certain, long enough time, which is certainly enough to realize the error, disconnect the circuit and insert a resistor into it. By the way, by making the resistor R adjustable, you can smoothly control the intensity LED glow. For example, as shown in Fig. Here, the minimum current through the LED is 10 mA, and the maximum current is 17.5 mA. In fact, Imax = (9-2) V / 400 Ω = = 7000 mV / 400 Ω = 17.5 mA.
But we can make a more interesting chain, for example, as in Fig. Here we see a MIXED connection. So, closing the switch SA2, we turn on the "red" LED, and by closing switch SA3 "green". But you can simultaneously close both switches. Then the total current I1 + I2 = 20 mA will flow through the resistor R. If you add another "combat deuce", which includes the "yellow" LED, you will get a light, which can be used for games.
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