| Electronics > Beginners |
| 2n2222 and voltage dividers |
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| Tom18991:
See my simple circuit below. If I remove the 3k resistor from the circuit, the LED will start to show a tiny bit of light when the input voltage gets to around .6 volts. When I put the 3k resistor in the circuit, I was expecting that it would act as a voltage divider, such that the LED would start to glow when the input voltage was around 2.6 volts (or higher), since 2.6 volts * (3/13)=.6 volts. However, the LED starts to glow at a slightly different voltage, making me think I'm missing something about how the transistor is interacting with the voltage divider. Can anyone explain for me? TIA... |
| Nerull:
Your transistor is backwards and 12V is likely enough to cause reverse breakdown. The ON datasheet specs 6V as the emitter-base breakdown voltage. |
| rstofer:
Flip your LED and resistor to connect between V+ and the collector. Then connect the emitter to ground. |
| Jwillis:
You can put the LED on either side of the transistor. Since its being used as a switch it make no difference which side (collector or emitter) the load is on .But in this configuration the LED is backward. |
| Doctorandus_P:
As said before, you have connected the transistor in reverse. When drawing schematics it is always a good idea to stick as much as possible to a few simple rules. * Signals go from left to right. * Voltages go from top to bottom. You can build a few variants of this circuit. In the left schematic I drew it as an emitter follower. The emitter follows the base voltage ( - 600mV) and the led grows steadily brighter with more input voltage. In the middle circuit the transistor is used as a switch. The tranistor does not conduct current as long as the base is below 600mV, and if the base voltage gets above 600mV the LED wil quickly light up to full brightness. If you use higher values for R4 & R5 (100k & 27k) then the switching behaviour will be less pronounced. In the circuit on the right the LED will not light as long as the base is below 600mV + the LED voltage (approx 1.8V, depends on color of the LED). If the voltage at the base gets above 2V4, the transistor will act like a switch again and the LED will quickly light up to full brightness. I drew the schematic with KiCad and also attached it to this post. KiCad is a Free & Open Source schematic and PCB design program with no built in silly limitations. |
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