I'm attempting to build the following circuit to better understand how to use a NPN BJT as a switch.
What I've Tried
I'm calculating the current across the LED (in order to light it up) as:
5.2V* - 1.7V (LED drop) = 3.5V
3.5V / 17mA = 200Ohms
*NOTE - The power source is 5.2V because I'm using 4 AA rechargeables in series (at 1.3V each).
The problem I see when I push the button to turn the circuit on is that the transistor becomes very hot. I noticed this the first time because I could smell something. Then I touched it. Ouch! :) I'm assuming I'm providing too much current on the be (base to emitter) circuit.
Things I've Tried / Additional Problem
However, when I attempt to add resistance into the be circuit then the LED doesn't light up, even when my resistor gets down to a value as low as 47Ohms.
- Is it possible (due to ratios of current needed) to even power both sides of the circuit from the same power source? Or is it ridiculously difficult or something and not done?
- Can you help me understand the additional calculation(s) I should be using to power the circuit so my LED will light when I push the button?