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I'm trying to make a circuit that will light the led with progressive intensity one second after the circuit is powered. For that I created an RC net with tau equal to 1 sec. At 1 second I expect the Vb to reach 63% of the dc voltage(1.2v) which is ~0.7v, that will forward bias the transistor and the led will start lighting. The problem I got is that the voltage at the test point gets stuck at 555 mv when I add the transistor. I'd expect it to eventually reach the 1.2v. Moreover, this doesn't happen if I reduce the RC values to, i.e 1K and 1mf, for maintaining the tau constant.

Why the voltage at the base of the transistor doesn't reach the 1.2v?

circuit

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    \$\begingroup\$ How much base current does your transistor need to light the LED? How much current can it get through a 1M resistor? Do you see the problem? \$\endgroup\$ – Dave Tweed Jun 8 at 11:04
  • \$\begingroup\$ I thought that the current at the led was determined by the 5v and the 470 ohms resistor, which gives 10mA. \$\endgroup\$ – perencia Jun 8 at 11:11
  • \$\begingroup\$ Well, yes, assuming that the transistor has minimal voltage drop. But what does it take to achieve that? \$\endgroup\$ – Dave Tweed Jun 8 at 11:14
  • \$\begingroup\$ You seem to have a fundamental misunderstanding about how bipolar transistors work. You need significant current flowing into the base terminal, and a 1 megohm resistor probably will not provide enough current. \$\endgroup\$ – Elliot Alderson Jun 8 at 12:22
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To get the BJT to work properly you need enough current to flow through the base terminal, and in your case due to the high resistance you are only getting: $$ I= V/R $$ $$ I = 1.2/10^6 = 1.2 \mu A$$

Thus the base current is too low to function. While when you reduce the resistance to 1K ohms only, the base current will be 1.2mA which is sufficient enough.

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