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I have a darlington Transistor circuit. Like in the schematic i attached. When i power up the 3V3 from my raspberry, the led is not on. When Doing Danke same with 12v instead of 5v, it Works perfectly.scematic the r3 is 82ohms for 5v And 470ohms for 12volts.

What is wrong With the circuit? Thanks

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  • \$\begingroup\$ How much current does LED need ? \$\endgroup\$ – Golaž Mar 13 '15 at 18:05
  • \$\begingroup\$ Well, to start, an MJE2955 is not a Darlington. And at 5 volts the LED will only get ~2mA. Less if it's a white. \$\endgroup\$ – WhatRoughBeast Mar 13 '15 at 18:57
  • \$\begingroup\$ What kind of LED do you have? What is its forward voltage drop & its current requirements? \$\endgroup\$ – brhans Mar 13 '15 at 19:12
  • \$\begingroup\$ Actually there are plenty of leds. The voltage drop is 5v @ 500mA. T1 in combination with T2 is not a darlington neither? What do I have to change to make the current flow? \$\endgroup\$ – Joel Mar 13 '15 at 19:16
  • \$\begingroup\$ No, T1 in combination with T2A is not a Darlington. A Darlington configuration looks like en.wikipedia.org/wiki/Darlington_transistor \$\endgroup\$ – alexan_e Mar 13 '15 at 20:18
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You probably want R2 to be more like 68~100 ohms than 7K for a 500mA load, and R1 more like 510 ohms to 1K. That corresponds to a beta of around 10 for the PNP and 20 or so for the NPN.

I presume there are many R3 or else you'd never see 500mA.

Even with those values you should be able to see some light from the LEDs, so maybe something else is wrong.

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  • \$\begingroup\$ R1 and R2 are both 1k. Why does there have to be a change for the resistors when switching 5v instead of 12v? What betas do I have to use when doing my calculations, if there are two or more mentioned in the data sheet? \$\endgroup\$ – Joel Mar 14 '15 at 18:23
  • \$\begingroup\$ Additionally, how do you calculate R2 to be ~100ohms? \$\endgroup\$ – Joel Mar 14 '15 at 18:26
  • \$\begingroup\$ The forced betas used in the Vce(sat) specifications (usually 10) can be used, you can usually get away with a bit less base current (higher beta) if the hFE is high (the 2955 is not very high gain) at that current (maybe 20) and still have the transistor well on. 100 ohms corresponds to a beta of 500mA * 100 ohms/(5-0.6-0.1)V = 50/4.3 = 11.6 (on the PNP), assuming 5V supply, 0.6V Vbe, 0.1V Vce(sat) for the NPN. 7K would be more like 800, which is not happening. \$\endgroup\$ – Spehro Pefhany Mar 14 '15 at 20:46

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