# 2.1VDC forward voltage needed for LED. What do I need to do to the circuit shown?

Discussion in 'Power Electronics' started by nbtone, Oct 14, 2016.

Oct 14, 2016
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2. ### ronv AAC Fanatic!

Nov 12, 2008
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It depends on the current you want for the LED.

3. ### SLK001 Well-Known Member

Nov 29, 2011
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What purpose do the transistors serve?

4. ### crutschow Expert

Mar 14, 2008
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Provide a return path for the LED current depending on which way the switch is positioned.

5. ### SLK001 Well-Known Member

Nov 29, 2011
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All that's needed for that is a ground. The transistors serve no useful purpose.

6. ### crutschow Expert

Mar 14, 2008
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And how would that work when you throw the switch?
The two LEDs are in one package with only two leads.

#12 likes this.
7. ### SLK001 Well-Known Member

Nov 29, 2011
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Oh, yeah... I was looking at only one path. It would be cheaper to use two separate LEDs, but that isn't the question and maybe isn't even possible for the application.

#12 likes this.

Oct 14, 2016
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9. ### ronv AAC Fanatic!

Nov 12, 2008
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Hmm, Seems like it should have kind of worked. But lets see.
The LED's want about 20 ma and drop about 2.1 volts. So basically 4.5 - 2.1 / .020. About 120 ohms for the top resistor.
For the transistors to turn on "hard" they need about 1/10 the collector current for the base. Since the LED will hold the voltage to 2.1 volts and the transistor base emitter drop is about .6 volts you can calculate R base. 2.1 - .6 / .002. About 750 ohms.

10. ### djsfantasi AAC Fanatic!

Apr 11, 2010
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Given @ronv 'S analysis, apply it to your circuit.

In the switch position as shown, you will pass the current through a 1k8 Ω base resistor. This will apply only 0.8ma to the resistor, insufficient for it to switch the 20ma the LED needs. Since it does not light, I surmise the LED needs more than 8ma.

Secondly, with an 82 Ω resistor on the LED, you'll have 30 ma of current. More than it's max rating. Have you damaged the LED?

11. ### nbtone Thread Starter New Member

Oct 14, 2016
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I tested the LED with 2 AA batteries, and it is okay. If I have 1.1VDC at the LED now, shouldn't I need a smaller resistor in place of the 82 ohm one? If I put a 120 ohm resistor, the voltage will be even lower, won't it?

12. ### ronv AAC Fanatic!

Nov 12, 2008
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The LED voltage should be 2.2 volts if the voltage on top is 4.5 volts like in your diagram.

13. ### AnalogKid Distinguished Member

Aug 1, 2013
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Two resistors in series from Vcc to GND.
One end of LED connected to the center node.
Other end of the LED connected to the switch pole connection
One switch throw connected to Vcc
Other switch throw connected to GND

ak

14. ### nbtone Thread Starter New Member

Oct 14, 2016
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Maybe I'll try putting around a 40 ohm resistor in place of the 82 ohm rsistor to see what happens?

15. ### ronv AAC Fanatic!

Nov 12, 2008
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Before you do that, mark up the drawing with the values you have then measure the voltage at each point and write it on the drawing.
Include the power supply voltage, each side of the led and the base of each transistor.
Or look at AK's idea if you don't need to keep this one.

16. ### nbtone Thread Starter New Member

Oct 14, 2016
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Wait a minute. I've been playing with the ground reference in other parts of the circuit that uses this, and it's -4.5 VDC and not +4.5VDC. Does that make it easier?

17. ### djsfantasi AAC Fanatic!

Apr 11, 2010
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So instead of 4.5v across the circuit, you have 9v. I think that makes a difference. Your LEDs are getting about 85ma. Not sure why they are still working at all.

18. ### eetech00 Active Member

Jun 8, 2013
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You can also use a DPDT toggle switch instead of transistors.

19. ### nbtone Thread Starter New Member

Oct 14, 2016
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Here is a more complete schematic. The relay shows only one side, and there are two circuits that it switches. I was a little confused about where I was finding the voltage. I went and spent some more time studying, and these are correct voltages. The two 100 ohm resistors used to go straight to ground, but I've had to do some ground referencing. Now the LED won't work. Sorry about the earlier posts.

20. ### ronv AAC Fanatic!

Nov 12, 2008
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Yeah, Your ground kind of disappeared.
What is the LED supposed to show? The state of the red/green relay control?