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Could you please help me understand below circuit in detail. What could be the voltage at testpointA if LED forward voltage drop is 2V?

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Micro
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  • Will all LEDs glow? or only D3 will glow and other won't ? – Micro Oct 20 '23 at 14:00
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    I’m voting to close this question because it appears to be a homework problem with no effort shown. – Hearth Oct 20 '23 at 14:07
  • How many LEDs will you need to buy, measure and sort to find three that have a forward voltage of exactly 2.00V as they warm up? it is likely that your LED #4 will soon burn out with nothing to limit its current. – Audioguru Oct 20 '23 at 14:08
  • @Hearth My apology for asking basic question. I am learning things.. – Micro Oct 20 '23 at 14:13
  • The simulation you have can tell you if the LEDs light up. Put a current probe on one of the legs of the LED. If even microamps of current are flowing, the LED would be illuminated. However, ideal LEDs (which are all identical) are not real, so that circuit would be hard to replicate from real devices. – user1850479 Oct 20 '23 at 14:26
  • @user1850479 Thank you. Yes I did run the simulation and it does shows few micro amps flowing through the D4. As Audioguru mentioned without any current limit it is most probably going to burn out .. That was I assumed or understood. – Micro Oct 20 '23 at 14:55

1 Answers1

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If you simulate it you will find that the voltage at TestPointA is a little bit over 2 V, 2.086 V.

If you remove one of the top LEDs, D1 or D2, the voltage will measure exactly 2.0 V. This is because the simulator will divide the supplied voltage equally across two identical components. With both D1 and D2 connected the current will divide between them, so they will each have half the current that D4 has. In the model for the LED the voltage drop will be somewhat dependant on the current, so the LEDs with less current, D1 and D2, will have a slightly lower voltage drop than the one with more current, D4. You will also find that the current through D4 changes if you disconnect D1 or D2.

If you measure the voltage across D3 you will find it is much higher than 2 V, around 3.28 V. This is because it is passing more current than the other LEDs. If you change R1 to 26K the current through D3 will be about the same as through D4 (~ 73\$\mu\$A) and the voltage across it will be close to D4's, about 2.08 V.

This is all in simulation with identical LEDs, in the real world you would not get these results as even in the same batch there is quite a bit of deviation in the parameters of each individual LED.

GodJihyo
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