3
\$\begingroup\$

I am new to electronics. I have been given 100 Leds project by my teacher at college . Can anyone here validate this circuit. Thanks in advance.enter image description here

\$\endgroup\$
3
  • 7
    \$\begingroup\$ That circuit is valid for some possible meanings of "desired result." However, it probably isn't valid for a large number of meanings I've usually encountered in practice. Would you mind specifying what you want to achieve? \$\endgroup\$
    – jonk
    Sep 17, 2017 at 18:04
  • 1
    \$\begingroup\$ Yes. Any explanation of driving LEDs will show why it won't work. \$\endgroup\$
    – user16324
    Sep 17, 2017 at 18:05
  • \$\begingroup\$ "homerwok" question and even clearly by someone who really didn`t study much :-) Jokes aside, do a quick search for "driving multiple leds" and "thermal runaway parallel leds"! \$\endgroup\$ Sep 18, 2017 at 7:20

1 Answer 1

25
\$\begingroup\$

No, that's a really bad circuit. You have 100 LEDs, all in parallel. Bad idea.

Since this is a assignment, I'm not going to give you a better circuit outright. However, consider that LEDs want to be driven with a fixed current, not a fixed voltage. The change in current due to a change in voltage is very large when the LED is lit. Conversely, the voltage changes little as a function of changing current.

Now also consider that every LED will end up at a slightly different voltage when run at the same current. If you put two such LEDs in parallel, then they both get the same voltage by definition of parallel. The small difference in forward voltage of the two LEDs leads to a much large difference in current between the two LEDs. Even worse, the LED that gets the higher current dissipates more power. That makes it get hotter, which decreases it's forward voltage, which makes it take a larger share of the current, which makes it get even hotter, etc.

Think of a way to run LEDs, or groups of LEDs, at a fixed or reasonably controlled current.

\$\endgroup\$
5
  • \$\begingroup\$ Then should I use strings of leds with a resister in each string to control the current, but then the input voltage can't be 3v i guess or (in this parallel circuit) should I add a resistor before each led , but that will make the circuit bigger. Sir I really appreciate your help..... \$\endgroup\$ Sep 17, 2017 at 18:23
  • 9
    \$\begingroup\$ @Jeff: Yes, these are all things you should consider, but don't stop there. Engineering is about thinking of various alternatives, then picking the best considering the value you place on different criteria for measuring "best". Sometimes the hard part is deciding the relative values of the various criteria. What is easier for you, using a higher voltage power supply, using 100 resistors, something else? There are a lot of other options. \$\endgroup\$ Sep 17, 2017 at 18:38
  • \$\begingroup\$ +1 from me (a lurker) for how much educational this answer and your approach is! \$\endgroup\$
    – yo'
    Sep 17, 2017 at 19:52
  • 2
    \$\begingroup\$ LED with the smallest forward voltage will hog most all of the available current, and either silently fail open circuit within seconds or burn/melt/pop. Then the LED with the next lowest forward voltage takes its turn ... repeat until all the LEDs are destroyed. And yes, there is a chance of LEDs actually failing pretty violently - A transformer with 1.5A rated current is likely to deliver far more if not limited by the load, even though it will eventually overheat. The bond wire in a typical LED is not unlikely to vaporize from a few amperes and blow the plastic case in half. \$\endgroup\$ Sep 17, 2017 at 20:45
  • 1
    \$\begingroup\$ @JeffZender: Does the transformer have to transform to 3V? Or can you change it to e.g. 30V which would allow you to run strings of 10 LEDs + resistor? \$\endgroup\$
    – Michael
    Sep 18, 2017 at 7:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.