So I just started tinkering with circuits online using circuits.io

First I made this circuit using 1.5v battery to power two LEDs which was insufficient and wouldn't glow.

Then I removed and added a 9v battery without a resistor which was over-sufficient for the LEDs. enter image description here

Lastly, I added both of them (9v+1.5v) and that seems to power the LEDs sufficientlyenter image description here

I thought that in the last case they both would be overkill for the LEDs but that didn't happen. Don't the voltages get coupled when supplied in series?

*Doing something like this practically is a bad idea but I wanted to know theoretically so experimented this.

Edit : The much preffered schematic enter image description here

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    \$\begingroup\$ Hey OP, while you're at it have a look at this. EEs are... engineers, and yes engineers are picky. They usually are picky for a reason, take the criticism in the good way and learn a new thing, it will help you down the road, I promise. \$\endgroup\$ – Vladimir Cravero Jan 2 '17 at 18:28
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    \$\begingroup\$ I once connected an LED directly to a 9V battery. The outcome was very exciting: the LED was glowing and melting at the same time! Never seen that before. \$\endgroup\$ – Nazar Jan 2 '17 at 18:46
  • \$\begingroup\$ @VladimirCravero thanks for the guidance ! BTW that schematic was generated by circuits.io \$\endgroup\$ – Ashok Jan 2 '17 at 19:09
  • \$\begingroup\$ Hmmm... In this case, I think I prefer the pictures to the (presumably) automagically-generated schematic. The schematic doesn't tell me which battery is which voltage, and is a horrible mess. \$\endgroup\$ – Peter Bennett Jan 2 '17 at 22:25

If I read your cartoons correctly, (real schematics are much preferred), it appears that you have the 1.5 volt cells in parallel with the 9 volt batteries. This is Very Bad - there is no telling what the actual voltage will be, as the 9 volt battery will try to charge the 1.5 volt cell, while the 1.5 volt cell tries to pull the 9 volt battery down to its own voltage.

LEDs must always be used with a series current limiting resistor - there are lots of questions about this on this site, and there are web sites that will calculate the resistor value for you.

Red LEDs typically have a forward voltage of about 1.8 volts - you need to supply a higher voltage, through a current llimiting resistor, to get them to light. Other colour LEDs require higher voltages, up to 3 volts for blue and white LEDs.


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