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9V LED Circuit: Only One LED Lights Up

thosmatthews

Sep 24, 2022
2
Joined
Sep 24, 2022
Messages
2
Hi there,

I'm an amateur when it comes to this so:

I'm making a circuit with 2 6V LEDs in series with a 9V battery. Both LEDs are tested and work, but only one comes on when I connect the 9V battery. Does this mean I need a 12V to power both of them?

I tried adding a second 9V to make 18V across the circuit but with the same result: only one LED lights up, the same one each time.

I imagine the answer is very simple!

Many thanks,
Tom
 

danadak

Feb 19, 2021
205
Joined
Feb 19, 2021
Messages
205
First their series threshold to turn on is 12V per your description.

But because they are exponential curved diodes in I-V characteristics they will turn on at a
lower V but lower current/brightness.

1664023681909.png

Because they are not matched one may have much higher turn on V so it stays
off but the other is just turned on enough to see. Modern LEDs will turn on with
uA thru them.

What is part number of the LEDs ?

You could hook them in parallel but with a 3V zener in series with each to drop
the excess 3V or a R to limit current to their max allowed specs.


Regards, Dana.
 

danadak

Feb 19, 2021
205
Joined
Feb 19, 2021
Messages
205
If you have a bench power supply, you could make a simple
plot of V vs I to see how closely they are matched. If you tried
18V on them and they still not operating correctly something
is bad with one of them. Note using > 12V is overstressing
the leds, as they are rated for 6V.

There seems to be no real electrical info on them, eg. datasheet
seems thin. Also seems like they may be a parallel array of diodes,
and no limiting resistor, relying on their dynamic resistance for
current limiting. Not a good design unless one drives with a
constant current source at a design current.

You could, to maximize battery life, do a switching power supply
to run them in series. One with constant current.


Do you have a datasheet for them you can post to see specs ?

A 9V battery, depending on current will not have a long lifetime. Look at discharge curves
in this datasheet.


A Lithium with a boost constant current driver might be a better solution :




Regards, Dana.
 
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