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Confirming whether this circuit will work

MeatySteak

New Member
Hi,
I am very new to circuits, as probably evident by the basicness of this one, and am especially new to LEDs. I bought some LEDs, specs are advertised as 3v, 20mA. So pretty standard. I'll link them down below this. They do come with their own resistors pre-wired, but I wanted to move things around. The pre-wired resistors are 1500ohm metal film, and there is one per resistor as you would expect. I'm using a 12v power supply, and as long as my calculations are correct, that would mean that the LED would receive 6mA of current each, well below the advertised 20mA. This setup does work however, 6mA seems to be enough to power it well enough. I'm assuming 20mA is just the maximum current aloud without it burning out.

So, what I wanted to do was instead of having a resistor for each LED, have only one to control multiple LEDs, specifically one for the blue ones and one for the white ones. I attached a very rough picture of what I plan on doing along with the basic calculations to figure out the resistance for the resistors. Hopefully I have everything right and it makes sense. Just want to confirm whether this will in fact work or if what I have done is just stupid.

In short, I want to connect 4 Blue LEDs to one resistor with a current output of 0.024A (so 0.006 for each LED), and 10 White LEDs to another resistor with an output of 0.1A (so 0.01A for each LED). Reason for this is so I dont have 14 resistors which I would need to try and fit into a very small space but instead only have 2 which is very manageable.

Please be gentle, I am not an artist and I dont work with circuits very often. :)

Links:
https://lighthouseleds.com/pre-wire...ngle-12v-pre-wired-led-blue-ultra-bright.html


Thanks!

1656815332495.png
 
Last edited:

danadak

Active Member
Your calc should go like

R = V / I = (9V - Vled) / .024 = (9 - 3)/.024 = 250 ohms

The above is basically a Kirchhoff's loop. So writing the voltages around the
loop starting at battery 9V - Vresistor - Vled = 0, and then you algebraically
solve that. You start at a point in the loop and then walk around the loop
back to your starting point. Many youtube videos on this topic.

The same should be done for the white LEDs.

LED voltages (nominal) -

1656843821085.png


So check your datasheet voltages. Also as LEDs are in parallel important to match them for
V so that one does not hog all the current because it turns on at lower V than the others.

Are you planning on doing this with a 9V battery ? Battery life will be a little challenged.


1656844331034.png



Regards, Dana.
 
Last edited:

MeatySteak

New Member
Your calc should go like

R = V / I = (9V - Vled) / .024 = (9 - 3)/.024 = 250 ohms

The above is basically a Kirchhoff's loop. So writing the voltages around the
loop starting at battery 9V - Vresistor - Vled = 0, and then you algebraically
solve that. You start at a point in the loop and then walk around the loop
back to your starting point. Many youtube videos on this topic.

The same should be done for the white LEDs.

LED voltages (nominal) -

View attachment 137695

So check your datasheet voltages. Also as LEDs are in parallel important to match them for
V so that one does not hog all the current because it turns on at lower V than the others.

Are you planning on doing this with a 9V battery ? Battery life will be a little challenged.


View attachment 137696


Regards, Dana.
Thanks for the response! I was planning on doing a 12V, so I factored in a 3V drop in my calculations. Didnt realise different colours had different voltage drops tho.

I ended up looking into it a bit more and found out this works, but only in concept since these LEDs arent consistently the advertised voltage, i.e. small discrepancies leading to large differences in current draw.

I’m just going to try fit everything and hope for the best!

Thanks
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks for the response! I was planning on doing a 12V, so I factored in a 3V drop in my calculations. Didnt realise different colours had different voltage drops tho.

I ended up looking into it a bit more and found out this works, but only in concept since these LEDs arent consistently the advertised voltage, i.e. small discrepancies leading to large differences in current draw.

I’m just going to try fit everything and hope for the best!

Thanks
As long as the LED's are from the same batch (or even identical types from different batches), then as long as you have at least three in parallel they tend to share the current 'well enough', and the more you have, the better the sharing. However, if the opportunity is there, always put them in series if you can.
 

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