LED Circuit and resistors

Thread Starter

MeatySteak

Joined Jul 3, 2022
3
Hi,
Hope this was the right spot to post, wasnt really sure.

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

https://lighthouseleds.com/pre-wire...gle-12v-pre-wired-led-white-ultra-bright.html

Thanks!

1656825694383.png
 

ScottWang

Joined Aug 23, 2012
7,244
You should follow the ee rules, one resistor for one led because the Vf of led is not the same for each led, if you only use one resistor then the led will be burned up fast than its spec.

What is the better value of the resistor for led, I will use it for less than 80% of the rated current, if you don't care about the money and time then you can do whatever you want.
 

ericgibbs

Joined Jan 29, 2010
15,735
hi MSteak,
If you could accept 9 White and 3 Blue LED's this would be a simple method.
More efficient than the parallel method you posted.
E
 

Attachments

Last edited:

Ian0

Joined Aug 7, 2020
5,556
Have you ever looked at LED tape? If so, you will find that the 12V version has three LEDs in series, then one resistor.
Just like @ericgibbs 's circuit.
 

Ya’akov

Joined Jan 27, 2019
6,046
Hello, @MeatySteak, Welcome to AAC.

I just wanted to welcome you to AAC. Aside from that, I concur with @ericgibbs since parallel operation of the LEDs will almost certainly result in differing output from each of them and not look any good, even ignoring the other troubles presented by it.
 

eetech00

Joined Jun 8, 2013
3,241
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.
If you want to use only two resistors then the circuit will need a higher voltage supply, and connecting the LEDs in series/parallel fashion will greatly reduce the current requirements. If you use a 30v wall wart, you can connect them this way:

1656860437552.png
The total supply current is 40mA (20mA each string). The resistors can be 1/2 watt, 5%.

The 30v wall wart will need to output about 100mA max.
 

Ian0

Joined Aug 7, 2020
5,556
R=1Ω will not be an effective current limiting resistor. What happens if the LEDs are at the minimum or maximum Vf? Or as the Vf varies with temperature.
 

Boggart

Joined Jan 31, 2022
16
Welll….I’ll let the TS decide.
It will work as I’ve shown.
No, it won't. For example, say the LEDs are on the low side regarding Vf. Some whites I tested this morning were 2.7V @ 20mA, so 10 in series would result in a LED Vf of 27V, and 3V across the resistor. With a 1 ohm resistor, that would be 3A! It wouldn't in reality be that high as the higher current would raise the LED Vf, but the final balanced current would be many times the rated current of the LEDs.

You can't cut that close to the specs with resistor values, you need a reasonable amount of voltage drop across the resistor, and select a resistor that will not allow current to exceed max value at minimum LED Vf. Remember, as LEDs heat up, their Vf goes down, so things get even worse.

If you want maximum efficiency and minimal voltage loss across the current limiter, you use a constant current IC instead of resistors.
 

eetech00

Joined Jun 8, 2013
3,241
. . . Provided that you select each LED for forward voltage and keep it at a regulated temperature.
Well....the first entry on their chart shows 1ohm for 3v@20ma.
Until I see an Vf vs If chart for the actual LED, I'll stick with what I've shown.
But I think the actual current will be less....more like 10ma.
 

Thread Starter

MeatySteak

Joined Jul 3, 2022
3
Oops, was meant to reply here a while ago.

Decided I'll just try fit all the resistors in and have one for each LED, if I'm efficient I'll get it done.

Thanks for all the responses! I did some research on this technique, and found some interesting stuff. How I understand it by going with my method is, not all the LEDs are going to have the same forward voltage, there will always be a tiny discrepancy and unfortunately, this discrepancy will cause much larger current draws so 1, lights will vary in brightness and 2, some might burn out very quickly. So while it works in concept, it just doesnt work in practice.

I'm glad this has triggered another discussion about other techniques though. I plan on doing some other builds anyway so this might be useful later on and it cant hurt as someone else with a similar idea might find it useful, so keep it going!!

Thanks everyone.
 

Thread Starter

MeatySteak

Joined Jul 3, 2022
3
hi MSteak,
If you could accept 9 White and 3 Blue LED's this would be a simple method.
More efficient than the parallel method you posted.
E
Hey Eric, unfortunately I cant, but for future reference, is it 3 each series because my power supply is 12V? If I was to up the voltage to 15 or something like that, would that allow an extra LED in series?

Thanks!
 

ericgibbs

Joined Jan 29, 2010
15,735
Hi Meaty,
That would work OK, adjust the series resistor to give the LED current you require.
E
If you need any help with the R value, just ask.:)
 

Audioguru again

Joined Oct 21, 2019
4,805
LED flashlights (called torches in a couple of countries) connect 24 to 60 LEDs directly in parallel but their forward voltages are all matched. I made some night lights using many paralleled LEDs that all have the same forward voltage.
 
Top