100 LED Parallel string resistor

[agentorange]

Hi, usually my Led strings are no more than 20 leds and with a 5 volt source I use a 3R30 (no matter what colour the Leds)

However, I've bought myself a couple of 100 led solar powered strings (leds connected in parallel) which I wish to power directly from a 5 volt supply (bypassing the solar panel and internal battery)

I realise each LED should have it's own resistor, and if I have just one per string each LED will have a different brightness etc, however it's a pre-made string, so I just need one resistor for each string, but the online calculators just refuse to calculate for 100 Leds.

They are standard Leds.

I quickly tried a 5 V power supply with the LEDs without a resistor and nice and bright, so it works, I just need a resistor value.

Anyone got any idea of what value I should use?

Many thanks for any replies.

Ali.

 

[DF96]

The value which makes them the desired brightness? (and keeps them within spec so they dont burn out)

Alternatively, how about Ohm's Law? I hear it is quite useful for calculating resistor values, given the requirements. What are the requirements?

 

[theAnonymous1]

For that many you should really know the LED specs to get an accurate calculation.

Just start high (10R?) And work your way down until they are bright enough for you.

As far as the online calculators, since you are only going to use one resistor, just multiply the individual led current by 100 and enter it as 1 led.

 

[macboy]

OK, so since they are in parallel, the forward voltage drop of the parallel combination of 100 of them will be the same as the forward voltage drop of a single one. You need to find out what that is in order to do accurate calculations.

Let's guesstimate that Vf is 2.0 V.
You have a 5.0 V supply.
The resistor, then has 3.0 V across it.
So R = V/I
What is I? It is the total current that you want to pass to the LEDs. Since they are in parallel, and are identical they will "equally" share the current (more or less), so you need 100x whatever the desired current is. The current limit per LED is commonly 25 mA or so, and 10 to 20 mA will give a nice bright output without excessive heat. Let's say 15 mA. Then the total current desired is 1.5 A.

3.0 V / 1.5 A = 2 Ohm.

You can certainly start higher and work your way down if you need more brightness, but 2 Ohm is nearing the limit.

Don't forget to consider the power dissipated in the resistor:
P=V²/R = (3*3/2) = 4.5 W
I'd use a 10 W resistor to give some margin.
If you use a larger resistor (less brightness), then the power will go down proportionately. Everything above is an estimation since Vf is an estimation. For red LEDs, Vf is around 2.0 V, but for white (or blue, violet, UV) LEDs, Vf is very different, around 3.5 V, so re-do the calculations.

 

[agentorange]

since you are only going to use one resistor, just multiply the individual led current by 100 and enter it as 1 led

and

Since they are in parallel, and are identical they will "equally" share the current (more or less), so you need 100x whatever the desired current is.

Ahh! Of course, that makes perfect sense now you've said it. Doh!

And thank you macboy for your comprehensive reply, that's straightforward enough for me to understand

Again, many thanks.

Ali.