Rear light LED on Givi Top Box

[davsato]

Second, and this is where it gets technical, the voltage drop across an LED is of the order of 2 Volts. So, on a 12 Volt supply there's 10 Volts to drop across the resistor .......................
So, what you do with a 12 Volt supply is put at least 3 LEDs in series (one after the other) - the effective Volt drop is now 6 Volts and the resistor only needs to be (6 Volts / 20 mA ) or 300 Ohms and needs a power rating of 0.12 Watts (about an eighth of a Watt). Less wasted energy.

On a 12 Volt supply you could put more LEDs in series, but I stuck with 3 as it made my wiring simple. However, I did put two groups of 3 LEDs in parallel.

Confused yet?

The total power consumption of my system is 100 mA, which is 1/20th of the original 2 Amps with the filament lamps.

If you want I can knock you up a drawing which will make the above much easier to understand. Picture / thousand words scenario.

confused yet? not quite, lets try harder.

so, if i put 6 LEDs in series i wont need any resistors? and i can put several of these strips in parallel and it will be ok?
thats interesting as the LEDs ive found come in a continuous strip in series, straight out of the plastic moulder i suspect, so without resistors the only soldering to do will be at each end.

im not bothered about consumption, i just want it to work.

[Lutin]

Ah no, sorry it doesn't work like that. If only it did.

Any LED will need a resistor to limit the current through it, in this case to a maximum of 20 mA.

Putting an LED straight across a supply will destroy, or at least permanently damage, the LED.

I suppose that you could get away with 5 LEDs in series, so your current limiting resistor would then need to be (2 Volts / 20 mA) 100 Ohms. Although this is risking the LEDs not working with the supply much below 12 Volts - but then again if the supply is much below 12 Volts the last thing you'll be worried about is the LEDs not working!

Sorry, I should have made this point earlier. It's another case of there being no free lunch.

[jarl]

Yes and no. 12V supply on a vehicle isn't 12V, it's more like 12 to 15V, so you definitely want that resistor in there to act as a buffer to limit current if the voltage increases- small voltage increases can destroy an LED. If you had a power supply that was exactly 12V (it isn't) and you had a string of resistors that were exactly 12V at their rated current (nearly impossible) then it would work. For real life applications, you need that resistor. 3 or 4 in series with a resistor sounds like a good amount to me

[davsato]

so to be on the safe side i will put a resistor on each LED. a lot more soldering but but im not bothered about that, it beats sitting with the mrs.

[Lutin]

so to be on the safe side i will put a resistor on each LED.

Yes, you can do that but, depending on the application, it shouldn't be necessary.

What are you intending to do exactly?

[Grendel]

The idea was not to fit the reflective tape behind the lights but behind the red lenses fitted to the top-boxes or side cases.

The red lenses on Givi boxes can be removed as they are held on by very small screws. Behind the lenses is white tape stuck over the black plastic of the boxes. It is this tape that is being talked about, as it is not great reflectivity wise.

My mistake misread one of the posts

[davsato]

Yes, you can do that but, depending on the application, it shouldn't be necessary.

What are you intending to do exactly?

same as on this thread, maybe put some lights in my topbox lenses. ive got some LEDs coming from that chinese bloke, and some stripboard lying around, so i thought i'd have a play.

the LEDs are only 3V, they come with resistors for 12V supply, its 15yrs since i did any electronics at college and i was wondering if i had to put a resistor with each LED, or to neaten things up can i have all the resistors together and all the LEDs? in series its ok to have a row of resistors then a row of LEDs isnt it?

[tendays]

various LED calculators

parallel series LED resistor Calculator - powered by led-seller.com

LED series parallel array wizard

The Best Current Limiting Resistor Calculator for Led's

[Lutin]

various LED calculators

parallel series LED resistor Calculator - powered by led-seller.com

LED series parallel array wizard

The Best Current Limiting Resistor Calculator for Led's

Not bad those. Mind I wouldn't run LEDs in parallel - all sorts of unbalanced current problems there.

Anyhow, the resistors that I got with my LEDs were 510 Ohms and 180 Ohms.

With a 12 Volt supply use the 510 Ohm resistors - gives an LED current of (near enough) 20 mA.

And then on 5 Volts use the 180 Ohm resistors - again gives an LED current of 20 mA.

These values of resistor are correct to give an LED current of 20 mA at the precise voltages of 12 and 5 Volts.

Unfortunately, you will very rarely find exactly 12 Volts on a motorcycle. When I did some checking a while ago, at a fast idle I measured 14.6 Volts on my Transalp. The voltage needs to be higher than 12 Volts in order to be able to charge the battery.

So, what we need to do is to build in some "slack". Now these LEDs from Sure Electronics are continuously rated at 20 mA, any more than that and you will stress the LED and shorten it's life. Now you might not think that the brake light will be on for any length of time, but when was the last time you stopped at a junction or a set of traffic lights and had to hold the bike on the brake? How long was that? 1 minute, 2 minutes? I sure you see what I mean.

But there's no need for use to go crazy. The difference in brightness of an LED at, say, 15 mA and at 20 mA is discernable, but it is not huge.

If we assume a supply voltage of 15 volts, instead of 12, we can calculate a new resistor value to use.

There is a Volt drop of 2 Volts across each LED. so, our resistance calculation looks like this -

(15 Volts - 2 volts) / 20 mA gives a resistance value of 650 Ohms, This is for a single LED, mind.

However, if we put two LEDs in series we get -

(15 Volts - 4 Volts) / 20 mA, which gives 550 Ohms, Which is only about 8% above the value of the 510 Ohm resistors that will come with your LEDs.

So, what you can do is this -



How does that sound?

[davsato]

cheers lutin that makes things a bit easier.
i will have a play when they come. getting 100 so ive got a few spare

tendays thanks for those, very handy, saves a lot of calculating