9 replies to this topic

[screech]

If i run 10 of these (click) in series, what resistor do i need to run them on 12v.

cheers

Mark.

[Guess-Works.com]

don't run them in series run them in parallel, as if one pops, they will all go out... (christmas tree light syndrome)

either attach a 470 ohm resistor to each annode ( this way if one goes pop each will still receive the right level ) or wire all annodes to same bus and use a 47 ohm resistor, but if one goes pop the current level to the other will increase.

reason I say 47 is it's a std resistor rating and also give you a bit of grace on the max forward current...

Edited by GuessWorks.co.uk, 16 May 2008 - 02:01 PM.

[Guess-Works.com]

what's this a high intensity brake light ???

try one of these for size http://www.maplin.co...e...er&doy=16m5

[screech]

don't run them in series run them in parallel, as if one pops, they will all go out... (christmas tree light syndrome)

either attach a 470 ohm resistor to each annode ( this way if one goes pop each will still receive the right level ) or wire all annodes to same bus and use a 47 ohm resistor, but if one goes pop the current level to the other will increase.

reason I say 47 is it's a std resistor rating and also give you a bit of grace on the max forward current...

The idea is that if one goes i want them all to fail anyway that why iv chosen to wire them in series,
does the same size resistor apply?

cheers

[Guess-Works.com]

if wired in parallel or series, you still have a .3amp current draw for the 10 LED's so yes a 47ohm resistor should be fine, but how the diodes will react I don't know 1st to last in the chain.

[DaveRob]

I think the problem here might be the forward volt drop of 2.5V@20mA...... if you wire them in series you might run out of voltage...... as your battery is only 12v...... All PN junctions passing a forward bias current have a volt drop across them..... usually in standard diode form its around the 0.6v... but in LED status where different wavelengths are emmitted from the junction they can be very different.... in your case it seems to be 2.5v...... so 10 in series is 25v.... in parrallel its only going to be 2.5v BUT to controll the current as GuessWorks says you will need a resistor.... to get your 25mA at running speeds of 13.5V say.... less the volt drop of 2.5V so 11V dc..... R=V/I so R=11/(10x0.025) this is 44ohms...... closest standard is as GW says 47R..... Power of the resistor is IsqR.... so (0.25X0.25)X47 makes 3W..... so you need a 47R 3 Watt resistor and the diodes in parralel.....

Just my 2ps worth..... usual disclaimer applies

Rob

[Guess-Works.com]

Blimey.... my physics A level from 23 years ago is still standing up....

All I remember is P=IV watts and V over I.R that's just about seen me through for the last 20 years of asring about with wiring looms

Edited by GuessWorks.co.uk, 17 May 2008 - 08:50 AM.

[Ethel]

That's worthy of a FAQ, DaveRob

Surely if one led fails and you can't tell just by looking it can't matter much?

But if it really is essential to have a 'one out - all out' set up could you run 2 banks of 5 leds with some sort of current comparator between them?

[Boomboy77]

Screech these need to be run in parallel for them to be of equal brightness.
The LEDs I have, I have tested and sorted the right size resistor for you (300ohms) theres 10 of each. (one resistor per LED)

The trouble with running them in series is that like said above if one goes they all go out, or should it go short you end up supplying too much current for the rest of the pack and burn them out............

If you show me what you are trying to do I will make up a pcb for you to mount the whole thing onto, will save on the wiring and be easier to work with I should imagine!

[DaveRob]

As boomboy mentions...it is actually better to use 1 resistor per LED.... prob best to get 470R and it makes the power of the resistor needed 0.3W.... a bit more work on the install and more space needed etc but a marginal increase in reliability.......depends on your space....skills and how you want it to look of course... but still better to have the lot in parrallel...... Id keep the current down to the 20mA draw per unit if you arnt going to fit some sort of voltage regulation...... if you dont go down the regulated supply route you may find some variation in brightness due to different battery voltage scenarios ie at idle... full load with lights on etc..etc

Rob