Led Advice!

[Lukeypoo]

Right here's the dilly, I've wired 15 4v white leds in parallel hoping to utilise the 12v feed to the sidelights on the car

The trouble is I have just tested three leds (wired in series) on a 9v battery and it wouldn't power them! Powers two very well

My thinking (excuse me if I'm wrong) that if 9v powers 2, 12v will power 3?!

What can I do?

Thankyou in advance!

 

[Sidewinder]

Are they constant voltage or constant current devices?

 

[Lukeypoo]

I don't know? How would I check? I brought 50 leds for a couple of quid on fleabay

 

[Bruspark]

With no Data on the LED's doing this properly is pretty hopeless  . .

If you want to play around  consider that LED's marked 4Volts will light brightly at 4V and dim rapidly below that.  Also consider they have a + and a minus - and will fail (even explode /stink dangerously) on too much power or wrong connection.  Do you know which end is positive ?       Series wiring is + LED - ----   + LED  -  etc

Connected in Series 2 x 4V LED required 8V to light to full rating.  You gave it probably 8 or 8.5 from a 9V battery under load so it worked

Connected in Series 4 x 4V LED requires 16V to light fully.  They will be Dim or out on the 9V battery and probably very dim or out on 12V

LED should NOT be directly driven from car '12V' as this will range from 11.5 - 14V according to charging conditions.  If driven via a large resistor you can ensure than neither condition over-volts the LED's

You initially said you had wired the LED's in Parallel (i.e all + together and  all - together ) so if connected directly to the 12V they will all die very promptly .   Please don't

You could connect them all in parallel and feed that circuit via a high wattage resistor and fuse such that the LED's measured 4v across them with the car supply at 14V

Or connect them in parallel with a low wattage resistor on one connection of EACH LED such that the LED's measured 4v across them with a fused connection to the car supply at 14V

In each case start with say 1000 ohms in series with the LED and reduce that in stages until you see 4V across the LED

If you don't have a multimeter, spec sheets and/or a box of resistors to experiment with you are going to waste a lot of time on this

 

[SLIPSHOD & SLAPDASH]

Check polarity & try a different one if you have 50.