Hey guys, I got a question about LEDs. Basically I want to wire a bunch of them (I got 100 on E-Bay) in the back of the vents and such to make them glow red at night for fun, but basically what I was wondering if I wire them in series do I still need to add a resister before them? Also, how many can I wire in a row?

Thanks,
Douglas

 

You will not need the current limiting resistor if you connect enough L.E.D.'s in series provided you have the proper number of L.E.D.'s in series.

To answer your second question, that depends upon the L.E.D.'s you are using. They are all different, however, they all draw similar current (and thus voltage).

The current range of a typical L.E.D. is ~20mA (0.02 amperes) but the range can be from 2-40mA or more. This means that an L.E.D. will need around 1.5 - 5 volts each. Again, it depends upon the color, size, type, make, etc.

An L.E.D. typically draws ~25mA (0.025 amperes) at ~2 volts (each).

The best way would be to use an ampmeter to determine the current draw of a number of LED's wires in series so you know how many LED's to use. If you presume 2 volts per LED, then six in series would need ~12 volts. Seven would probably be better since that would translate to ~14 volts (engine running voltage). Add or remove LED's as necessary to obtain the proper current (20-25mA is generally okay if you don't know the rating).

If you don't have an ampmeter, start by connecting eight or nine LED's in series (no current limiting resistor needed) to 12 volts. Remove one LED at a time until you get the brightness you believe is correct (not too dim or bright).

Doing it this way, if you don't get the correct current per LED, the worst that will happen if you have too many LED's is that they'll just be too dim. If you don't have enough LED's, the worst that will happen is that they will be too bright, thus resulting in a shorter life for the LED.

FYI, connecting one LED directly to 12 volts (without a current limiting resistor) will work, but only for a short period of time. Some LED's will just flash before burning out, others will illuminate for 30-90 seconds (or more) before they burn out due to the excessive current.

You could connect all the LED's in parallel. This would require just one current limiting resistor. The wattage (and thus size) would then have to be larger. 100 LED's in parallel, each drawing 20mA equals 2 amperes total. With a voltage drop of 12 volts (figuring supply 14 volts) will equal 25 watts. Double that for a safety margin and you'd need a 50 watt resistor. The value would be ~500 ohms.

Using that method, if one or more LED's burn out, then the other LED's will then get more current because of the absence of the burnt out LED. This will then shorten the life of the rest of the operating LED's. The more LED's that burn out, the shorter the life.

Does this make sense?

 

It's been my experience that connecting LEDs in series without some type of curent limiting resistor will cause the LEDs to fail after some period of time. For a 12 - 14 volt system, it would be better to run approx 5 LEDs in series & use a current limiting resistor on the order of 130 to 200 ohms, or so.

------------------
Tim

1992 Z28, 305 TPI 5 speed, 12.1 sec @ 114.7 mph

1984 Berlinetta 305 TPI Auto, 13.8 sec @ 99.7 mph

 

Thank you for the replies. I don't have the LEDs yet, so I am not sure what voltage, etc they will run at.

That did make sence, for what both of you said. Right now I am running single LEDs with a 680 ohm resistor. So far they are nice and bright, and one has run for 20+ hours so I think they are okay.

Thanks,
Douglas

 

They should have a long way to go, then. most I've seen have a life-span of 50,000 hours (unless overloaded).....that's almost 6 years of continous usage.

Pete



------------------
Pete's Place
HiOutput Tech Page
83 SC..former V6...now 213+k mile LG4/5-speed.. 14.891@95.54
89 Formula..ultimate project car..fast as I can push it with one flat tire
79 Lemans..267/auto..no brakes

My karma ran over my dogma