140Views2Replies

# leds.strip lights?

22 inch long strip surrounding an exhaust tip.want an led every inch.my main prob is i dont know how physically big is an led.5 mm?3mm?dime? pencil eraser?that odd looking mole on my....,well you know. how big are the darn things?also,do i put a resistor every 2 or 3 bulbs according to volts and ohms and such or one really badass resistor at the begining.next ,the resistor drops voltage for the following leds in line.if the led in front is getting proper watts or voltage,then the one after that is getting less and so on,isnt it?i dont know help?

## Discussions

8 years ago

i still dont understand about required resistors.i have read about resistors.i know i need em,just dont know if i put resistor,led,led,led,led,resistor,led,led,led,led and so on.if i have the required amount of juice at the begining of the first group of leds,then how much juice will be going into second resistor....or maybe i should run a second power wire past first group to next resistor\led group[parallel] and so on .seems simplest to me.guess ill do it that way.just involves a little more wire.5 leads or so,each one longer than the one before it.appreciate the answer.

8 years ago

So, look at it this way - an led is essentially a short circuit (they have some resistance but very little). V=IR.

Volts = amps * resistance. if you put 1 volt across a 1 ohm resistor you'll get 1 amp of current. If you put 2 volts across 1 ohm resistor you'll get 2 amps forcing their way through.

Now let's look at it another way: No resistor. The voltage drop of an arbitrary led, say 3 volts. After that voltage drop, it's essentially a short circuit. The diode element is saturated and fully conducting. If you feed it EXACTLY 3 volts, then its internal resistance makes it draw the correct amount of current of say 20 mA (.002 amps). Then something goofy happens: if you feed it 3.1 volts, thats .1 volts at nearly zero resistance which as you saw before causes a spike in current. Since the resistance is so low, the amperage jump is VERY high. THIS is what destroys leds. If you hook it up to say 12 volts, then its 3 volts dropped (fine) and 9 volts at nearly zero resistance = multiple amps of current = cooked led.

Enter the resistor. Since it has a non-zero resistance that is relatively constant over the operational temperature spectrum, it will ALWAYS drop the same voltage, and/or ALWAYS let the same amount of current through (at a given voltage). This makes our circuit safer because if we go over in voltage, the circuit won't see 'nearly zero' ohms, it will see whatever the resistor is. This prevents that astronomically high amperage spike when even a small over-voltage occurs.

What does the resistor do with that voltage? The energy has to go somewhere -- newton said so -- turns out it's heat. SO another way to look at it is you want the bare minimum resistors as they use up energy (defeating the purpose of high efficiency LED lighting). You want as many LED's as possible in series to drop the voltage, and then a resistor at the end to use up the last of the juice.

Odds are in a car application you're getting around 14 volts (from the 12V system). That means you can have 4 3 volt leds in series and a resistor designed to drop the remaining 2 volts at .02 amps (20 miliamps). I recommend using an led calculator.

If you need assistance knowing how big a 5mm, 3mm or 'other' size led is, I cannot help you there. That's one of those things you'll have to RESEARCH and perhaps even READ.