I give up.

 

Over 10 years ago I built some small Christmas light strings. They were powered by a pair of C cells in series. All the LED's are in parallel and the only other components in the circuit is a on/off switch and a 3A schoky diode.
There are 53 LED's in the two strings. Current draw from the battery is 820Ma or 15Ma average current LED current.

During the same period of time I wired a horse(LIVE) with over 300 LED's for a night parade. I used 2 battery packs and the the LED's worked for a number of hours.

The Christmas lights still work to this day, and have been used almost every year since they were built.

Attached Images

christmas.jpg (69.8 KB, 626 views)

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The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

[quote="Nigel Goodwin"]car voltage output is not steady, do this method and hit ur car rpm to 5000+ ..then check how many led's survive :lol:

 

thats nice stuff k7elp60..

anyway for everyone those led's are 5 volts.. bcuz they are BIG. ok even if they are not.. just assume they are 5 volts bcuz the core of my question was if i am supplying the same volts as required by the LED's ..do i still need a resistor in serious or would the LED's pull max amepre from the powersupply and blow off?

i got my answer anyway.. thanks to everyone. :lol:

 

It doesn't matter how many RPM your motor does, the alternator's output will remain quite steady, it'll change half a volt or so, nothing more...
if it does then you've got a realy old car...

 

Don't go away.
There are lots of maybe's about your "5V" LEDs. Please post a datasheet or manufacturer and part number so we can determine:
1) Maybe it is a standard 2.2V red LED with a built-in resistor having a value to provide a reasonable current from a 4V to 6V supply.
2) Maybe it is two 2.5V LEDs in series. If yours is like this then if your 5.0V supply is actually 4.8V they will be dim, and if the supply is 5.2V they might burn-out. Because there is no current-limiting resistor. The range of voltages from 4.8V to 5.2V is a very good tolerance of only 4%, your supply could very well be worse.

The only "true" 5V LED that I have ever seen is my blue one with a 150mA current pulse through it. Its continuous current rating is only 35mA, so if I connect it to a 5V supply its life will be only milli-seconds.

 

all of them

 

with a difference of 0.2 or 0.3 volts only, u can considerably reduce the LED life.. or even blow them.. i want to be on safe side.. i attached a volt meter at one of the tail light circuit and rev the engine..the voltage sure jumps a little up and down... if u have volt meter built onto car dash..that would tell u the same too.

i just want to be safe from burning the LED's or reducing their life.. the led is probably not 5V.. it might be 3.3 or something.. anyway i just used 5V as refrence...my question core was wether u do need or dont need a resistor in series when they supply voltage is same as LED rating

if i need to replace the LED's.. i would probably have to buy a new tail light and mod it again.. it can run down very expensive for a lexus :lol:

 

The voltage from a car battery is relatively stable, certainly far more than becomes a concern feeding LED's - the tiny changes in voltage will have no effect on the life of a correctly connected LED array.

As you have been told repeatedly! - YOU NEED A SERIES RESISTOR - unless the LED in question has one built-in, in which case it will be clearly advertised as such.

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Boss,

Why dont you buy a bunch of LEDs (and resistors) and just try building a test version to see if it does what you want. You obviously have no experience in this area and dont seem to be able to listen to good advice.

So, try it out first before you wreck the lights on your Lexus.

Oh by the way, YOU NEED SERIES RESISTORS.

JimB

 

Not really. A car battery reads around 1 to 1.5V higher when running than off. Also voltage drops across the power wires as major accessories (headlight, AC blower, taillights) are turned on, the line could lose 0.5v or so.

It depends on the value of the resistor you put in. If your series string of 2 or 3 LEDs drops 11v and you put a resistor in there designed to drop 1.6v to make 12.6v, regulation is extremely poor. If your car goes to 13.8v when running, the LED current will be close to double (smoke results). Or you could adjust the resistor size for the correct current at 13.8v, but then it'll be dim at 12.6v.

A regulator is not a bad idea, but it should be a 12V low dropout reg. Still, you musy have resistors in every series string or it's doomed. The regulation problems associated with low resistor-to-LED voltage drop ratios are not all due to supply voltage, thermal and mfg variations in the LEDs themselves can mean they don't have the exact drop specified.

For a non-switching reg to make 5V @ 10amp from a 12V car system could mean dissipating at much as 90W! This is far, far too much heat.

 

In digital class using a regulated 5V supply we were still required to use limiting resistors on each LED and on each of the seven seg display led's.

I found that the standard Led will go over its rating at 5volts.

If you want your LEDS to be bright, but not blow up use a resistor at each bank and use a Voltage regulator to reduce 12v to 5V, or use a down step transformer. To use a transformer you must invert DC to AC then step it down and rectify.

An Alt in a car spits out 14.4 volts off the regulator. Which then is loaded to about 13.6V or so.

Another way to do this is to divide the voltage like you would for a transistor. 'Voltage divider' then to divide the current, use the center going into a parrallel bank of resistors to divide current down to 25mA. I don't have time to go into figures of the resistors off a 12v supply. In theory it should work this way, but voltage spikes can still get through. This may require some huge wattage resistors on both voltage and current dividing.

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I did a little study and well I came to the conclusion that you must still limit the current even on a stepped down power supply of 5 volts.

It would be pointless to divide the voltage and current for every LED. You would need to bank divide the V and I for a 2.5A or more current to supply enough current for just 100 LED's, thats a huge resistor not to mention that the resistor would disipate tons of wattage and create and huge drain on the system.

So it may seem harder and more time consuming to bank a resistor for every LED, but this way it doen't go against the LED's theme of low voltage low current devices. They are suposed to run at lower consumption ratings.

Why don't you go out and buy a LED buld that APC makes take it apart and study the set up and carry it out to a larger scale.

To disipate 8 volts or more off a 5 volt regulator you might have to use several in parrallel to speard out the disipation.

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There is one other way to do it. You can stack all these in long series strings- which, depending on what you want to do, may be far easier to wire. You get a cheap-ass power inverter, it'll put out around an 80v square wave. Rectify that and you can drive a big series string. Now you'll still need a current limiting resistor and it needs to drop maybe 20v of that, so in practice you've got around 60v of source voltage (lots of LEDs).

Now you can actually do MUCH better with a current regulated string, it's too much trouble with only a couple of LEDs per string but this is easier. Small current shunt resistor, an op amp, a voltage ref, and an appropriately rated output transistor. Or, simpler yet, just a voltage ref and a bipolar transmitter emitter follower. With an accurate current reg, you don't need to drop all the voltage you would with a resistor.

 

I'm with Nigel on grouping leds. There is some fluctuation of the
voltage in a running car but if you are concerned about dimming
LEDs just make sure to put three in a group instead of four and
use a bit bigger resistors. This would give you all "steadines" you want.
By grouping them, you also reduce current draw. If you have 3 LEDs
in a group, your current will be 1/3 of max. etc.
Besides it reduces parts count compared to 500 LEDs with individual
resistors.
Wiring LEDs in parallel sure can work on small scale and with less
than perfect results because you will always have some of them
brighter than another. This can be somewhat reduced by using all
LEDs from same batch but no guarantee.
If you think they are all working and they are all prefect in parallel,
just put them side by side in one cluster to make one illuminated screen.
I tried it and even with few LEDs there was significant difference in
brightness.