CAR DC with Alternator

[AtomSoft]

Ok i want to power some LEDs, lets say 6 leds

I know that the car outputs 12v from the battery. But i hear that there is a spike from the alternator. I wanted to know whats the best way to power them without using a voltage regulator.

The LED has a 3.4v (VF) drop ... so if i connect 3 in series thats 10.2v ...

12v - 10.2v = 1.8v / 20mA = 90 Ohm Resistor... But what can i use to protect against the spike?

 

[Reloadron]

Hi Ya Atom

Actually when running the automotive voltage can be 13.8 to about 14.4 volts or greater. Just sitting there (not running) you are actually looking at about 12.6 volts. The problem is when running the voltage of the system varies. Without a regulator I don't see an easy way out.

Ron

 

[AtomSoft]

So should i use a 12v regulator or something, i was tring to keep cost down but cant i guess

 

[AtomSoft]

Can i simply use a 12v zenor? The mA is only 20mA since in series

 

[Reloadron]

The next ***** is that a 12 volt regulator requires at least 14 volts to operate (called a 2 volt dropout voltage). There are regulators like the LM2940 series known as a "Low Dropout" regulator which has a .5 volt dropout so it needs a minimum of of 12.5 volts placing it real close to the line, likely too close with an engine not running.

If you went with a 9 volt regulator like the LM7809 (good for an amp when heat sinked) the best you would do with the mentioned LEDs would be two. Automotive systems can be a ***** with projects like this.

Ron

 

[AtomSoft]

Would this work?

 

[Reloadron]

Yeah, hell that would work if you only need 60 mA or so.

Ron

 

[AtomSoft]

yeah i need to setup about 7 of these through out the car. Image 7 voltage regulators... with heatsinks.. would increase cost a ton! Im replacing some bulbs in a friends car... internal bulbs... and would like to keep cost down so i can make more $$$ from this heh... Thanks a ton!

 

[crutschow]

LEDs are very tolerant of any spikes generated in an automobile. I don't think you have to add any any protection for that (but you could add a capacitor after the resistor but before the LEDs if you feel you need to do something).

 

[AtomSoft]

Thanks! Nice to know, but would like to be safer then sorry, besides 20 caps and 20 zeners comes out to less than $2 which is great!

 

[k7elp60]

Most LED's will illuminate at 1/2 the maximum current, I would figure your resistor for 14.8V-LED V/.020. When the voltage is down to 12.6 then the current would be about half. I did a calculation as follows: 14.8V-10.2=4.6 volts drop accross the resistor. 4.6/0.020A=230Ω I would use a 240. When the voltage is 12.6-10.2= 2.4 volts. I=2.4/240=10mA.

 

[AtomSoft]

To simplify this i might move the resistor to after the Zener.. would make more sense to me... since before zener voltage is unknown sometimes. and after it should be 12v or very close.

 

[audioguru]

To simplify this i might move the resistor to after the Zener.. would make more sense to me... since before zener voltage is unknown sometimes. and after it should be 12v or very close.

The zener diode must be fed the limited current that comes through the resistor. If you connect the 12.0V zener diode directly to a 13.8V to 14.4V battery then the zener will instantly burn out.

Who says that your LEDs are 3.4V? No they aren't, they might be 3.4V because LEDs are all different and have a range of voltages. They might be 3.1V or might be 3.7 or any voltage in between. They need to be measured then have the current-limiting resistor (from the 12V zener diode) calculated for them.

 

[ronsimpson]

A LED is much like a Zener diode. Below their voltage they pull very little current. At their voltage they start to pull current. The current goes up steep with very little more voltage. Pick a resistor that gives 20mA at 14.5 volts. If you use 4 LEDs, about 12V, then voltage variations (12v to 14.4v) will cause the light to change form 0 to full. If you use 2 leds, (about 6 volts) the voltage variations will cause the light to change about 30%.

 

[AtomSoft]

The zener diode must be fed the limited current that comes through the resistor. If you connect the 12.0V zener diode directly to a 13.8V to 14.4V battery then the zener will instantly burn out.

Who says that your LEDs are 3.4V? No they aren't, they might be 3.4V because LEDs are all different and have a range of voltages. They might be 3.1V or might be 3.7 or any voltage in between. They need to be measured then have the current-limiting resistor (from the 12V zener diode) calculated for them.

I understand what you mean but as for the led dropout ... im telling you what the datasheet says. I dont have all day to go check led dropouts and calculate on that so i follow the data available.

Also, some of the boards im making for the leds may contain 6 to 9 leds all which will be sets of 3 leds in series so if the boards has nine leds then i would need 60mA at least for brightness. And at full 35mA i would need 105mA for 3 sets...

so for 60mA i need 76 ohms or something close to that for 20mA each set
and for 105mA i need 43 ohms or something close to that for 35mA each set

BLUE LED: https://www.mouser.com/ProductDetail/Cree-Inc/C566C-BFN-CS0W0352/?qs=D3QG1iK7FqhS/t3xpngBIg==

 

[kinarfi]

From my experience, just go with the 3 LEDs and the 90 ohm resistor, rig one set up and hook it to cigarette lighter plug, plug it in and go for a ride, take you amp meter and watch the the current. LEDs are quite tolerant to momentary over currents, I've run them up 50% over for short periods with no ill effects.
Another way to limit current is the use of a 3 terminal voltage regulator with a resistor between adj and out and current drawn from the adj terminal, this can be put ahead or after the LEDs. The value of the resistor is 1.24 volts divided by the current, generally good for 3 LEDs on a 12 - 14 volt circuit, if your running around 20 - 50 ma, you probably don't need a heat sink.
Kinarfi

PS lm317 in to92 package, 100 ma, $.42 each https://www.mouser.com/ProductDetai...317LZG/?qs=sGAEpiMZZMtqO%2bWUGLBzeIrZQMrk8AIN
resistor $.06 https://www.mouser.com/ProductDetai...=sGAEpiMZZMtlubZbdhIBIDOajCjqTKn2XXYRZnxQefg=

 

[audioguru]

im telling you what the datasheet says.

No you are not, you are saying what Mouser says. Mouser is not the manufacturer.
CREE is the manufacturer and they show that the typical forward voltage is 3.4V and might be as high as the maximum of 4.0V. The minimum voltage is not stated.

If your LEDs are actually 4.0V and you have 3 in series and you use a 12.0V supply then they will be very dim or will not light.
If your LEDs are actually the unknown minimum voltage and you have the current-limiting resistor set for 3.4V LEDs then the LEDs might burn out.
Threfore you must gamble about the LEDs voltage, or measure the LEDs voltage and calculate a suitable current-limiting resistor or use only 2 LEDs in series for each group.

 

[AtomSoft]

No you are not, you are saying what Mouser says. Mouser is not the manufacturer.
CREE is the manufacturer and they show that the typical forward voltage is 3.4V and might be as high as the maximum of 4.0V. The minimum voltage is not stated.

If your LEDs are actually 4.0V and you have 3 in series and you use a 12.0V supply then they will be very dim or will not light.
If your LEDs are actually the unknown minimum voltage and you have the current-limiting resistor set for 3.4V LEDs then the LEDs might burn out.
Threfore you must gamble about the LEDs voltage, or measure the LEDs voltage and calculate a suitable current-limiting resistor or use only 2 LEDs in series for each group.

audioguru no disrespect but what the hell are you talking about? "No you are not, you are saying what Mouser says. "

THE TYPICAL VALUE IS 3.4V so how is that not what the datasheet says? Mouser may not be the manufacture but i thought they take data from the datasheet or from manufacture directly.. You think they just put any value?

I connected 3 led in series here at home and hit it with 12v and its bright as hell! Ill do a test with multimeter in a minute on the drop out measured.

audioguru i always like your advice tho so dont take this wrong

 

[AtomSoft]

Also as a side note the LEDS wont be on while driving. Only happens when door is open or someone manual turns the inside lights on. Like the reading light.

 

[audioguru]

audioguru no disrespect but what the hell are you talking about?

I am talking about the unknown forward voltage of your LEDs and the current-limiting resistor calculated only if they have the typical voltage of 3.4V each.

If you use only two LEDs in series then their current will have less difference if their forward voltage is high or low. Or you can measure their forward voltage and calculate a suitable resistor value.