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Topic: How can I get a clean 12v from a cigarette lighter receptacle? (Read 1 time) previous topic - next topic

MrJaWapa

I know the alternator causes the battery to give off more than 12v, and that the battery can sometimes spike up to high voltages.


I'm not sure what to do to prevent my lights from burning up. I'm afraid to wire them to run on 12v and then burn up under higher voltages; or wire them for 14v and the lights be dim due to the lack of voltage.


I've looked for in-line voltage regulators, but everything I have come up with needs a bit of buffer.

Johnny010

More details needed.

What lights?
What voltage do they REALLY need (are they LEDs?)
What sort of current do you need to draw?
Datasheets if possible?

INTP


MrJaWapa

I didn't give the LED info because I didn't think it was necessary.


They are 5mm LEDs

RED
20mA
fV = 2.0-2.2v

WHITE
20mA
fV = 3.0-3.2v

I don't have data sheets, sorry.


I'm wiring the LEDs into groups of 24. I used an online LED calculator (http://ledcalculator.net/) and got this:

RED
http://ledcalculator.net/default.aspx?values=12,2.0,20,24,0

WHITE
http://ledcalculator.net/default.aspx?values=12,3.0,20,24,0

Which leads me to another question, can I ignore the 1ohm resistor? 1ohm doesn't sound like a lot, but I want to make sure.

INTP

 :smiley-confuse:
Car, red and white LEDs. . . . are you making indicators?
5mm LEDs are not bright enough for that purpose.

MrJaWapa

I'm making an emergency response dash light.

A single 5mm LED is not bright enough, but 24 is very bright. I have 3 "blocks" of each color (making 72 white, 72 red).

Wawa

Can't put that many LEDs in series, and use a low value CL resistor.
Then current depends too much on supply voltage variations.

There must be about the same voltage drop across the CL resistor as across one LED in a string.
So a maximum of three white LEDs or four RED LEDs in series.
Leo..

INTP

If it's just to shine at your own face, sure. They'll be washed out in daylight.
A 5mm LED gives like 3 lumens.
A 5050 SMD LED gives you about 20 lumens with a better distribution.
You'll run out of adjectives if you think 24 5mm LEDs is 'very bright'.

All 144 of those LEDs are as bright as just 20 5050's.


Anyway, if you already did the worthless work of soldering up 144 LEDs and 144 resistors, that many LEDs will handle the extra voltage just fine if you wired them up for 12V. Throw in a 3A inline fuse.

MrJaWapa

I've never heard of 5050 SMD LEDs.

I only done this to tinker around with something new; learn something, I guess.

Thank you for answering my question!


EDIT: Op, The LED calculator said I would need 1ohm resistors, can I get by not using one?

wvmarle

EDIT: Op, The LED calculator said I would need 1ohm resistors, can I get by not using one?
1 Ohm resistor or not, you WILL burn your LEDs the moment you have the tiniest of power spikes! 20 mA on 1 Ohm that's 0.02 V you drop there (6 LEDs on one resistor is what the calculator suggests).

Now what would happen if your voltage goes from 12V to 12.5V? Then your 1 Ohm resistor gets to handle 0.5V, and that results in a current of 500 mA through your LEDs. They'll burn very brightly for a short amount of time - until the first burns out and breaks the circuit. The resistor will probably handle it nicely for quite some time as it's just 0.25W it's getting in this scenario.

Also those LEDs never have the exact same drop. 2.0V for one LED is a bit high a number anyway, I'd expect 1.6-1.8V. So that's going to be your first problem: not 0.02V on your resistor, but a whole lot more, and currents to match.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

MrJaWapa


wvmarle

That's going to be really really hard - and for your LED project not the way to go.
Four in series with appropriate series resistor (ideally one resistor per LED but it's a bit impractical for these numbers) and you'll be handling spikes and changes in voltage just fine. If you've got a particularly dirty supply with severe spikes add some capacitors to smooth it out.
Or if you really insist on getting the power clean, get yourself a standard 9V regulator (a beefy one with big heat sink as you're soaking up some 9-10W here) and wire your LEDs to 9V. You may be able to find 10V regulators as well, saves a bit on power loss.
You still need a proper current limiting as LEDs are near-resistance-free conductors.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

Paul_KD7HB

I know the alternator causes the battery to give off more than 12v, and that the battery can sometimes spike up to high voltages.


I'm not sure what to do to prevent my lights from burning up. I'm afraid to wire them to run on 12v and then burn up under higher voltages; or wire them for 14v and the lights be dim due to the lack of voltage.


I've looked for in-line voltage regulators, but everything I have come up with needs a bit of buffer.
Spikes on the 12 volt line are caused by the starter motor when you start the car. The power to the cigarette lighter is turned off as are all the other accessories when you start the motor. So the big spikes won't be problem.

Paul

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