Power Up LEDs

Maybe.

All the following ASSumes you have a charger for lead-acid batteries.

Depends on circuit used to monitor and regulate the charging of the battery. Some chargers will "look" for the 12V (or something in the 8-14 VDC range) before it will enable it's output. If the voltage is too low, the charger might go into a mode where it behaves as a small valued current source (50 mA-100 mA) where the voltage is not regulated until the voltage reaches a "safe" value like 8 or 9 volts. In this way a totally dead battery is slowly brought up to voltage, allowing the charger to make sure the battery will safely take a charge. This mode prevents damage to batteries with shorted cells or if the charger is hooked into a dead short to prevent fires.

Another issue you may find is that the charging output while technically is considered DC, it is not necessarily designed to produce "clean" DC because batteries don't care. You may notice a lot of pulsing or other objectionable artifacts in the light produced.

You could try hooking it up and see what happens. If you happen to have a 16 to 20 volt zener diode, you could hook that up across the output just in case the charger tries to output a dangerously (to the LED's) high voltage.

If the charger does successfully illuminate the LED's, but you have objectionable pulsing or other nastiness, hook up a fairly large aluminum electrolytic capacitor to the output, paying attention to the polarity of the capacitor. I would use a capacitor that has a voltage rating of at least 25 VDC. Hopefully it will be 500 uF or larger in capacitance, but you could try anything you can find to see if it helps if you have objectionable pulsing. Several thousand uF would probably be ideal.

If you still have a Radio Shack in business in your local area, they will have exactly what you need. If not, there is always Newark, Digi-Key (they will ship anything less than 13 oz. first class for a VERY reasonable cost), Mouser, Jameco, Marlin P. Jones, or you might find a surplus house selling just what you want.

Good luck with your quest.

P.S. If you happen to have a small lead acid battery for a motorcycle or lawn tractor or something, you could also have your very own UPS for your lighting and that would negate all the problems I described above.

no

Many certainly can be. It depends on the type of battery the charger was supposed to charge, the design of the 12V LEDs and how clever the person was to connect each of these.

There are so many types of battery chargers.

Some are not even good to leave the battery on charge all the time.

I would prefer to check that out first and connect a small lead acid battery to get some stable source for your leds.

Perhaps you can tell us what the charger looks like?

Dear Mr.ronseto

I am of the opinion that the answer for your question is YES.

A battery charger can be directly used for 12 Volts LED provided the Battery Charger - DC VOLT-AMPERE is higher than the VOLT-AMPERE needed for the LED.

If the Battery Charger is under-sized, it will burn out.

DHAYANANDHAN.S

We have a pertinent "NO", a "YES, without restrictions, and some more other approaches, pointing out restrictions or conditions.

12 Volts DC have been designed to work from a DC source with average, say 12 Volts as nominal value.

The cheapest battery chargers are just a tapped transformer with a selenium rectifier (2 diode bridge configuration and center tapped secondary) and provides 2 rectified sinuses without any smoothing. The voltage is controlled with (sometimes) only a selector switch, that provides between 16 to 30 Volts at the output.

(the selenium rectifier's characteristic is somewhat influencing the output/voltage/current as you can see on the rectifier's data sheets)

You supply a 120 Hz strobe voltage and this will have its impact on the led life. Is actually switched on and off this amount of times, according the sinus curve.

Our eye doesn't see that happen, but it occurs. You will see the led glow, giving more light output the more you go from low, to medium and the most light you will get at the "start" position. Just don't know for how long.
Perhaps you can tell us?

All the battery chargers that need a battery as reference, simply will not work, because a led is not a battery and is not acting as one.

The very cheap trickle chargers may work when a few small leds are connected.

A regulated DC supply could take better care of it. (like the ones used on CB radios)

he'd blow the little diodes to smoke!, it wont work, once he exceeded max forward voltage it would be over

Thanks for your response. I didn't go in detail too much, considering the few words question.

Ron talks about 12 Volts led arrays. One small led will just flash once, because it gets like more than 10 times the voltage (and admissible current)

The array however will make it for a while.

I was a bit sarcastic when asking for how long.

It won't be long, yeah?

you can only if the the D.C supply current of your charger is less more than LED's drawing current and the charger voltage should be stable in your case should be 12 volt

with -/+ 0.5 volt error ratio

Thank you all. I was trying to find a cheap way to provide light using LEDs without using batteries. I probably should be looking for a regulated power supply. The other idea would be to use a car battery for power and charge with a charger. I wonder how long a 12V LED would keep burning on a car battery?

https://learn.sparkfun.com/tutorials/light-emitting-diodes-leds/leds-without-math heres the math on sizing Leds

The question that follows is: how many Amps has the car battery you want to use.... and how many watts does the LED draw?. On the other hand, depending for how many hours on a row you expect to use the LED´s as a lighting source, you may consider getting a "Deep Cycle Battery"

He also did not mention how many wheels are on the car and if it is a toy car, or electric vehicle. I pass.

Yet another cautionary comment.

I have in front of me an LED spotlamp fitting described as "12 Volts" but this is Chinese for "Needs a current limited supply of 350 mA and 8+ volts". I also have the separate mains PSU that was supplied with it by the retailer, the label of which says "Load 2...3LED 1W" DC 350mA 6...12V". Hmm.

If your LED lamp(s) have built-in current limiters you might get away with feeding from a charger but most battery chargers supply un-filtered, un-regulated rectified AC, so beware!

Depends on the power of your 12V DC LED lamp assembly.

12W lamp draws 1 amp.

A 100Ah car battery will keep a 12W lamp burning for over 100hours. Do the sums....

Fitting out a boat or RV Ron?

A constant current source can be made with a voltage compliance using the load, a transistor and two resistors. This circuit is normally used in the input LED drive of an SSR. I have one with input range of 3 to 32 Volts DC.

Gajanan Phadte

I used a 60w 12v garden light transformer with a diode bridge added to light 380 led luminaire I built. Blew up the diode bridge so I fitted a 30A version with heat sink and all seems good.

Far better to go down this path;

http://www.aliexpress.com/wholesale?catId=0&initiative_id=AS_20150323000248&SearchText=led+power+supply+50w

As is, usually "NO".

As many here have already posted......you should believe them.....

Usually the LEDs connected to a 12 Lead Acid Battery charger (assuming such a charger is meant!) will burn out, either immediately or after a very short life span.

Some LA Battery chargers, when unloaded (no battery) may output as much as 18 volts! In my limited experience!!

But even with a battery connected, as the battery nears full charge, the voltage could easily approach 14.5 volts.

12 volt LEDs need 12 volts. Period! That is for a proper life span....you can "under drive" them with slightly less voltage if that helps.....(probably not!) safely....

I figured they'd be able to work up to at least 13.8V eg automotive, but I found one ref that says 13.5 is the maximum and automotive charging circuits may kill them. A bit bog.

A naked car battery would be ok.

Other lamps had other limits. Needs googling and checking by OP.

Seriously, a 12v SMPS monoblock is pretty cheap. Why mess around with a battery charger?

Exactly.....

Some people are destined to be disappointed!!

OK, let's take a look at bulb that does not have an active switching regulator. I needed to replace two incandescent bulbs above the dinette in my RV and they have the BA15d base, which is a bayonet base with two contacts on the bottom and the shell is not electrically connected. This bulb manufactured by GRV and is available for sale on a certain major on-line retailer and it looks like this:

This unit has no capacitors, inductors, IC's or transistors. Just four or so resistors to provide current limiting for the series strings of LED's. If you peek inside the unit between the "cracks" in the LED PCB strips you can see the resistors.

This particular unit does NOT have a constant power profile as seen by the plot of current vs. voltage below:

Here is the plot of power vs. voltage:

As you can see, the power consumed by this particular LED module rises very quickly as a function of voltage. As every EE should know, the power dissipated by a (linear) resistor goes up as the square of the voltage and this curve certainly suggests a second order function.

There are four (I think) resistors packaged in this bulb. Each appears to be a 1/4 watt resistor. At 15V, this bulb consumes approximately 9W. At the higher voltages, I only powered the bulb long enough to get a current reading, then I powered it down as I was getting the distinct odor of "hot electronics" on the verge of releasing the magic smoke. Doing the math, you can see at 9 watts dissipated by four 1/4 watt resistors is not a formula for long life.

This particular bulb, must be operated at normal 12 VDC automotive voltages or you will let out the magic smoke.

This particular bulb exhibits a changing brightness over the range of voltages tested. At the lowest voltage, you could just barely see the individual 'pixel' LED elements in each of the 5050 LED's.

I will say this though in defense of this particular design, the positive current consumption coefficient (or positive impedance if you will) makes this particular bulb more likely to "play better" with battery chargers that use switching power supplies.

LED bulbs that have constant power profiles, tend to exhibit a negative impedance, meaning that as the voltage rises, the current drops in proportion. This characteristic can play havoc with the control loops of switching power supplies and oscillation may ensue. A little series resistance may be inserted to dampen this response. Since LED bulbs draw so little power, a small series resistance does not exact a large power consumption penalty.

Excellent.

As I said earlier, it depends on the type of battery charger, the LED package one is using and how clever the installer is in recognizing or installing a suitable SMPS.

The LED shown should be replaced with the load to be driven.

Gajanan Phadte