Hi All.

Could some one out there explain how lamps and power needed etc works.

I recently posted that I had purchased a couple of 1watt leds, and was told that they would be very bright and far to powerful for a 12v car battery. What I cant understand is how come a headlamp bulb is 50/80 watt 12v and is fine?

The unit I am hooking these lamps up has a max power of 800mA of current , any idea how this relates to a lamp rated in watts , just totally confused by the whole issue and could really do with a basic explanation of how to sort what lamps and power needed and how it all relates to each other.

MikeF

Power (WATTS) = Volts x Amps
so 80Watts / 12V = 6.6 Amps (your normal main beam tungsten lamp )

The LED in simple terms will be 1W / 12V = 0.08 A or 80mA, so you could have 10 on your 800mA unit

However, you need the tech spec, because I think that some 12V LED's actually run around 300mA, you would need to check or measure.

Would be interesting to elaborate on why these would be thought too bright and powerful for a car battery though

I think bulbs wattage is measured on how much power they consume, and how much they emmit. Conservation of energy says whatever they take in is converted to other forms of energy, light and heat

Bulbs taking 80w will convert some to light, and a load to heat
LEds are much more efficient, and convert almost all to light

Leds only use a tiny current, and need a resistor to protect them from the voltage. This converts some energy to heat
Basic terms, a light bulb is a resistor which glows when electric is passed through
LEds aren't, they're different, but a bit of a mystery to me lol

Obviously a bulb and an LED work in differnt ways...

By its nature a bulb gives off light by getting hot (so it is a resistive load)....Apply 12V across the bulb and the internal resistance controls the current draw.

An LED on the other hand works by black magic and trickery... If you apply a 12V battery to it the current draw will be huge, it has little internal resistance and will gobble up power until it melts...

Also worth noting that big power LEDs tend to have a very specific current they will work at to give the desired life/color/intensity. If you just plug a power supply in as mentioned the current will do whatever it likes, as the LED gets hotter more current will be drawn etc.

Using a proper driver will restrict the curent supplied to the LED by varying the voltages.

A small LED can be current limited using a resistor in series... But as the LED gets bigger and the current is increased the power rating of the resistor required gets silly.... So you start using driver circuits instead that limit the current through switching.


This website has some useful info..

http://led.linear1.org/1led.wiz

It's all to do with efficiency of conversion, an LED will convert far more electricity into light compared to a standard filament bulb. Filament bulbs generate more heat than light, it's only the inert gasses in the bulb that stop it instantly vapourising.

Here's something I found through Google that illustrates the point;

http://www.designrecycleinc.com/led%20comp%20chart.html

Is the driving done through Pulse Width Modulation (PWM)?

I think you can vary the brightness of LEDs through varying the pulse width (but I'll wait for an expert to come along)

yes you do need resistors and electronics for a bare LED, but these days there are many 12V LED's avaialble off the shelf, (which i presume the OP is reffering to) which have all that built in to them, hence the power consumption and output being different and they wont go pop if they are connected directly to the 12v battery

LEDs expressley sold for automotive use such as LED side repeaters, high level brake lights etc will already have the right value resistor built in.

General purpose LEDs are fine for car use, you just need to use the right resistor in series with them.

Resistors are nice and cheap. Theres certainly no need to delve into pulse modulation or wave harmonics to make a bog standard LED work.


Use this calculator http://led.linear1.org/1led.wiz to calculate the value resistor you would need.

Enter 14.5 volts for the source voltage (battery voltage when charging- dont put 12volts!)

If the 'diode forward voltage' isn't stated click on the ? beside this value to get the help screen.

Enter the LED's current rating. click the Find R button.

There is also a link on the page to help you work out the right resistor to use if you are building an array using several LEDs.

An incandescent lamp (i.e. one with a filament) converts somewhere between 2% and 4% of the electrical power into light. Quartz halogen lamps (the type used in headlamps) will be somewhere close to the top of that range.

Modern LEDs can convert maybe 25% of the electrical power into light, which is a vast step up in efficiency. As an example, the really bright LED lights you see on the back of pushbikes usually use a 1 watt emitter. Imagine one of those shining at you from the dashboard.

A modern LED designed for lighting applications puts out between 50 and 200 lumens per watt, an incandescent lamp around 10 all down to the efficiency as said above.

The LED you've bought looks like a lighting class LED in an automotive holder - you can tell if it's a white output from a yellow phosphor on the front (underneath if you scrape off the phosphor 99% chance you'll find a very bright blue LED - it's done that way to allow the selection of a suitable white colour temperature)

I run two 1watt white leds for the side lights/ running lights, low current drain and far brighter than the 4 or 5 watts tungsten lamps they replaced. Had them on all my cars for about 10 years now, never had one fail either, resistors run warm, but then they will do.

[Edited on 7/3/13 by Westy1994]

Thanks all.

Bit clearer now , understand the difference between LED and std lamps.Thanks for the links etc . Big help.

Mikef