HOW TO HOOK UP LEDS - PROPER LED WIRING - choosing the correct current limiting resistor and verifying performance

Copyright © 1995-2010 Quickar Electronics, Inc - click here to return to our web site

The basic principle is to choose the correct components and wiring based on the most accurate information at hand, and then using a simple, inexpensive meter, measure and verify performance, and make adjustments as necessary to achieve 100% performance within manufacturers operating guidelines.

In almost all cases leds will need a current limiting resistor

to keep them from blowing up

To make sure you dont blow up your leds by accident

CLICK HERE FOR OUR CURRENT LIMITING RESISTOR CALCULATOR for leds

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LEDS LIKE ALL OTHER ELECTRONIC COMPONENTS HAVE THEIR OWN SPECIAL ELECTRICAL PROPERTIES THAT ARE GOVERENED BY A SET OF MATHMATICAL FORMULAS KNOWN AS OHMS LAW.

Ohms law governs the relationships between VOLTAGE (V) - CURRENT (A) for amps –

WATTS OR POWER (W) – AND RESISTANCE (R) measured in ohms and watts

CLICK HERE FOR OHMS LAW PAGE

While you don’t have to COMPLETELY know and understand ohms law ,because there are formulas on our web site that will help you with the math, you do have to understand that ohms law will guide you to select the correct value components needed to safely operate your lads; specifically the current limiting resistors needed.

LED BASICS:

On most leds with leads, the longer lead is positive.

Leds are current sensitive devices, meaning that if the current flow through the led isnt limited, the led will burn out.

Usually this is prevented by using a resistor to limit the current thru the led.

Leds are also sensitive to polarity, meaning the correct wire must be on the positive and the correct wire on the negaive lead of the led or it wont work. We suggest you add the correct current limiting resistor first if you dont know which is the positive and negative wires on your power supply....the resistor will prevent the led from blowing up if it is hooked up backwards at first.

Because leds are semiconductors, you must apply a certain voltage ( called forward voltage drop) before the led will be fully turned on, but when you reach that forward voltage drop, you must then make sure that the current is limited to the manufacturers maximum rating or under.

When you buy leds from us, we tell you the forward voltage drop and maximum current. You can then use our current limiting resistor calculator to calculate the correct resistor based on your power supply voltage.

CLICK HERE FOR OUR CURRENT LIMITING RESISTOR CALCULATOR for leds

Another essential part of proper led wiring, is first of all to verify the actual voltage output and current capability of your power source.

Using your inexpensive meter, set the meter to read VOLTS DC, and put the red( positive) lead on the positive of your power source, and the black ( negative) lead on the negative of your power source, and the measure the voltage of your power source.

Once you know your power supply voltage, you then make your calculations based on the led information supplied by the manufacturer ( voltage drop and maximum current)

After wiring your leds with the proper current limiting resistor(s) in place, then using the same simple, inexpensive meter, you measure the current, buy put the meter to the MILLIAMPS or AMPS setting, and placing the meter in line ( in series) with the led(s) measure the current flow to make sure it is within the maximum operating current of the leds. If the reading is too high or too low, you can either adjust your power supply voltage, or adjust the resistance value of your current limiting resistor(s) until you reach your desired reading.

Please note: if your meter goes the wrong way - reverse the wires.

 

SERIES VS PARALLEL WIRING

Leds can be Wired In Series : where the electricity flows from positive to negative thru each led one at a time in a row

( you dont want too many leds in series, because the voltage drops add upin series, and with too many leds, the voltage required will be too high to be practical )

In series wiring the forward voltage drops are additive, but the current requirement stays the same..........for example, if each red led has a 2 volt forward voltage drop and a maximum 20 milliamps, and you put 2 in series, then you will need 4 volts at 20 milliamps to drive the leds to maximum brightness........If you had 6 red leds in series, then 12 volts at 20 milliamps is required

or Wired In Parallel : where each led gets the same power at the same time

( not a good idea for more than a few leds because of minor differences between leds, some will get too much power, and some wont get enough)

In parallel wiring, the current requirements are additive, but the voltage drop remains the same; for example, if each red led has a 2 volt forward voltage drop and a maximum 20 milliamps, and you put 2 in parallel, then you will need 2 volts at 40 milliamps to drive the leds to maximum brightness........If you had 6 red leds in parallel, then 2 volts at 120 milliamps is required

CLICK HERE FOR OUR CURRENT LIMITING RESISTOR CALCULATOR try some calculations for yourself

The object is to find which method or combination of methods gets your power requirements in your led wiring scheme, to closly match the power source.

Having voltage much higher than needed results in the need for very large, expensive, hard to find, and inefficient current limiting resistors that waste energy by getting real hot.

By using a power supply that is just slightly higher voltage than needed,or by putting leds in series will result in a more efficient design, and the current limiting resistors needed will be much smaller and very inexpensive and easy to find .

If you need to hook up many leds The correct way is to place several leds in series, and then several series strings in parallel with each other, thereby needing neither the high voltage needed with series wiring, or the high current needed with all parallel wiring.

OSRAM OPTOELECTRONICS has a great article about this in more detail - click here for osrams article

A NOTE CONCERNING USING LEDS IN AUTOMOTIVE APPLICATIONS: Automotive voltages fluctuate from under 12volts dc to as high as 14.8 volts dc. We suggest using current limiting resistors based on your highest battery reading with alternator running full blast .The leds wont be as bright when the car isnt running, but you wont blow them up . Alternately, you can use an IC as a Buck Regulator and put your set point at 12vdc and calculate resistors based on 12 volts.

click here to return to our QUICKAR ELECTRONICS web site SHOPPING CART SYSTEM

MODEL RAILROADERS - NEW - CLICK HERE TO LEARN HOW TO WIRE LEDS AS TURNOUT (SWITCH) POSITION INDICATORS

 

BELOW ARE SOME COMMON CIRCUITS YOU MIGHT ENCOUNTER:

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proper way to connect multiple leds is either each one with its own current limiting resistor

or see the picture immediately below for another safe alternative

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rather than wire every led in parallel with its own current limiting resistors as shown above, you can use the

series/parallel circuit matrix shown below use 120 ohm 1/8th watt resistors for the diagram below with white leds and a 12 volt dc power source

click here for an interesting paper published by osram optoelectronics detailing wiring groups of leds together

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leds for model railroaders

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for lighting HO SCALE BUILDINGS

USING WHITE INVERTED CONE LEDS

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for lighting HO SCALE BUILDINGS

USING SUPERFLUX ( ALSO KNOWN AS PIRANHA) LEDS

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DRIVING LEDS WITH PWM ( Pulse Width Modulation)

Although you can use a potentiometer or rheostat or use a transistor as a variable resistor to brighten and dim leds, both methods have serious drawbacks. In all the above cases the parts can get real hot, the circuit is very inefficient, and with batteries it can lead to seriously decreased battery life, and in some cases, more power is being wasted limiting the current than is going to the leds themselves

without getting too technical

pwm takes pure dc, and chops it up into a series of pulses in which the duration between pulses can be varied using the potentiometer on the pwm kit. Some people also refer to pwm circuits as a duty cycle controllers

by varying the width of the pulses, we can vary the brightness of the leds

by varying the pulses, this has the effect of raising or lowering the average current delivered to the leds ( making the leds brighter or dimmer) This can be measured accurately enough with a simple VOM meter set to read milliamps - make sure you do not exceed the manufacturers guidelines for driving your leds -PLEASE NOTE PWM IS NOT A LED CURRENT LIMITER - PWM OR NOT, YOU STILL MUST LIMIT THE CURRENT TO THE LEDS TO THE MANUFACTURERS MAXIMUM RATINGS

The advantage of PWM over Pure DC is, it is much more efficient, resulting in lower power consumption, longer battery life, less heat in the pwm circuit versus the other circuits mentioned, less led heating, smaller heatsinks can be used on parts requiring heatsinking

for sake of electrical calculations for current limiting resistors for your leds, you can consider the pwm circuit as if it wasnt even there, in that you still have to choose and use the correct power supply and/or current limiting resistors so you dont blow your leds up.

see diagram below:

The one fascinating thing pwm will do, happens due to the human eyes inability to see that rapid 235 to 2000 flashes per second, so that by using a smaller than normal amount of current, your eyes can be fooled into thinking it is just as bright as pure dc.

That is how they get the backlighting on cell phones to be so bright and last so long, they are lighting the backs of the lcds and keypads with pwm driven leds !

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A sample calculation of the dropping resistor is below,

based on a 5v supply and a 1.7 volt red led driven at 10ma

connecting light emitting diodes (LED's) to supply

Most leds operate at 1.7 - 3.6V although this is not always the case and it is wise to check. The dropping resistor is simply the net of supply voltage minus the led voltage then divided by the led brightness current expressed as "amps" (ohms law). Note the orientation of both cathode (negative) and anode (positive) with respect to the ground end and the supply end. Usually with a led the longer lead is the anode. (positive)

 

HERE IS A GREAT ARTICLE FROM OSRAM OPTO ELECTRONICS THAT EXPLAINS HOW YOU HOOK UP LARGE GROUPS OF LEDS

THE CORRECT WAY http://www.quickar.com/ledstherightway.pdf

DISCLAIMER:

WHILE WE BELEIVE THIS INFORMATION TO BE SAFE AND CORRECT WE ASSUME NO RESPONSIBILITY FOR ITS USE

BEFORE USING ANY OF THE INFORMATION CONTAINED HEREIN PLEASE CLICK HERE TO READ OUR COMPLETE WARRANTY AND DISCLAIMER AND TERMS OF SALE

LED WIRING - choosing the correct wiring scheme, the proper current limiting resistors and verifying performance

Copyright © 1995-2010 Quickar Electronics, Inc - click here to return to our web site