I need help with LED and the resistor calculation

[bigjack]

I need some advice. I have had my first attempt at using LED's in wagons. or thought the white light is a bit on the bright side, I'm quite pleased with the result.

My question is this, I've looked at a couple of LED resistor calculators
This one http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator#series and this one http://ledcalculator.net/ . Now if I use a 9 volt supply, and 2 LED's do I take an average of the LED voltage drop ie Red 2.1 and Orange 2.2 making an average of 2.15v and put that into the voltage drop box and then 2 LED's into it's box?

The next question, one calculator shows the resistor on the positive side and the other shows the resistor on the negative side. Which is right or, are they both correct?

Any help, much apprediated.

 

[TerrySoham]

You add the forward voltages together.

You can place the resistor on either side of the LEDs.

Regards

 

[bigjack]

You add the forward voltages together.

You can place the resistor on either side of the LEDs.

Regards

Thanks for that, Do I do the same with the forward current?

 

[TerrySoham]

I responded without looking at the web sites. Now I've looked I need to alter my response.

Yes average the two values and enter the value in the input box.

Also enter 2 in the number of LEDs

Sorry for the confusion

The actual formula is (input voltage - total forward voltage) / current in milliAmps

 

[TerrySoham]

What are your forward currents? If they are similar use the lower value in the formula,. Selecting the correct value resistor need not be that precise!! (IMHO)

 

[bigjack]

What are your forward currents? If they are similar use the lower value in the formula,. Selecting the correct value resistor need not be that precise!! (IMHO)

Various values actually , generally 25ma or 30ma.

Thanks for the help

 

[Neil Robinson]

What are your forward currents? If they are similar use the lower value in the formula,. Selecting the correct value resistor need not be that precise!! (IMHO)

Various values actually , generally 25ma or 30ma.

Thanks for the help

To me those values look a bit like the maximum forward current values. I would suggest that you design using the current quoted for the typical values of luminous intensity. This is often between 33% to 66% of the maximum forward current.

 

[tramcar trev]

This resistor calculator is ASAD (all singing all dancing) has options for leds in series or parallel.... I'm told its best in theory to have the resistor on the anode or + side of the LED but I can't see why, the resistor limits the current so could be anywhere.....
I also discovered that to imitate incandescent lighting use warm white LEDs at a slightly lower current than normal and Fluros are imitated nicely by white LEDS... I buy mine via Ebay international, most from HK or PRC @ prices per 100 that are close to the price each here.
Another tip that will infuriate the purists is that I use these, multiturn trimpots and use these as a mini dimmer, adjust with a screwdriver to get exatcly the degree of illumination you want then either leave it in place or measure the rersistance and replace with a fixed resistor... these are 1K which will be about right for a 12V supply.... : http://cgi.ebay.com.au/10PCS-1K-Ohm...al_Components_Supplies_ET&hash=item45f9f70ad9
Oh nearly forgot the URL: http://www.bennyinternational.com/lib/ledcalculator.html

 

[TerrySoham]

Using your data, the formula gives a value of 156.6 ohms. Use the nearest value you can find.

 

[bigjack]

What are your forward currents? If they are similar use the lower value in the formula,. Selecting the correct value resistor need not be that precise!! (IMHO)

Various values actually , generally 25ma or 30ma.

Thanks for the help

To me those values look a bit like the maximum forward current values. I would suggest that you design using the current quoted for the typical values of luminous intensity. This is often between 33% to 66% of the maximum forward current.

So, by entering a lower forward current if it states say Max forward 30ma then I enter 10ma, the light will not be as bright. Am I assuming correct?

 

[Neil Robinson]

What are your forward currents? If they are similar use the lower value in the formula,. Selecting the correct value resistor need not be that precise!! (IMHO)

Various values actually , generally 25ma or 30ma.

Thanks for the help

To me those values look a bit like the maximum forward current values. I would suggest that you design using the current quoted for the typical values of luminous intensity. This is often between 33% to 66% of the maximum forward current.

So, by entering a lower forward current if it states say Max forward 30ma then I enter 10ma, the light will not be as bright. Am I assuming correct?

Basically yes, but you may not notice that much difference until the current reduces to quite a low value.

 

[bigjack]

What are your forward currents? If they are similar use the lower value in the formula,. Selecting the correct value resistor need not be that precise!! (IMHO)

Various values actually , generally 25ma or 30ma.

Thanks for the help

To me those values look a bit like the maximum forward current values. I would suggest that you design using the current quoted for the typical values of luminous intensity. This is often between 33% to 66% of the maximum forward current.

So, by entering a lower forward current if it states say Max forward 30ma then I enter 10ma, the light will not be as bright. Am I assuming correct?

Basically yes, but you may not notice that much difference until the current reduces to quite a low value.

Thanks for all of your help. At long last (because of everyone's help) I'm starting to get my head round this subject