In search of equations so i may figure this out myself.

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thisisjaye
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Joined: Thu May 12, 2011 4:19 pm

In search of equations so i may figure this out myself.

Post by thisisjaye » Sun Jun 12, 2011 2:13 pm

Hello,

I am not very versed with resistors, diodes, and transformers and im trying to figure out a site that may have a place to enter variables to determine what i need to light 6 different leds from AC power. I have done similar things in the past but all the leds were of the same variety. this time around i have one that is 1.7V 25ma one thats 3.4v 700 ma 3 that are 3.4 25 ma and a 2.7 that is 25ma. im also putting in an ac led light bulb and would like everything to work off of one switch. im not looking for someone to do the math for me, just to point me in the direction to figure it out my self.

thanks ya'll

Jaye

esklar81
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Re: In search of equations so i may figure this out myself.

Post by esklar81 » Sat Jun 18, 2011 7:15 am

thisisjaye wrote:I am not very versed with resistors, diodes, and transformers and im trying to figure out a site that may have a place to enter variables to determine what i need to light 6 different leds from AC power. im not looking for someone to do the math for me, just to point me in the direction to figure it out my self.
Jaye,
I respect you for undertaking to learn how to do this, rather than asking for someone to do this for you.
thisisjaye wrote: im also putting in an ac led light bulb and would like everything to work off of one switch.
I suggest you get yourself a switched outlet strip and plug a lamp socket adapter, such as this for the "ac led light bulb" into one outlet and a 1 A, 5Vdc wall wart into another. That will save you the trouble of re-inventing a couple of wheels and should limit your work to the relatively benign 5 W of 5Vdc.
thisisjaye wrote:I have done similar things in the past but all the leds were of the same variety. this time around i have one that is 1.7V 25ma one thats 3.4v 700 ma 3 that are 3.4 25 ma and a 2.7 that is 25ma.
The simplest and most reliable way to do this is to provide a DC voltage that is greater than the highest Vf (3.4 V, in your case), then arrange the LEDs in parallel, each with its own current-limitting resistor.

I don't know of a plug-and-chug calculator for this, here's the calculation for the resistor to protect an LED. If you're interested in the derivation, look here.
thisisjaye wrote:thanks ya'll
You're welcome!

Eric

Mee_n_Mac
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Re: In search of equations so i may figure this out myself.

Post by Mee_n_Mac » Sat Jun 18, 2011 10:22 am

I'll add to the prior post that you can put LEDs in series and use a single current limiting resistor for the string but (you knew there'd be one) ...

1) the current through the string will be the same for all the LEDs so you need to make sure they all can handle that current.
2) because not all LEDs are created equal, the light output (brightness) from each will be different, especially for differing colors or types or vendors. This may or may not be significant for your usage.
3) doing this often results in a smaller value for the current limiting resistor. While this means less power dissapated in the resistor (a good thing) it also means that variations in supply voltage can lead to significant changes in the current supplied to the string of LEDs. You need to do an analysis for the highest and lowest voltages you expect and make sure the current doesn't exceed the max allowed. FWIW you need to do this even if each LED has it's own resistor. You also need to calculate the power dissapated in the resistor and get one that can handle that power.

You can use an LED driver chip and avoid most of the issues above but you mentioned using an AC source. Can I assume you're converting that AC to a DC voltage (aka using a "wall wart" or similar) or are you doing something "special". Just as diodes are used in rectifying AC (turning AC into DC), you can use LEDs this way as well. They are, after all, diodes ! :) But there's some more to think about if you're doing this.

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