LED clculators not working..

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dacflyer
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LED clculators not working..

Post by dacflyer » Wed Mar 16, 2016 7:06 pm

i was playing around with a 7-seg led and to get the desired brightness to match some other leds i have i needed the right resistor.
i have a 5v supply, voltage to the led with resistor is 1.8v approx. at 1Ma.
with this info i thought i'd play with the led calculator ( several online ) and i put in the info. and it tells me i needed something like a 3.3K resistor.. this is not right..it would not even light the led at this value.
anyone know what is going on with these led calculators ?
seems that they do not do well with low number readings

example ---> http://led.linear1.org/1led.wiz?VS=5;VF=1.8;ID=1

and another http://led.linear1.org/1led.wiz?VS=9;VF=1.8;ID=1

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Casey1947
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Re: LED clculators not working..

Post by Casey1947 » Wed Mar 16, 2016 8:22 pm

Hello,
5 - 1.8 = 3.2
3.2 / .001 = 3200
3300 Resistor is the closest value.
It should light up but you may only see it if it is dark and the lights are out. Why are you only using one mA of current? If you use 10 mA of current the resistor should be 320 ohms.

Casey

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Re: LED clculators not working..

Post by dyarker » Thu Mar 17, 2016 4:50 am

Casey is correct. Where did you get that 1mA figure? That just about the curve in an LED's voltage to current graph where the line changes from almost horizontal (and dark), to almost vertical (increasing light).

The 1.8V drop on the LED is correct for a red LED.

The calculator worked with the numbers fed in.

A typical graph: https://en.wikipedia.org/wiki/Light-emi ... -Curve.svg

The whole page: https://en.wikipedia.org/wiki/Light-emitting_diode

Cheers,
Dale Y

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Re: LED clculators not working..

Post by dacflyer » Thu Mar 17, 2016 8:17 am

i have a high brightness led 7-segment display, using it with other 7-segment displays of same size, but the brightnesses are way different, so i needed to dim it down to match the others.
so i used a 220ohm resistor to get the match i wanted, but my meter showed it drawing 1ma, and the voltage to the led was about 1.8V

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Re: LED clculators not working..

Post by dyarker » Thu Mar 17, 2016 9:33 am

Something wrong somewhere with 1mA reading, because -
1.8V is the correct drop for a red LED, so
5V - 1.8V = 3.2V across the resistor
3.2V / 220 Ohms = 0.014545A, which = 14.5mA
which is consistent with small LEDs that have maximum currents of 20mA or 30mA.

What value resistor did you start with that was too bright?
Dale Y

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Re: LED clculators not working..

Post by dacflyer » Thu Mar 17, 2016 9:38 am

if i remember right i think i used 100 Ohm with 5v.
when it says voltage drop is that asking what is needed to power the led ? or the voltage drop across the resistor ?

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Re: LED clculators not working..

Post by dyarker » Thu Mar 17, 2016 3:14 pm

That would be the LED's forward voltage, because that is pretty well stable over the current range of the LED. The color of light emitted from a LED is set by which doping chemicals (elements) are added to the basic semiconductor chemical (often silicon). And the chemistry physics sets the forward voltage.

You pick the current to set the brightness you want, as you discovered. The current is determined by the resistor. To calculate the resistance of the resistor, divide the voltage across the resistor by current. The voltage across the resistor is the supply voltage minus the LED forward voltage.

In general (I think there are a couple exceptions), the LED forward voltage increases as the frequency of the light increases. (or wavelength decreases). So infrared like 1.6V, red about 1.8V then amber yellow green each a little voltage than the one before. Blue at 3.8V to low fours. White LEDs are also 3.8V because they really blue LEDs with florescent coating inside the lens.
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Re: LED clculators not working..

Post by dacflyer » Fri Mar 18, 2016 9:14 am

ok, so what was i doing wrong to get the weird info i got ?

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Re: LED clculators not working..

Post by dyarker » Fri Mar 18, 2016 10:04 am

??? I don't know either, just that the 1mA reading is wrong somehow. You might have mis-counted positions from decimal point on meter display, knob on meter in wrong position, one of the test leads in high current shunt jack on meter. Something that caused reading to be times ten off.
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Re: LED clculators not working..

Post by haklesup » Fri Mar 18, 2016 10:43 am

Using an online calculator for a specific device is probably going to result in inaccuracies. Most diode calculators probably assume a single Si junction but a display may have many in parallel. While the forward voltage may remain similar, the forward resistance can be quite different than the calculator.

1mA is probably about right for a single LED in a T1 package like the calculator seems to represent. Check the specs for the display or similar one or just determine the resistor value by experimentation.

Consider using a Current source. you can get them cheap on ebay either loose or as part of an LED light fixture. For example

http://www.ebay.com/sch/i.html?_from=R4 ... e&_sacat=0

http://www.ebay.com/itm/SD-C017C-3A-1-3 ... SwT~9Wjdx8

http://www.ebay.com/itm/6W-9W-12W-15W-1 ... jn2b2BbHGw

I've been getting this last style and they are cheap and really easy to use.

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Re: LED clculators not working..

Post by dyarker » Sat Mar 19, 2016 7:34 am

1Ma is not even close for typical LED (maybe in closet at midnight during a power outage)

A current sources would be overkill for indicators, or segments in displays.
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Re: LED clculators not working..

Post by dacflyer » Sat Mar 19, 2016 6:50 pm

well my meter has the AMP meter section ( 20A MAX ) and it displayed .01A

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Re: LED clculators not working..

Post by dyarker » Sun Mar 20, 2016 4:51 am

.01 is ten milliAmps when the units are Amps. So it is just a simple mis-read of the meter.

C U L
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Re: LED clculators not working..

Post by dacflyer » Mon Mar 21, 2016 12:36 pm

ahhh stupid me, i read it backwards..never was good at decimal readings..
thanks... so 10Ma sounds more logical to you all now ? and a 220Ohm resistor @5V

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Re: LED clculators not working..

Post by dyarker » Mon Mar 21, 2016 1:10 pm

No sweat! One off boo-boo. The math says the actual current should be 14mA, but your meter only has 2 digits after the decimal point so the "4" is cut-off.

Cheers & CUL,
Dale Y

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