The basic help for leds please

Thread Starter

questilong

Joined Apr 19, 2016
10
Hello,
I am just starting electronic, I tought I was doing well until I came to this problem.
I absolutely want my led to be powered of my 6.1VAC - 5A transformer (other component runs from this tap).

I have this led :
http://www.cml-it.com/index.php?p=34&a=19270251&c=89

My transformer outputs 6.1VAC @5A (I don't want to use a bridge rectifier to get DC and power the led)
This led require 12VAC (+-10%) @9ma to light @60mcd.

IS IT CORRECT to assume that my led will run from my 6.1VAC transformer but will draw 18ma (double) to run @60mcd ?
OR the led will run from 6VAC but will still draw 9ma and run at about @30mcd ?
OR this will not even light the led ?

THANK YOU VERY MUCH I tried to find the answer alone but failed...
 

dl324

Joined Mar 30, 2015
11,240
Welcome to AAC!
My transformer outputs 6.1VAC @5A (I don't want to use a bridge rectifier to get DC and power the led)
The LED includes a half wave rectifier
This led require 12VAC (+-10%) @9ma to light @60mcd.

IS IT CORRECT to assume that my led will run from my 6.1VAC transformer but will draw 18ma (double) to run @60mcd ?
OR the led will run from 6VAC but will still draw 9ma and run at about @30mcd ?
OR this will not even light the led ?
No. Assuming it will operate from the lower voltage, current will be closer to 5mA; based on the 9mA spec when operated from 12VAC.
 

#12

Joined Nov 30, 2010
18,210
I see your indicator has its own rectifier inside it. It will not convert low volts to higher amps, it will just use less current because there isn't much voltage. I predict it will work, but be dim.

and...dl324 got there 10 seconds ahead of me.:(
 

Thread Starter

questilong

Joined Apr 19, 2016
10
You don't know how much you helped me, Thank you,
because I did not know it includes a half wave rectifier (now I see it on the specs !

So to clarify the led will run from the 6VAC but will draw 5ma of current and still light @60mcd ?
(Why it draws less current with a lower voltage ? Ohm's law ?
Or did you said it will run from 6VAC and draw 5ma of current and light @30mcd ?

Thank you for the welcoming (I love this forum I read a lot of helping post before signing up :)
 

dl324

Joined Mar 30, 2015
11,240
So to clarify the led will run from the 6VAC but will draw 5ma of current and still light @60mcd ?
Brightness will be about 25%. The note says brightness operating from 12VAC will be 25% less than DC.
(Why it draws less current with a lower voltage ? Ohm's law ?
It depends on the internal circuitry. We know there's a diode and a series resistor, don't know if there's more than 1 LED.
 

dl324

Joined Mar 30, 2015
11,240
Additional info. If the LED was operated at 12VDC with a 50% duty cycle, brightness would be reduced by 50% because luminous intensity is time averaged.

When operated at 12VAC, the peak voltage is around 18V; but for most of the positive AC cycle it's less. That's how they came up with the 25% reduction when operating on 12VAC (instead of 50%).

Relative luminous intensity vs current isn't linear, but for smallish variations, it's close enough. This is the graph from a random datasheet:
upload_2016-4-19_14-34-15.png
 

Thread Starter

questilong

Joined Apr 19, 2016
10
Wow that's awesome of you !
Ok it start to make complete sense thanks again for this,
You really made me understand better how leds works (I noted that led only work on DC too that's valuable to me)

So now I will digest the information and get back if I have another problem.
(I know it seems simple but I am just starting so sorry for this ;)
 
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