# how measure watt of lighting LED

Discussion in 'General Electronics Chat' started by sendbad, Mar 13, 2015.

Jan 8, 2015
43
1
HI
i have White lighting LED 3,7 volt 75lm

but Unknown wattag

how can i know wattag of thes led

2. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,150
769
put 3.7V across it and measure the current.

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3. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,789
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How do you know the Vf and how do you know the lumen output? Did you measure them?
Without a datasheet/part number/spec it would either be trial/error (aka run it at some current and measure the temps) or possibly measuring the physical size of the LED and comparing to others..

Post a picture of this LED..
In general "wattage" really isn't a number you need to know.. Vf and max current rating are the real numbers to use.. With that you can calculate the "wattage".. But IMO "wattage" really isn't a number you need to know to use an LED at all..

4. ### MCU88 Member

Mar 12, 2015
360
35
And...

Wattage = (3.7VDC x Imeasure)

Imeasure is the current that you read on your multimeter.

Jan 8, 2015
43
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i put it with battry 3.7 v 4.8ma it work for ,30h then lighting drop dawn

Jan 8, 2015
43
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vf and lm Printed in Base of it

7. ### BobTPH Active Member

Jun 5, 2013
806
121
No, it is 17.7 mW. And if you are getting 70 Lumens from 17.7 mW you have the worlds most efficient LED.

Bob

Jan 8, 2015
43
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how Calculat it

9. ### MCU88 Member

Mar 12, 2015
360
35
lol gawd you actually pulled an calculator out and did it?

Jan 8, 2015
43
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so for 70 lumens what suitable waat

11. ### BobTPH Active Member

Jun 5, 2013
806
121
On the order of 1W.

Bob

Jan 8, 2015
43
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so with battry 3.7v 4 ah it work for 12 second
no it work for .3h

13. ### BobTPH Active Member

Jun 5, 2013
806
121
1W at 3.7V is 270mA or 0.27 A. A 4AH battery should last about 14H at 1W.

Bob

Jan 8, 2015
43
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Direct connection with the battery without resistance work for .3 h

15. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,902
878
And why did it stop working? Was the battery dead? Was it a new battery when you started your test? Or without a resistor, was the LED dead?

Jan 8, 2015
43
1
without a resistor battery discharg after ,3h and led not dead

17. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,902
878
Note I asked about the resistor because connecting an LED without one is a sure way to destroy the LED. There are some very specific exceptions, but generally an LED requires a current limiting resistor.

Did you make your measurements at the points illustrated in the picture below?

Is this how you got your measurements of 3.7 VDC and 4.8 mA?
Ignore the two AA batteries... I just used them to represent your battery. (What are you using for a battery?)

@BobTPH, where are you getting on the order of 1 W? If we are to trust the OP figures (!), your original calculation seems right to me.

There is so much conflicting /extraneous information here and little hard facts. We need to know
• What is the LED
• What is its forward voltage
• What is its rated current
• What is the power source
• How are the measurements being taken.
• Forgot about the lumens

18. ### BobTPH Active Member

Jun 5, 2013
806
121
The 1W comes from that fact the state of the lighting LEDs produce about 60 to 100 lm / watt. If the LED produces 70 lm, it is somewhere on the order of 1W.

Bob

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19. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,902
878
Hence my conclusion that there is conflicting information. From the data given, there are two widely divergent, yet plausible, answers to how much current is being used...

20. ### BobTPH Active Member

Jun 5, 2013
806
121
Yes, there are two conflicts, power used vs lumen output.
And current used vs battery life.

Bob