mypiandi
Posts: 13
Joined: Tue May 13, 2014 5:15 am

LED Watt / Amp question

Tue May 28, 2019 12:10 pm

Hi, I'm controlling five 12V LED lights via a relay from my Pi.

These are the lights: https://www.ebay.com/itm/10W-LED-Flood- ... wEzxYc74E

The LEDs are rated 10 Watt each and I'm trying to work out what type of 12V power adapter I require.
My calculation is that each light consumes 0.833 Amp (10 Watt / 12 Volt), i.e. a total of 4.17 Amp for all five of them.

Rummaging around in my box of old power adapters, I could only find 12V ones rated between 0.5 and 1.0 Amp.
I hooked the 1.0 Amp one up and it runs the lights for extended periods without even getting warm.

I was expecting it to get hot rather quickly. So my question is: would the 10 Watt LED rating only be an equivalent value, similar to the LED downlights in my house, where the Watt figure indicates what incandescent bulb it resembles?
Or am I approaching this wrong altogether?

pcmanbob
Posts: 6968
Joined: Fri May 31, 2013 9:28 pm
Location: Mansfield UK

Re: LED Watt / Amp question

Tue May 28, 2019 1:14 pm

As there is no way for you or us to know if the 10W actually refers to the actual wattage of the device or some equivalent bulb rating ,
I would suggest you measure the actual current draw by the LED lamp in question , that way you will know exactly what's be drawn.

Even a cheap digital multi meter with a DC amps range will be able to measure the current draw from 1 LED lamp..

And yes your calculations are correct if its actually 10W at 12V
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ptimlin
Posts: 88
Joined: Tue Jan 08, 2019 7:44 pm

Re: LED Watt / Amp question

Tue May 28, 2019 8:56 pm

Getting about 800-1000 lumens for 10W on LEDs sounds about right, so I am guessing the 10W is the real power being used by the LED light.

I assume when you say you hooked up the transformer and the "lights" ran without the transformer getting hot, you were actually talking about ONE light? Not lights, right? I can not imagine you were lighting all 5 off a 1Amp 12V transformer.

On the other hand, did you measure the voltage on the transformer when things were powered up. It could very well be that the transformer was putting out all the current it could, say an amp but the voltage may have dropped very low. So while all the lights turned on, they may have been running at only a quarter or so of their full power, but being LEDs still looked rather bright.

As PCMANBOB mentioned, this would be a good time to pull out a meter and take a few measurements to see what is really going on.

mypiandi
Posts: 13
Joined: Tue May 13, 2014 5:15 am

Re: LED Watt / Amp question

Thu May 30, 2019 2:30 am

Thank you, those were very helpful suggestions!
I used a multimeter and measured 0.71 Amp, which seems close enough to the calculated value.
I had indeed hooked up all 5 lights, so it may well be that the power supply restricted the power somehow to not heat itself up.
I will buy an appropriate power supply, just to be safe.
Once again thank you for your help!

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