12V transformer

[periag]

Hello all,

I bought some 10W 12v LED that i want to test.
I have a 12V 850mah transformer that i retrived from and old device.
When i plug this transformer and test it using a multimeter, i get 19.5V
Can i use this transformer to power my 12v LED?

Thank you

 

[(*steve*)]

No, not without something to limit the current. You might destroy the transformer, the LED, or both.

Also, transformers are not rated in mAh, maybe it's mA. Is it just a transformer? Or is it a power supply?

The LED will get hot when operating at its rated power, it will need to be connected to a heatsink.

Tou can probably test it (the LED) by connecting it directly to a 9V battery. At 9V, the LED will not draw much current, but you should see a glow. Even if the LED is actually for a lower voltage, the 9V battery will not be capable of supplying enough current to damage it quickly (but remove the battery if the LED is very bright).

 

[periag]

Thank you for your answer,
I didn't gave you enough detail...
In fact it's a power supply. And the led isn't just a led. it's a flood light see here : http://i.ebayimg.com/00/s/MTAwMFgxMDAw/z/w2IAAOxyUgtTOOhj/$_57.JPG
So it already have an heatsink
I've been able to test it. I had a 9V power supply( that showed 15V ) i give it a try and it power the light correctly.

But just for my information, why does a power supply give more voltage then what is show on it? does the voltage drop when under load?

Thank you

 

[davenn]

hi periag
welcome to the forums

But just for my information, why does a power supply give more voltage then what is show on it? does the voltage drop when under load?

if it is an unregulated PSU, then yes, it will have a higher no load voltage that will drop when a load is applied

for a "rule of thumb" ....
most unregulated PSU's will have a current rating lets say 500 mA (0.5A) and a voltage rating, say 12V
IF the load is drawing 500mA, then the output voltage will be 12V, it may be a bit higher if a lower current is being drawn or conversely a bit lower if a little more that 500mA is drawn

cheers
Dave