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240v to 12vdc driver/transformer, what wattage output do i need?

i am trying to power 12no 3w 12vdc bulbs starting with 240v mains. i have found a 240v - 12vdc driver/transformer meant for LEDS but not sure what wattage output i need, 40w?

I think you are doing the math right.  If each bulb requires 3 watts of power, then 12 of them require (12bulbs)*(3W/bulb) = 36 watts.

Assuming these bulbs really do have a voltage of 12V DC across them, then 3W = 12V* 0.25A.  So in terms of current that is 0.25 A = 250 mA, per bulb.  Then if you wire up 12 bulbs like this, in parallel, then the sum of all those currents together  is 12*0.25A = 3A, for all 12 bulbs.

Also notice that 12V* 3A = 36 W.  So that's reassuring.

Anyway, the math is easy.  But is your power converter going to live up to these simple mathematical assumptions?  Is your gizmo intended to supply a constant voltage of 12 V DC?  Or is intended for driving LEDs?  If it is truly intended for driving LEDs then it may be a constant current regulator.  Constant voltage and constant current are NOT the same thing, in fact they are as different as Yin and Yang.

So it would be helpful if you knew more about your power converter, whatever it is.  If you cannot decode the hieroglyphics on it yourself, then please post a (clear) picture of it to this forum, and then I, or one of the other volunteers here, may be able to tell you what your artifact is,and what it is capable of powering.

ciderpig (author)  Jack A Lopez5 years ago
jack, thanks for yhour response, i wasnt expecting to get it right but it sounds like im on the right track. I have attached a picture of the driver and the bulbs for clarification. thanks again ian
Vseries40W3.jpgindex.jpg
Thanks for the pictures.  That helps. 

The bad news is I think connecting those two things together, will cause each one to try to kill the other.

If you like, I will explain why I believe this to be the case.  It has to do with the fact that LED drivers try to supply constant current, and this one looks like it will try to supply 3.3 amperes to whatever gets connected across its output terminals.

What the lights bulbs would prefer is something that is trying to supply them constant voltage.    So I am going to recommend that you seek out a 12V DC supply that is NOT an LED driver.  Attached is a picture of what I expect such an artifact would look like.

In the big version of this image, here
https://www.instructables.com/files/orig/F0X/WSTP/H1JUIUO0/F0XWSTPH1JUIUO0.jpg
you can almost see the little marking and symbols on it.  It says 1.5A, and that is an indicator of the maximum current it can comfortably supply, and that amount of current will change depending on how many light bulbs you have connected to it.  For example if each bulb draws 0.25A, then two bulbs in parallel draw 0.5A.  Four in parallel draw 1.0A. Six draw 1.5 A.  In contrast, the voltage supplied by an adapter like this one should be approximately constant; i.e 12V when one light bulb is connected, 12V when two light bulbs are connected, 12V when six light bulbs are connected.

BTW, they make 12V power bricks (ac adapters) like this that can supply 3, or 4, or 5A, etc, maximum current.  They tend to get more expensive as the maximum amount of power they can supply (Pmax = V*Imax) increases.  I just picked a 1.5A one for the picture, because it seemed like a nice representative picture.  Although they all tend to look like black plastic bricks. I found the picture via Google Images ( http://www.google.com/imghp?hl=en&tab=wi )

12v-4A-power-brick-supply.jpg
Ignore what I said about the max output current of this thing being 1.5 A. It actually says it is 5A. I misread that, and the 1.5 A is actually the spec for the input current. Don't ask why I think the 5A out is a more believable number than the 1.5A in. I mean without faith in the people that make these things, you might wonder if any of the numbers are believable.