Question re PSU current

[Mash]

Hi all!
In my efforts to understand how things work, I am playing around with Cree LEDs and driver boards.
My DX driver boards are meant to supply 800mA from a 3.6~9V input.

Now here is my question: If my PSU wall-wart is labelled lets say 6V and 400mA, what current will the board supply to the LED?

ie if my input is 400mA will the board be able to "UP" it to 800mA since thats its spec?
What if I had a 6V, 1A supply?

I am trying to see if my input current has any effect on the output current?
This question relates to drivers in general, not just the DX ones I am playing with. Most of the spec I see for driver boards, do not mention input current, so I am trying to get my head around this.
Hope Ive made my question clear, and thanks in advance for the answers!

 

[zipplet]

The board will try to pull as much as it can in order to generate the rated output current and voltage - remember this also includes a loss for the boards inefficiency (they are never 100% efficient).

Looking at the spec it supplies 800ma at 3.7V. With for example 90% efficiency (they don't quote the actual figure), at 6V input it would draw around 542ma from the power supply. This is higher than your PSU is rated at so you might burn it out - either that or the PSU will limit it's output at 400ma (not so likely) and then your driver board will not drive the load at the full rating.

 

[Mash]

Thanks for your reply!
So should my PSU's current rating always be higher than the output current of the board? Or is it better to work it out everytime using your calculations above?
Trying to nail this down once and for all, so additional comments are welcome!

 

[zipplet]

I worked it out by calculating the wattage that your driver board will output - 3.7V * 0.800A - and then adding another 10% to cater for it probably only being 90% efficient at best (it could be worse). Then you know roughly how many watts it will require (in this case, assuming 90% efficiency, 3.256W).

Then you need to find a PSU that can supply this wattage in the input voltage range the driver is happy with. You quoted a 6V supply, so if you were to use a 6V supply, it would need to be 524ma (3.256W divided by 6V = 0.542A).

If you were to use a 9V supply for example, it would only need to supply 361ma in order for the driver to supply the full 800ma rating.

Please take this with a pinch of salt as I have no idea how efficient that driver is!

 

[LEDependent]

Thanks for your reply!
So should my PSU's current rating always be higher than the output current of the board? Or is it better to work it out everytime using your calculations above?
Trying to nail this down once and for all, so additional comments are welcome!

Wall warts like yours are voltage regulators. The 6V figure is the voltage it will output. The 400mA figure is the maximum amount of current that it can safely deliver.

Looking at the user reviews, you can expect a current draw under 500mA when you use a 6V supply.

 

[Mash]

Thank you all for your valuable input, it would be impossible to learn all this without your knowledgable help!
I am getting this now, so to achieve the rated drive current, I need to do the calculation and have a high enough voltage to make up for lower the input current.
On another side question, I have a laptop psu, which the label says, input 100-240V, 35-48VA. What does the VA mean? Volt Amps, since it is a multi voltage PSU?

 

[Fallingwater]

Here's a suggestion - get an old or trashed computer and take its power supply out. Use the 5v rail to power the board. It has ample current capacity and the voltage is just right.

 

[Mash]

I had that idea at the begining of my explorations Falling water, then I thought is it really efficient to use a 200-300W PSU, for running a few LEDs? Seems to defeat the efficiency point of LEDs. But for just experimentation I totally agree with you, good voltage range, solid power, and usually steady output.

 

[Fallingwater]

A "300 watt" PSU won't actually eat 300 watts all the time.
It'll get from the mains as much energy as it needs to power itself and whatever is attached to it.