Newbie voltage question

[Magic Matt]

What do I need in terms of circiutry to run one of the high powered Luxeon V LEDs nice and brightly from a single AAA battery? ...or can't you?

Do I have to use batteries with V higher than the Vf of the LED?

I'm sure somebody mentioned something about a Buckboost(?) to me a while ago but I have no idea what one does or where you get one.

 

[Mr Happy]

What do I need in terms of circiutry to run one of the high powered Luxeon V LEDs nice and brightly from a single AAA battery? ...or can't you?

Do I have to use batteries with V higher than the Vf of the LED?

I'm sure somebody mentioned something about a Buckboost(?) to me a while ago but I have no idea what one does or where you get one.

I think you need one of those power up-converter circuits that can boost the output power to a higher level. Not one of the big ones that can run a whole house from a single AAA battery, but one of the smaller ones at least. Something that will boost the 1 watt from the battery to the 3-5 watts needed by the Luxeon should do it...

 

[Magic Matt]

I don't suppose you have any links do you? I've tried Google and the forum search, but I think I must be using the wrong terminology as I'm coming up blank.

 

[Mr Happy]

Seems like you are not quite getting the joke...

What you need is a boost converter. But see, a single AAA battery only has a certain available power output. You obviously can't run a house on a single AAA battery or everyone would do it, and then they wouldn't need to buy electricity from the electric company.

A single AAA sized alkaline cell is limited to a reasonable current of about 300 mA, and an NiMH rechargeable cell about 800 mA. For the NiMH cell that would equate to about 1 watt of power output, and that means you cannot expect to drive an LED with more than about 1 watt of power input, say 300 mA drive current. That will make it light up quite brightly, but you cannot hope to make it run at maximum brightness with such a small power supply. It usually needs about two AA cells in series, or a CR123A cell, to achieve enough power for that.

So to recap, the keyword to search for is boost converter. You will find lots of info about boost converters (or boost drivers, or boost pills) in the forums here on CPF.

 

[45/70]

Awww, ya had my hopes up, Mr H.

Dave

 

[Magic Matt]

Yeah... jokes are sometimes lost on me... :duh2:

...a single AAA battery only has a certain available power output.

Didn't realise that... I didn't really get past I=V/R and as you can't have R=0ohm (even a wire has resistance) the limiting current would be from the circuit resistance... so if the resistance was 0.01ohm it could deliver 150Amps... although as the cell has (according to the side) 950mAh it would only be for a few seconds.

If the NiMH is limited to 800mA output, that means the cell should last for at least an hour... since it can't discharge quicker than that... cool!

I consider myself duely educated... thank you sir!

So to recap, the keyword to search for is boost converter. You will find lots of info about boost converters (or boost drivers, or boost pills) in the forums here on CPF.

Yup, found lots of things under that phrase... I shall start reading like mad now!
:thanks:

 

[Mr Happy]

I didn't really get past I=V/R and as you can't have R=0ohm (even a wire has resistance) the limiting current would be from the circuit resistance... so if the resistance was 0.01ohm it could deliver 150Amps... although as the cell has (according to the side) 950mAh it would only be for a few seconds.

Almost, but not quite. The cell itself has resistance, and eventually this dominates the total resistance of the circuit. In your example with 0.01 ohms in the load you might find the cell has an internal resistance of 0.3 ohms -- giving a total resistance of 0.31 ohms and a current of only 4.8 amps. Since the power in a load is given by W = I²R, that would be (4.8²)(0.01) = 0.23 watts delivered to the load. At the same time the power wasted in the cell as heat would be (4.8²)(0.3) = 6.9 watts. So you see, for maximum power delivery increasing the current does not always help. The two limiting cases are volts but no current (zero power) and current but no volts (also zero power). Maximum power is somewhere in between.

 

[Magic Matt]

I see! :thumbsup:

Also found this on Wiki - http://en.wikipedia.org/wiki/Internal_resistance


I'm going to use a theoretical 1.2V 1Ah NiMH with 1ohm internal resistance in this...

1 cell alone gives me 1.2V, 1Ah capacity, max (1.2V/1ohm) 1.2A current

I connect two batteries...

In series I would get 2.4V, 2Ah capacity, max 1.2A current
In parallel I would get 1.2V, 2Ah capacity, max 2.4A current

 

[Mr Happy]

In series I would get 2.4V, 2Ah capacity, max 1.2A current

2.4 V, 1 Ah capacity

The capacity remains the same when you put cells in series. You can double the capacity, or you can double the voltage, but you cannot double the capacity and the voltage with just two cells. If you could do that you would be getting something for nothing.

 

[Magic Matt]

Ok... in which case... would it be better to connect several cells in parallel and use a boost converter to drive the LED... or would it be better to put some cells in series and use a voltage regulator?

 

[Mr Happy]

Ok... in which case... would it be better to connect several cells in parallel and use a boost converter to drive the LED... or would it be better to put some cells in series and use a voltage regulator?

If you have a choice between series and parallel it is usually better to arrange cells in series. Then when the battery voltage exceeds the LED voltage you need a buck regulator rather than a boost regulator, and to drive LEDs properly the current needs to be regulated rather than the voltage. All the LED drivers out there do the right thing though, you just need to pick one with suitable specs out of the many available.