Question about battery voltages on LED's (I'm confused)

[Scougar]

Hi guys, first post

I'm just waiting for my two "Cree 5W" (not actually 5W) ebay cheapies to turn up, and have already ordered an external battery pack to hold 4xx AA rechargables, an XR-E R2 WG with star to try out, and very stupidly a £15 controller for it that takes upto 5.5v input (to give me 1000ma and a load of other modes).

A few things I'm confused about:

1) With this controller I've got http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=110632352901&ssPageName=STRK:MEWNX:IT if I use 4x AA's (Sanyo Eneloop) will I burn out the LED because the voltage is too high? I admit I'm slightly confused by the voltage side of things vs the ampage. (So for example, with 4x AA's and I measured them at 1.33v when taken straight from the packet), that would give me at least 5.32, and I've seen people say they are 1.44v's when fully charged = 5.76v. Will the controller handle it (I assume some loses from the battery pack though), and what voltage will the XR-E LED see? I don't want to burn it out because I haven't understood something.

2) I've just realised this morning that I have a 12v lead acid battery pack (WP1221W) from a UPS I rarely used. I then saw that some people are powering single XR-E's from a 12v power source. Going on question number one.. I thought 12v would almost blow the LED instantly? How are people doing this? (See here for a guy running 12v with 1 led and 10x AA's. http://www.ghostgum.com.au/misc/BikeLight.htm

Sorry if they are newbie question's, really just want to try some things out, but don't want to keep blewing parts from not knowing the basics.

 

[Mike Painter]

If LEDs (or anything) are in series then the amperage is constant and the voltage needed is the sum of the voltages across the LEDs.
If LEDs (or anything) are in parallel then the voltage is constant and the amperage needed is the sum of the current across the LEDs.

Most regulators regulate and keep constant current and feed the LED whatever voltage it needs.

The controller might or might not handle it and at least the top half looks like one from kaidomain or deal extreme only a lot higher.

A single XR-E under 12 volts will let the magic smoke out and there is probably a resistor in the circuit to limit the voltage or they are using a "Buck" (reducing) driver.

 

[Al Combs]

:welcome:

The link you provided says 5.5 volts max input. So 4 eneloops might blow the regulator. You could use 3 and a dummy. For some reason dummy cells cost more than real batteries. Use a 2" brass screw, some 1/2" washers and nuts to make your own. That's if you already have a 4 cell AA holder.

As far as a regulator to drive an LED off a 12 volt battery, take a look in the driver list for buck regulators. As an example from the list, there is a 1 amp version of the SOB that goes up to 16 volt input. The SOB is a single mode regulator if that's an issue.

 

[Scougar]

Thanks for that guys. OK, well I went out and got a 4000mah li-poly pack that is 5v (saw it on offer randomly), and I can power the flashlight/torch from that instead of 4x AA's (I could use a diode to reduce the voltage I believe on 4x aa instead of using a dummy on the 4 pack. It's prob cheaper to get a 3 pack from hong kong to be fair).

So that's 5v's I'll be supplying to a single XR-E LED. Now.. I 'could' split this across two flash lights, but it's just an experiment for now, so sticking to one LED. Given the controller I listed (the ebay link), Do you think it's safe to run an XR-E safely on that controller?

BTW: The controller says it is a synchronous buck-boost controller.

 

[Al Combs]

Here is a good reference thread on NiMH batts. The 2000 mAh AA eneloops are a little more than half way down the page. The 1 amp curve is a little higher than 1.4 volts initially. So 4*1.4=5.6 volts or too much for your regulator. The regulator you listed looks like it would be OK for an XR-E. Maybe a little on the pricey side but otherwise OK. Li-Po's are 4.2 volts per cell. I can't imagine what sort of Li-Po puts out 5 volts unless it has a built in voltage regulator.

Here is another good reference thread where an XR-E R2 was tested. The particular sample he used had a Vf of 3.63 volts at 1 amp. Important to note is just 0.03 volts was enough to cause a 10% change in the power level. LED's are very sensitive to changes in voltage. Which is one reason why you need a constant current regulator. Another good reason is LED forward voltage drops as the temperature rises. A voltage regulator would end up cooking an LED. I definitely wouldn't try to use a diode to drop the voltage to the correct value.

 

[Scougar]

Battery pack is an Energizer XP2000 or XP4001 (I have both). They both supply 5v's as it's a USB socket output, but the XP2000 is 2000mah and supplies a max of 500ma, whereas the XP4001 is 4000mah and supplies a max of 1000ma, or that can be split from the two outputs to 2x 500ma max. They were on offer, what can I say

I'm still trying to work out whether or not supplying 5v in, will cause 5v's out (i.e. blowing the LED). Using 3x AA's (4.5v) would technically be too much for the XR-E, so I'm hoping that 5v will be stepped down to the XR-E R2 level, I'm just not sure.

EDIT: Ideally what I think I need to do is get my Energizer XP4001 battery pack back to 3.7v's output rather than it automatically upscaling it to 5v, which is reducing battery life, and causing me issues. I wish I could find a way to open the darn thing so I could at least look at modifying it!

 

[Al Combs]

Are you talking about using an Energizer XP4001 along with the regulator you mention in the first post, or instead of it? The XP4001's predecessor (XP4000) had a 8.4v 1500ma jack in addition to the 5v 1000ma USB port. That would seem to indicate some kind of current regulation. But perhaps they are just echoing the port spec.:shrug:

If you have a DMM you could wire a resistor across the output of your XP4001 and see what reading you get. If you wired a pair of 5 ohm power resistors in parallel and put that as a load across the XP4001's output, it would be 2 amps at 5 volts (E=I*R). If it still only reads 1 amp, you'll know the battery also has internal current as well as voltage regulation.

 

[PapaLumen]

I'm still trying to work out whether or not supplying 5v in, will cause 5v's out (i.e. blowing the LED). Using 3x AA's (4.5v) would technically be too much for the XR-E, so I'm hoping that 5v will be stepped down to the XR-E R2 level, I'm just not sure.

The driver you mentioned will take the 5v in and put out whatever the led needs, thats what it does (along with providing a constant current.) There are cheaper drivers out there though. If you can be bothered to order from china you can get a driver that will take 3-18v and put out whatever voltage the led needs at 1amp for about $2 but its a buck driver so you would need at least 5v going in. I use them with 8.4v going in.

 

[Scougar]

Are you talking about using an Energizer XP4001 along with the regulator you mention in the first post, or instead of it? The XP4001's predecessor (XP4000) had a 8.4v 1500ma jack in addition to the 5v 1000ma USB port. That would seem to indicate some kind of current regulation. But perhaps they are just echoing the port spec.:shrug:

yes, I was originally going to try 3 or 4 x AA's (Nimh Sanyo Eneloops), but saw the XP4001's on offer so thought I could give those a go along with the buck-boost regulator I mention I'm trying to break open the XP4001 to see what batteries are in there to see if I can reduce them to 'just' 3.7v, but I want to try it without much damage, and there are no screws on it annoyingly. I want to be sure of whether it's 8.4 reduced to 5v, or 3.7v upped to 5v.

If you have a DMM you could wire a resistor across the output of your XP4001 and see what reading you get. If you wired a pair of 5 ohm power resistors in parallel and put that as a load across the XP4001's output, it would be 2 amps at 5 volts (E=I*R). If it still only reads 1 amp, you'll know the battery also has internal current as well as voltage regulation.

I have a DMM so I can measure the output of the power unit (Although I'm not experienced with electronic/electrical stuff). I don't have resistors to test it with yet.

The driver you mentioned will take the 5v in and put out whatever the led needs, thats what it does (along with providing a constant current.) There are cheaper drivers out there though. If you can be bothered to order from china you can get a driver that will take 3-18v and put out whatever voltage the led needs at 1amp for about $2 but its a buck driver so you would need at least 5v going in. I use them with 8.4v going in.

My XR-E LED is coming from deal extreme, and the buck-boost driver comes from Hong Kong so I can certainly be bothered to order from there


Ok guys, so I've assumed up to this point that the LED will be overpowered by the controller as such supplying too much voltage, but what actually happens is that the LED calls for a certain amount of voltage and the driver supplies it? (So it won't "blow" the LED as such?)

UPDATE: Well, still waiting for my lights to turn up so I can have a play. I was getting bored of not finding anyone who had taken an Energizer XP4001 apart (or any energi to go product), so I took mine apart (breaking it in the process, it will need to be glued back together lol). The batteries appear to be 2x li-po batteries on top of each other and when measuring the voltage directly from the battery output lead, I get 4.14V. I wonder if the XP2000 uses the same battery, just a single one rather than two? I have still to measure the output from the USB lead itself.

I took photo's I will get them up in a little bit

1) If the XP2000 uses the same battery (single), I could actually fit another inside and give myself a 6000mah @ 3.7V battery pack. I assume the electronics will handle it for charging. (or perhaps I shouldn't bother and just keep the XP2000 as a complete unit rather than wreck it.

2) Once I have confirmed the output voltage on the connector, I need to decide whether I then take a direct feed from the lipo battery for 3.7 Volts, and keep the casing for charging it. It's more than doable for a direct feed, I could 'possibly' use one of the existing USB out's for 3.7v's and the other for 5V I don't care about feeding two devices lol.

 

[Scougar]

Just been looking at the circuit board on my break. There appears to be a variable resistor (see picture 3, it's the one in the centre that looks like it will take a tiny phillips screwdriver) on the circuit board! Hmm.. I wonder a few things: 1) Does this control input or output? 2) What affect will this have on output voltage (and possibly input (charging!) voltage? 3) How will current be affected?

I would point out that I have no idea what the vast majority of components on this circuit board are for!

 

[Scougar]

Adjusting the variable resistor seemed to make no difference to the output of the USB ports. 4.98V the whole time :-( Also, the PCB is double sided and marked as a TennRich BA0225-1 Rev1.1 circuit board made on 2009-2-16, I can't find any info on it though.

I believe I will simply have to splice in to the battery cable with a secondary wire, and then rely on the circuit protection from my buck-boost controller. I'm not particulary happy about that as it drops to 2.7V before protection cuts in, but I can't seem to find a simple controller to protect li-po batteries to not drop below 3V.