(8 x AA NiMH) + (3 x XML) = Success?

Hi,
I've got hold of a non-working Gilan Pocket Torch. Originally it has a 35mm 35W Halogen bulb but I'd like to upgrade it to a triple XML board with the Ledil Cute triple spot optic.

The torch takes 8 x AA batteries (in a standard 4 x 2 holder). It might be possible to put a different battery pack in there, but the space is 37mm dia x 115mm, so I doubt it.

I've modded a couple of torches (an XML in a 3-D Maglite and a Maglite based umbilical dive torch) but I'm very much a beginner and I'm a bit confused about the power supply I need.

My eight-cell NiMH pack will give me 9.6v. This should be enough to run the triple XML, but will the batteries be able to provide 2-3A (even if just for 45 minutes)?

If I decide to use a constant current driver at say 2A do I use a buck or a boost, or does it not matter? As I understand it for a buck the voltage has to be at least 1v above the required voltage, I assume there has to be a similar difference with a boost driver - so in this case is the difference too small?

How about using two XMLs? If I use the triple optic with just two LEDs is it going to look odd? It would mean that I could need a lower voltage and make it easier to select a driver.

Thanks,

Neil

Quality NiMHs are good for a "2C" current, or twice their capacity. That rate may warm them up, but it won't decrease your charge cycles or capacity too quickly. You will want to eyeball the temperature on the LEDs for the first few runs to see if they are running hot. A direct-drive build will start out bright and dim every second it's on. Wire it up on an exposed heatsink (AA-goo it to a CPU heatsink or something) and see what the heat, voltage, and current draw do.

OK, thanks. I'll give that a go.

I've just taken the torch apart to have a look at the magnetic switch - I'm getting ~9.6v going into the switch, but only 8.4v coming out when it's on is that normal? The reed's in a little circuit with a couple of resistors and a Mosfet.

Could I just remove that and put in a bigger reed switch?

Neil

If the switch is sucking up 1.2v, that's not good for overall output. Also at 2 amps it's a 4W heater, and so should get warm. I don't know much about how magnetic switches are supposed to work, though.

Just to clarify you have 9.6 volt at the batteries while the torch is on but only 8.4v after the switch? Sorry that is not voltage sag. Is the MOSFET getting hot? If so you could try swapping it out for a better MOSFET that has less resistance ( sorry cant recommend one but someone else may)

If 9.6 is battery volatge when off and 8.4volt when on then you are losing it from voltage sag. The batteries drop in voltage while under load.
I recently built a 6 (3s2p) XML running 9 NiMh cells. I used a 0.2ohm resistor to drop the current down.
The Vf of the LED's set up was 9.9volt


I cant see your set up delivering enough voltage to run the XML's at full power even with out voltage sag.

When the light is on the voltage going into the switch is 9.6 and going into the lamp it's 8.4, so I guess it's lost in the MOSFET. The MOSFET itself isn't getting noticably hot, but the lamp is )

I'm not really bothered about getting full power, I think I'll be happy with 2-2.5A - it should be bright enough and I'll get longer battery life. I'm just not sure if I need to use a constant current driver. Unfortunately my limiting factor is the battery pack.

2A at 1.2V is only 2.4W, and if the FET is secured to an aluminum body, it won't get noticably warmer than the body does. But that much drop across the FET is not good. It could be a poor choice of FET, or there could be something else wrong. If it is the FET and you can get us enough information to know what the circuit is like, I'll recommend a MUCH better one you can buy for a few bucks.

Can you measure the voltage on each of the pins of the FET while it's running?

Sorry guys, I'm an idiot. My last post was made with the confidence that comes after a visit to the pub.

I've just checked the voltages again, and the voltage from the batteries with the switch off is 9.6, when I switch it on it drops to 8.6, so it looks like Packhorse was right and it is voltage sag.

Bugger. If I'm only getting 2.9v across each LED the current will only be about 700 mA - my battery will last for ever, but the output will be rubbish. Is there any point using a boost driver?

Neil

A boost driver would help, but you wouldn't be able to shut it off if it's just a simple boost circuit. When you shut it off, you'd still have direct drive through the driver. You need a buck-boost or buck with blocking FET. Some of the drivers sold as 'boost' may have the features you want, but most probably don't.

You may consider using Nickel-Zinc rechargeable batteries and buck driver. They have higher voltage and 8 of them should give you above 12V under load. See an example discharge curve here. They will have less mAh than NiMh but voltage is higher so energy stored may be similar, depending on particular batteries.

It may even be good enough to use linear driver (like LFlex or something AMC7135 based).

This will give you also possibility to use them with primary Lithium AA cells. I know it is expensive but you can keep set of 8 cells as backup while normally using rechargeables.

Of course for Nickel-Zinc batteries you need to get new batteries and new charger so it may be not what you would like.

I'd never heard of NiZn before, they look like a good choice but it looks like people have concerns about their reliability and they don't seem to be available over here (UK). On the PowerGenix website it says "PowerGenix is developing next generation AA cells. Stay tuned – availability is expected in second half of 2012." - maybe I'll wait until then.

I was thinking about Lithium Ion 14500s, but as I understand it I'd need to use unprotected batteries to get enough current, and that doesn't sound like a good idea.

OTOH it's probably best if I just use Alkaline batteries.

I have a technalite with one of those 8xaa holders and was able to put 10 aa solder tab enelops in with a kd p7 buck driver and 3 Cree xre r2 stars. The driver is mounted in the bottom with a switch and barrel male connector all in a copper slug. Bump the light for off/on, soft shake changes levels. The footprint of the pack is a pentagram with the female barrel in the middle. Wires lead up the side of the pack to the LEDs /sink. Been using it for two years several nights a week to walk the dog. It was given to me initially after it had flooded so it doesn't see use diving but does in the rain. Very bright light. If you could use Packhorse's poor man driver for 3 XML 2s1s, then 10 AA might work. 3 cells per led + 1 for good measure.
Scott

Thanks for the suggestion but the barrel of the torch is too narrow for that. In fact the 8 x AA holder needs a bit of force to get it in when it's got NiMH cells in, although non-rechargeables are less tight.

I've actually tried just using it as it is (ie no driver) with eight alkaline or eight NiMH and it works OK. I don't have any way to measure the output, but it is much brighter than the 35w halogen bulb that was there originally and brighter than my single XML lights so I'm happy with that.

At the moment I've got the medium beam optic in it (the spot has been out of stock), so I'm going to see how I get on with that before I try any other changes. I'm off to Pembrokshire for a week so I'll have a chance to startle some sealife!

Neil