How to regulate 3.6v down to 3v

[jbrow1]

I have a set of foot warmers for my shoe that require 2 AA batteries to run. I am using nimh batteries so plan to install a 3 cell holder in place of the 2 cell holder it comes with. This will increase the voltage and possibly damage the warmers, but if it doesn't it will likely make my feet to hot anyhow. The 2.4v of the nimh's doesn't power them as well as I'd like and even with 2400mah batteries they go dead after an hour.

So my question is what is the best way to regulate the voltage down to 3 volts, hopefully with something small because the battery pack and whatever I need to do this will be clipped to the top of my sock.

I can provide a link to the wamers if anybody wants to see what they are or the specs, but won't post it initially due to being thought of as spam.

Thanks!

*edit* after searching google it looks like one of the little black regulators is what I'm after. Seems a bit difficult to find specific for 3v though. Also seeing that I should have an input voltage of at least a couple or more volts above what I'm looking to reduce it down to. So food for thought for me I guess. Do these little regulators also act as a low voltage shutoff then as well? If that were the case I would maybe be interested in using a lipo battery instead of nimh's.

[pebe]

The simplest way would probably be to use a series Schottky diode. That would drop about 0.5V, but still leave you with 3.2V, and that would vary as the battery discharged. It would also be wasteful as power would be dissipated in the diode.

Your idea of using a voltage converter is the best way, but you would need to supply it with at least 5V. Fortunately these devices are almost 95% efficient, so you could feed it with 7.4V from 2 x Li-ion cells and be able to set the output voltage accurately to 3V, or whatever voltage makes you feel comfortable.

Here is a suitable buck converter:
http://www.ebay.co.uk/itm/1-DC-DC-Buck- ... 5d44cfdcc7

If you decide to go that route, then be aware that cells can be ruined by discharging them too far. To avoid that, use ‘protected’ cells that cut off to protect themselves against overcharging and over-discharging. I use ‘Ultrafire’ BRC18650 cells with a capacity of 4000mAh each, and I can recommend them (beware of some of the cheaper cells). Here is a supplier of 5400mAh cells with charger:
http://www.ebay.co.uk/itm/UKAj890-2-x-U ... 2c6d9f5e1d

They are a bit larger than AA size, but if you want AA size ones then you could use two 14500 cells giving 900mAh each like these, with a charger:
http://www.ebay.co.uk/itm/UltraFire-138 ... 46015097c5

I hope that helps.

[jbrow1]

pebe - thanks a lot for the informative post and the links I did not know they made the lithium batts with internal voltage protection, that's very nice. I've dealt with the different types of batteries a lot back in my rc car days, and it has me kind of hesitant to strap a lithium battery to my leg while wearing lycra type riding clothes! Although I was looking into lifepo4 possibly since they don't burn up in an inferno or go off like a shotgun.

I am curious about these things, they look small and simple. I assume they must waste power through heat while they are doing their work. I will have to google buck converter to learn more about them.
http://www.ebay.co.uk/itm/121132647825? ... 1423.l2649

Thanks again!

[pebe]

If your 2400mAh batteries only last an hour, then your warmers must be drawing at least 2amps. The little regulator you refer to is only good for 30mA so it would fry. Also it is a series regulator so the power lost by it would be completely wasted.

By contrast, the buck regulator only wastes about 5% power in the conversion process. It is analogous to an AC mains transformer where Pout = Pin x %efficiency.

Using a 7.4V 4000mAh battery to feed a buck converter giving a 3V 2A output will theoretically drain 810mA from the battery. Assuming a 95% efficiency, that would increase to 850mA, so a 4000mAh battery would last 4.7hours. Not bad, eh?

You may be interested in this link about protected cells:
http://www.lygte-info.dk/info/battery%2 ... %20UK.html
You can see they give good protection and are pretty safe devices.