Use 14.4V NiCad battery through a L7812 regulator?
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From: John Doe on 5 Dec 2009 13:59 I need to power three Cree XLamp XRE LEDs. The standard sealed lead acid battery is 12 V, but it is heavier than I would like to carry. I have some spare rechargeable 14.4 V DeWalt tool batteries. The L7812 regulator says it needs at least 14.5 V. will a 14.4 V NiCad tool battery be acceptable for that? Thanks.
From: Tom Biasi on 5 Dec 2009 15:20 "John Doe" <jdoe(a)usenetlove.invalid> wrote in message news:000798a4$0$2217$c3e8da3(a)news.astraweb.com... >I need to power three Cree XLamp XRE LEDs. The standard sealed lead > acid battery is 12 V, but it is heavier than I would like to carry. I > have some spare rechargeable 14.4 V DeWalt tool batteries. > > The L7812 regulator says it needs at least 14.5 V. will a 14.4 V > NiCad tool battery be acceptable for that? > > Thanks. Why do you need the 7812? Tom
From: ehsjr on 5 Dec 2009 15:30 John Doe wrote: > I need to power three Cree XLamp XRE LEDs. The standard sealed lead > acid battery is 12 V, but it is heavier than I would like to carry. I > have some spare rechargeable 14.4 V DeWalt tool batteries. > > The L7812 regulator says it needs at least 14.5 V. will a 14.4 V > NiCad tool battery be acceptable for that? > > Thanks. LEDs are best driven by a constant current supply, like the one drawn below. ----- +14.4 ---in|LM317|out---+ ----- | adj [R] | | +---------+---[LED]-[LED]-[LED]---+ | Gnd --------------------------------------------+ Choose R for the current you want by R = 1.25/I According to the LED data sheet, the maximum current is 1 amp, so R must be no less that 1.25 ohms. The resistor would dissipate 1.25 watts at that value, so 2 watts or higher should be used. The LM317 will dissipate ~3.3 watts nominal at that current, but over 6 watts if the battery is at full charge voltage (over 17 volts) and taken immediately off the charger and connected to the circuit. So, size the heat sink accordingly. You could use a 7812 in the same configuration with R = 12/I, but the resistor would need to be about 10 times the wattage of the resistor used with the LM317. I would recommend running the thing at well under maximum current for longer run time, less heat, and longer LED life. If you use 3.5 ohms for R, I will be ~357 mA, R will need to dissipate about 1/2 watt, and the LM317 dissipation will be under 3 watts worst case, and about 1.6 watts nominal. You'd get more than triple the run time that way, too. Ed
From: Tim Wescott on 6 Dec 2009 16:04 On Sat, 05 Dec 2009 18:59:49 +0000, John Doe wrote: > I need to power three Cree XLamp XRE LEDs. The standard sealed lead acid > battery is 12 V, but it is heavier than I would like to carry. I have > some spare rechargeable 14.4 V DeWalt tool batteries. > > The L7812 regulator says it needs at least 14.5 V. will a 14.4 V NiCad > tool battery be acceptable for that? > > Thanks. In case you haven't figured it out, DO NOT connect a plain-old LED to a stiff voltage source like a NiCd or lead-acid battery. At least, unless you want a burnt out LED and an unhappy battery. You can kinda sorta get away with it if you have an LED that drops 1.5V and a dry cell battery, but it's still not good practice. LEDs want constant current sources -- which is what the other posters are offering up to you. -- www.wescottdesign.com
From: Tom Biasi on 6 Dec 2009 18:53
"Tim Wescott" <tim(a)seemywebsite.com> wrote in message news:uL6dneaZrNjegYHWnZ2dnUVZ_vZi4p2d(a)web-ster.com... > On Sat, 05 Dec 2009 18:59:49 +0000, John Doe wrote: > >> I need to power three Cree XLamp XRE LEDs. The standard sealed lead acid >> battery is 12 V, but it is heavier than I would like to carry. I have >> some spare rechargeable 14.4 V DeWalt tool batteries. >> >> The L7812 regulator says it needs at least 14.5 V. will a 14.4 V NiCad >> tool battery be acceptable for that? >> >> Thanks. > > In case you haven't figured it out, DO NOT connect a plain-old LED to a > stiff voltage source like a NiCd or lead-acid battery. At least, unless > you want a burnt out LED and an unhappy battery. You can kinda sorta get > away with it if you have an LED that drops 1.5V and a dry cell battery, > but it's still not good practice. > > LEDs want constant current sources -- which is what the other posters are > offering up to you. > > -- > www.wescottdesign.com I was assuming he was using the Cree XLamp used as a replacement bulb operating at a nominal 12 VDC. They are used routinely in RVs and routinely operate at 14+ volts while the vehicle is running. Tom |