power 12v led from 72v battery.

[jimmyhackers]

essentially i have an adapted poundland led bulb that runs on 12v and draws around 300ma. so 3.6watts power total

my new batteries are 72v. and id like to power them off of that rather than a seperate battery.

my initial idea is to just put a power resistor in series and be done with it. although this would be very inefficient id like someone to overlook my calculations.

with v = i x r ive come to a resistor value. 12v divided by 0.3amp makes 43.33 ohms resitance of the bulb. i want to take up the other 60v with a resistor so. 60v divided by 0.3 makes 200ohms.
60v multiplied by 0.3amps makes 18watts.

so i need a 200ohm resitor of 18watts or above

is this correct?

 

[wesnewell]

Looks right to me according to this calc.
http://www.gtsparkplugs.com/Dropping_Resistor_Calc.html

 

[dnmun]

you may wanna use a smaller led.

 

[jesusjesus]

why add the complexity and power usage?

solar or led-self power generation IMHO

 

[izeman]

Take a 12v walwart and you're done. Most efficient.

 

[dnmun]

does not have to be a 12V source to power the led either. a regular 5V cell phone charger would work fine. still need the current limiting resistor though.

 

[ebent]

I have a 48v pack and a 12v converter from thunderstruck. I added a 12v power bus. Any need for 12v gets wired to the power bus.

 

[meeki]

I use this converter on my bike 72v li-ion. Works great and gives you 10Amps

http://m.ebay.nl/itm/DC-24V-36V-48V...Battery-Car-/331045796298?txnId=1173554317014

Cheap and fast shipping

 

[MadRhino]

Why not use 6 in a series. Leds are pretty small and voltage tolerant, and they last very long for cheap too. 3 red under the saddle and 3 white or blue for the front, eventually replace the middle one with a zoom lens flashlight. Switch this with a toggle on the handlebar, you have a very compact, robust and simple light system. You can even wire flasher and brake switches later if you want, for you'll have enough units already mounted to the bike.

I am using 8 led series on 24s Lipo, got rid of the DC-DC converter and this proved perfectly reliable in all weather and conditions.

 

[izeman]

How do you limit current to the leds?

 

[MadRhino]

I don't. All 8 of them together are pulling less than 30W.
I am using those small 12v Eagle Eye automotive LED lights.

 

[jimmyhackers]

thanks for the responses everyone.

theyve given me some ideas. i know a power resistor arrangement is technicaloly very wastefull. but its also the neatest smallest solution i can think of. in theory i will only be wasting 18 or so watts as heat in the resistor. then 22 or so in the whole lighting arrangement.

compared to the 1000watts or so my motor drains, 22 watts isnt a lot comparatively. my batteries are seeming to be around 16ah capacity(still in testing).
im not too worried about a 22w power drain.

a smaller led just wouldnt be as bright. i do a lot of night time riding and i need something bright. suprisingly these cheapo poundland 220v gu10 led bulbs "hacked" to run on 12v are incredibly bright and one is more than enough.

the idea of 6 superbright bulbs with no resistor would be awesome, and kind of the least wasteful. 6 would blind oncoming traffic and make me a hazzard not to mention clutter up the front of my bike.

ive ordered a few power resitors in haste....

and ive been wathcing far to much occ.....i think im going to get inventive.

thanks again guys. will post up some pics

 

[Gregory]

How do you limit current to the leds?

Given the low price on the ones I've seen, I'd assume they have a current limiting resistor built in rather than a CC driver board..

 

[sn0wchyld]

essentially i have an adapted poundland led bulb that runs on 12v and draws around 300ma. so 3.6watts power total

my new batteries are 72v. and id like to power them off of that rather than a seperate battery.

my initial idea is to just put a power resistor in series and be done with it. although this would be very inefficient id like someone to overlook my calculations.

with v = i x r ive come to a resistor value. 12v divided by 0.3amp makes 43.33 ohms resitance of the bulb. i want to take up the other 60v with a resistor so. 60v divided by 0.3 makes 200ohms.
60v multiplied by 0.3amps makes 18watts.

so i need a 200ohm resitor of 18watts or above

is this correct?

sounds good, but i'd strongly recommend a switch mode power supply (they work fine of DC). it'll cost only a little more than the ~20w resistor, particularly if you look at any 2nd hand electronics store (really you should use a 25+W resistor for 18W disipation, otherwise it may not last long...) and it'll reduce your power usage for reducing the voltage by a factor of about 20 . Also means you dont have a very, very hot object on your bike that you might accidentally touch too... your light will use about 4w vs 22ish and no chance of burning yourself on a hot resistor (20W in a small object is quite a bit of heat!).

 

[r3volved]

I am thinking of doing the same...subscribed

 

[jimmyhackers]

have installed my device.... works wonderfully.

i hot glue gunned in the power resistor inside the bulbs casing.....

and it hasnt gotten hot enough to melt the glue again.

so all is well...

 

[dnmun]

except it is a very inefficient and old fashioned method of regulating voltage and current. heat is of no value and it costs money for the battery to make that heat.

 

[jimmyhackers]

i guess ill just have to cope with the 21.6watts of energy wasted as heat.........

or mount the resistor in my seat for a warm behind on a cold nights