simple to build dc/dc voltage converter for 12v lighting ect

[nebriancent]

hello all
i have one main problem since i started running my e-bike
i had no way to run 12v led lighting off of my main bat as i have been running 48v nom voltage and durring full charge the battery it reading 55v so i could not use any 12v automotive led lighting on my bike unless i ran off of one of my 4 batteries and i have found this to be a bad idea

i started off thinking i will just keep an eye on the one batt and swap out wen i notice a voltage drop in comparison to the other batts ... but im sure we all know how that turned out

so the solution i came up w/ is by no means a new idea... and it is verry wastfull at higher currents but ITS CHEAP
and i know for menny like myself on a first ebike that dose come first untill we figure out if the ebike sceen will work for us

another solution i am noting is the use of small battery powered lighting made for bikes but i dont realy like the idea of replacing batteries

so i will give you the tools needed to create you own xv-12v converter
(i will build one complete w/ pictures over the next couple of posts here)

first thing you need is a staring voltage (how many volts is my battery pack)
2nd thing you will need is a target voltage (12-14v)

parts list

1 @ 14v 10w zenner(overkill but i like to play safe) i use pn 1n2978b
1 @ resistor (wattage and value dependant on build)
1 @ npn transistor (i get mine out of dead computer power supplys)

first off i suggest lower then 10w ttl draw from this circuit so 10 / 14 = .71 amp
i also suggest you build the circuit for more then you plan on using eg build for 1 amp draw

(side note) do not at any time short the positive and neg leads together you will destroy the circuit and give the load full voltage

the calculations

what you can draw is heavily dependant on the zenner transistor and resistor you use
first you need to find the dc gain and max power dissapation for the transistor

once you have the dc gain figured out divide the ttl amperage buy the gain and we now have a target volt ref amperage input

for exaple you may have a dc gain of 1000 so for say 1 amp draw you would need at least 1ma or .001 amp draw from the zenner/resistor pair
but remember over shoot over build and it will last

so now we need a easy way to figure out the zenner and resistor value (i use http://www.calculatoredge.com/electronics/zener.htm )
just input max volt(fully charged bat pluss a lil extra for safty)
min volt (battery is fully drained voltage)
zenner voltage(you target voltage)
and load current in ma (.001 of a amp)

next post i will show how to put these components together and i should have some pics of a fresh build
and how to find info on the trasistor ect

ps please please please use a heat sink ... if you dont you could burn your circuit and anything connected to it

 

[nebriancent]

so i have decided to go with the 2sc2625 transistor as i have just found a pair in a old computer psu
and since the salvaged transistors only have a dc gain of 10 i have decided to go with a Darlington pair setup
that is were the output of one transistor goes into the input of another so that the gain of the combined transistor setup is the 2 separate gains multiplied
so 10x10 = 100
so to get a 1 amp draw of of such a setup i would need to give it .01 amps (10ma) from the zener setup
so for this to work since my voltage on my bike pack will go from 60v down to 42v i need a 1.4k resistor @ at least 1.5w
but...

i dont do bare min on the wattage for anything so my resistor will be most likely 5w

i have taken pictures along the way of my build but i am held up by the resistor as of now and i cant figure out how to get the pics off my phone
so..
i will post a basic schematic of what i have done this far and the end result

 

[nebriancent]

here is a screen-shot showing how the circuit is put together

 

[velias]

Sometimes its just easier and cheaper/faster to buy a commercially available DC-DC converter.

 

[Doctorbass]

i've been talking about that idea back in 2007 when i had this dc-dc problem.

I had found that the AC-DC CAN work as DC-DC

Most of the switching power supply we see normally that are made for AC to Dc WILL accept DC at the input.

The most recent one will dont care about the polarity at the input and some of the old one will need to be connect on the right polarity otherwise they will just not start.

Many AC-DC adapter that use switching power supply inside WILL start at around 30VDC and up.

What is good is that it mean they will accept from 30 to 350VDC with still great efficiency!!

ex: my 4 Magicshines of 10W each all connected in parallel to the same AC-DC output of 14.5V will draw 54Watts TOTAL of DC power at the input with 24s ( 88.8V nominal)

not bad at all! for a "DC-DC" that cost me 9.95$ brand new.. most of the time they are sold in the surplus electronic store or ebay as 12V power supply

and there is great probablyty that if they are INPUT : 100 - 240V 50 - 60HZ they do work with DC at input!

They look like this one: http://cgi.ebay.com/Power-Supply-Adapter-12V-5A-iMAX-B6-B5-LCD-MONITOR-/170616320188?pt=Laptop_Adapters_Chargers&hash=item27b98670bc#ht_1804wt_935

btw.. just to let you know.. my 1500W Meanwell psu START and work if i connect his AC input to my 24s pack!

Doc

 

[nebriancent]

will they all run reliably at 30vdc?? and if so at their rated output? or how close to the rated output?

as far as i understood a switching psu takes main current .. rectifies it to dc(so would take strait dc for sure) .. re oscillates the current to high frequency .. steps down the voltage through a transformer .. rectifies the current again.. then uses a buck converter to get your target voltage.

or.... did i understand the principle wrong .. if so how?

and if i am correct how is it able to have such a wide input voltage?

or do they use some sort of pwm b4 they rectify the output to get the target voltage?


i know thats a lot of questions. but for me its not enough to know that it dose work .. i want to know why and how

ps i am currently using the above circuit and have been for about 8-10 months so if i can do it easer i would love to know and in learning help others though thees posts

 

[nebriancent]

Sometimes its just easier and cheaper/faster to buy a commercially available DC-DC converter.

the one i have been running on my bike for 8-10 months cost me nothing to build and it took me 30-40min to build using scrap parts all salvaged from dead psu's

 

[CoffeeLuke]

An AC.DC step down works as a DC.DC step down!!! ones with a large input ie.100V-220V will work as low as 40V.have no probs running lights and music.