r there any voltage controllers to step down voltage from 12-16v DC to regulated 5v DC. i require 5AMP of current so i can run my car led tail lamps.. they r 300 led's in parallel.. each 20mA

 

You probably will need a switching converter for that current. Take a look at National's SimpleSwitcher line. You want a "Buck Converter" which reduces input voltage.

Brent

 

L4970 from ST microelectronics.

 

Not necessarily better but possible- use 1 ohm resistor followed by LM338K set for 5 volts with proper heat sink.

1 ohm resistor at 5 amps will drop voltage by 5 volts so your input to the voltage regulator is somewhat tempered. At 16 volts the voltage drop across the regulator is slightly over the specs however it's supposed to be thermally limited internally. While system voltage could run that high it probably doesn't most of the time. 5 amps is at the limit of the 338 though it's supposed to handle brief overloads.

1 ohm resistor has to get rid of 25 watts. Rather than purchase one large resistor make it up by taking a number of resistors of same value in parallel.

This approach might be technically simpler though it has some risks (fully loading 338) and requires a way to get rid of the heat from the resistors and the 338 itself. Note also that a pass transistor or two can be wrapped around a voltage regulator. Note that getting rid of the heat is still a problem. The switching approach is less demanding in this respect.

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stevez

 

i did look at national website and had a deep look into their

LM2676 and LM 2678 but as i live in saudi arabia.. it is very hard to find the rest of the parts to complete the circuits..they have to be purchased in bulk usually.. .. and rather then spending money ordering everything from national website.. i rather get some ready made solotuion or atleast an IC which would not need extra components to complete the circuit like some produced by infeneon ( http://www.infineon.com/cgi/ecrm.dll....jsp?oid=24099 ) but the highest rating one is like 500Ma and wouldnt drive more than a few tens led's in parallel..

where as i need to drive like 300 led's in parallell in the red tail lamp and additional 50 led's for the turn signal..

radio shak might be willing to sell those parts as retail rather than bulk but i havent check up there.. incase i find them there.. then i woould sure go ahead to build it on my own..

where the heck i get portel or a prog which can open portel drawing available for download on national website..so i can print my pcb :?:

 

Protel is a PCB Layout tool. Its expensive but you can get a 30 day evaluation for free.

 

12 hours and counting... im still waiting for an email to arrive with download link... for portel!!

:cry:

 

too many resistors is like having a 300 * 25 = 7500 watt heater in my trunk and then there r 300 led's in each side tail lamps so that like 600 * 25 = 15000 watt heating in my trunk.. whoaa.. that would sure melt pretty much plastic in the trunk.. :lol:

 

excellent solution.. i think it would let me hook up all the lights on one circuit rather than making two since it can support a total load of 10A :wink:

 

If this tail-light not a ready-made device, You can connect seriel minimum 6 pieces red LEDs and one resistor for higher efficiency.
If really need all LED connect in parallel, i recommend to use lower voltage as 5V ( for red LEDs 2.5V so the serial resistor have low dissipation.)

 

but voltage in car varies between 11.8 to 16 volts.. wouldnt it be a problem for led's... bcuz as far as i think it would frown up led's designed for a 12 circuit

 

A '12V LED' isn't really any such thing, it's just a normal LED with a resistor built-in, so if the voltage applied increases slightly the current will increase, and if it decreases the current will decrease - no problem in either case.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Wow, just got caught up on this thread and the heat calculation caught my eye.

16 volts at 5 amps is 80 watts total.

A bank of resistors, regarless of number and size, will only dissapate 25 watts if it provides a 5 volt drop at 5 amps.

25 watts or 80 watts can be a lot of heat and is sufficient to fry stuff but it can be managed. Wasting that much power via resistors and linear regulators might not be the best but it is easily implemented.

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stevez

 

You could use a simple method such as burning off the excess voltage,
but as you can see, at 16Vin, 5v@5a out, you need to dissipate 55watts
of power. Not a good scenario. Your overall system efficiency will be 31%.

Save yourself time and money, and buy a point of load converter.

The synchronous buck topology is simple and very efficient. The entire supply will be about the size of a cigarette lighter. (volumetrically).

Plus, these run at typical efficiencies(%) in the low to mid 90s!

Look at Wall Industries, Datel, Tyco....just to name a few.
S.P.

 

all right.. can i have some links to site selling load convertor online.. so i may have a look at it..

why i need to custom make tail lights and not buy ready made's one's is that firstly they r unnecessaarily expensive, secondly they r boring solid pattren ie: dim when on, bright when brake is thrown..... what i want is a custom pattren for different situations plus i will be using very bright led..