# About cheap Led Drivers (with MOSFET) and even cable losses



## richon (Oct 11, 2011)

Hi, this is my first post:

I just bought some 3W leds (3.5V and 800mA) and I plan to use it with my Solar Panel and Battery for lighting the house with 12V instead of 220v (here in Chile we have 220V)

I just did some test with 2 x 3W and a LM317 , as a LED driver: the LEDs keep really cool but the LM317 gets very hot (more than 40°C , but less than 60°C)

I know I couldn't use 3 x 3W as 10,5V would be over the limit that the LM317 would work with 12VDC (of my car battery)

is there a way to find a better (and DIY and cheap) led driver o way to loose to much from the LED driver ?




(For cable Losses read down)


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## evilc66 (Oct 11, 2011)

*Re: First Question: About cheap Led Drivers*

LM317s in current limiting mode can work, provided that the source voltage (from the battery) and the total forward voltage of the LEDs is very close to each other, allowing for the 1.2v(?) drop off voltage the LM317 requires. This isn't always the best approach for battery applications, because you have to plan for the worst case battery voltage, which usually means the driver has to burn off a lot of voltage when the batter is charged.

There are some other simple alternatives like this one. Not much more expensive than the LM317 driver, but more efficient. You will still need to heatsink the FET if you are running a lot of current though, but the heat generated is proportional to the current, not the voltage difference like the LM317. I've run these at 700mA without too much issue, and just a simple TO-220 heatsink attached to the FET.

Another alternative, if you are feeling adventurous, is to build yourself a real buck driver. I've always liked drivers by Zetex, as their datasheets are very detailed, and the parts are cheap. National is also another good choice (LM3404 is a common driver chip), as they have some great tools on their website. These will all be surface mount parts though, so it adds to the difficulty. The cost per driver isn't that much though, but efficiency is much higher (which equals less heat), and can be dimmed easily enough if necessary.


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## richon (Oct 11, 2011)

*Re: First Question: About cheap Led Drivers*

I was just looking at the driver you recomended:

I plan to use the IRF520 instead of the JFET as it will work to, besides with my 12V battery:

1.- using two 3W leds with the (constant current) led driver gets 4W heat loss (10W consumption)
2.- using three 3W leds with the (constant current) led driver gets 1,2W heat loss (10,2 compustion)

so , clearly (2) gets the same consumption and even usind 3 leds (33% more light).


right?


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## evilc66 (Oct 11, 2011)

*Re: First Question: About cheap Led Drivers*

That's correct for the LM317, or any other linear type driver. The driver I linked to is a little different. It's still linear in it's basic sense (it's more like a current mirror), but with the way it's designed, the LED is not on 100% of the time. Step 2 starts to explain a little how the circuit works. I wouldn't worry too much about the difference in source and forward voltage on this one. The heat you have to deal with is more related to the current flowing through Q2 and the sense resistor.


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## richon (Oct 11, 2011)

*Re: First Question: About cheap Led Drivers*

well now i run into more questions:

using three 3W leds (in series) makes 10,5V total. The Battery is a little more than 12V

Dan (on the link you posted) asumes 0,5V dropout voltage of the JFET he used.

How could I know the dropout voltage of another JFET or MOSFET looking at his datasheet (online), 'cause if the dropout voltage is more than 1,5V my setup "3 x 3W leds to a 12V battery" won't work as it is suppoust to do

(sorry for some mistakes writting in english, as Spanish is my "mother" language)


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## fnj (Oct 11, 2011)

*Re: First Question: About cheap Led Drivers*

A MOSFET has a ridiculously low "on" voltage due to very low resistance and no junction drop. It can easily be below 0.05 V. The dropout of the circuit depends on the entire schematic, which I don't see at the link provided.


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## richon (Oct 11, 2011)

*Re: First Question: About cheap Led Drivers*

this is the link: http://www.instructables.com/id/Cir...tep6/The-new-stuff-Constant-Current-Source-1/ 

as I see... everything looks good for building this...

so... tomorrow i'll buy more parts and test it... with some pictures too.


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## richon (Oct 12, 2011)

*Re: First Question: About cheap Led Drivers*

Hi, i just tested it with a IRF520...


works fine, too cool really


the IRF520 (at 800mA calculated R3) was "cool", no heat at all


so I tested the consumption of the three 3W leds I use... only 540mA with R3= 0,66 ohms


so I changed R3 with a New Voltage Drop value... to get close to 800mA. R3= 0,45 ohms. But Consumtion was only 570mA.


could it be that with IRF520 or any mosfet I would need to use a really low R3?


Changing for other NPN (MPSA18, 2N5088 which has much more gain than 2N3904) made no difference.

still 570mA max....

should the R3 be reaaaaally low?


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## MikeAusC (Oct 13, 2011)

*Re: First Question: About cheap Led Drivers*

If you don't get at least 1 amp with a 0.45 ohm sensing resistor there is something else wrong with the circuit. As soon as the voltage across this resistor reaches 0.5 volt the transistor will start to conduct, reducing the voltage on the Gate of the FET.

Measure the voltages across different parts of the circuit to see what's happening.

You need to have a big capacitor across the supply - at least 100microfarad to make sure it isn't oscillating - this will reduce the average current.

If the LEDs have 3.5 volt across them, they will drop 10.5 volt. Plus 0.6 v across the sensing resistor means 11.1 volt needed - is your supply providing enough voltage under load ?


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## richon (Oct 13, 2011)

*Re: First Question: About cheap Led Drivers*

I'm using a Car Battery (12,63Volts) so enough voltage and I guess I won't need a cap for the supply, isn't it?

I tried changing R1 from 100K to 22K, and even 10K... no luck... still giving me 570mA.
Voltage at GATE from MOSFET is 10.2Volts
and measuring the value "across" the Sensing resistor shows only 0,29V ... so maybe here is the problem, right?


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## MikeAusC (Oct 13, 2011)

*Re: First Question: About cheap Led Drivers*

Switch your Multimeter to AC and see if there is any oscillation.


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## richon (Oct 13, 2011)

*Re: First Question: About cheap Led Drivers*

Mike: check oscillation where? on the baterry or on the Gate of Mosfet or on Source of Mosfet?

could it be that the dropout voltage of the mosfet, by being so low, ask for a really low R3 (0.1R or less?)?


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## richon (Oct 13, 2011)

*Re: First Question: About cheap Led Drivers*

after some investigation:

IRF520 has a Rds of 0,27 ohms... and at 800mA the voltage drop should be only 0,216 volts.

so... if i need 800mA for the leds , R3 should be 0,27 ohms , right?


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## zzonbi (Oct 13, 2011)

*Re: About cheap Led Drivers (with MOSFET)*

Don't forget the cables. Increase R3 and try with 2 leds only.


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## richon (Oct 13, 2011)

*Re: About cheap Led Drivers (with MOSFET)*



zzonbi said:


> Don't forget the cables. Increase R3 and try with 2 leds only.



what could the cables do???

I just used a really short cable (1mts) with my 0,27 ohms for R3... 1,1A consumption!!!!!

so... cable loss... (it was 15mts long cable in my first test (yes i know, don't kill me))


will test with new value to have a final value for R3 (so far, i'll test with 0.4 ohms


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## richon (Oct 13, 2011)

*Re: About cheap Led Drivers (with MOSFET)*

new test:

0,55 ohms Total consumption: 0.97A (still to high right (leds are rated for 800mA, how much should I consider for another consumption of the mosfet or the NPN?)

More update: with 1,1 ohms --->490mA

More Update: with 0,66 ohms--->770mA (GOAL!!!!, funny is that started at 810mA and after two minutes, as the temperature increased the current drops to 770mA and stayed there)

the IRF520 with his 2,5W heatsink is barely over room temperature... 

everything worked fine

with 12,5V from car battery
10,5V drop from leds= 10,5V*0,77A=8,085 Watts
2V drop from mosfet= 2V*0,77A=1,54 Watts
total consumption: 770mA*12,5V= 9,625 watts
84% efficiency.... pretty awesome for a dolar (cost of parts here in Chile)


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## richon (Oct 14, 2011)

*Re: About cheap Led Drivers (with MOSFET)*

More info on cable loss (all theoretical):
60 ohms/ Kms = 0,06 ohms per meter... and 15mts is 0,9 ohms
then 12,5V / 0,8A = 15,625 ohms.

Lamp and cable in series gives a total of: 16,525 ohms
so 12,5V / 16,525 ohms = 0,756 A (only 0,05A less)

then voltage drop at a given resistor: 0,756A x 0,9 ohms = 0,68 Volts

so 12,5 V - 0,68 V = 11,82 Volts to use at 15mts

after all this math... it doesn't explain the 0,55 A as a top current value with the 15mts cable and not with the 1mts cable....


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## richon (Oct 15, 2011)

*Re: About cheap Led Drivers (with MOSFET)*

i did measure the cable , 15mts where 1,7 ohms each... so a two cable 15mts is 3,4 ohms
then 12,5V / 0,8 A = 15,625 ohms

lamp and cable in series gives a total of 19,025 ohms
so 12,5V / 19,025 ohms = 0,657 A (142mA less)

then the voltage drop a a given resistor would be: 0,657A x 3,4 ohms = 2,23 volts

so 12,5 V - 2,23 V = 10,27 Volts if I use the 15mts cable

So now it explains how with the 15mts cable I couldn't get to the normal voltage use of the leds+currect regulator and how I couldn't reach 0,8A as expected.


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## idleprocess (Oct 15, 2011)

*Re: About cheap Led Drivers (with MOSFET)*

Many of these simple drivers come with the caveat that one should locate them as close to the LED's as possible - immediately adjacent is commonly recommended.

I plan on experimenting with the circuit from instructables in future builds, myself - located on the fixture centimeters away from the LED's.


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## richon (Oct 15, 2011)

*Re: About cheap Led Drivers (with MOSFET)*

the driver was always "no more than 5cms" away from the leds

the 15mts (before) were from the 12V car battery to the "driver+leds"


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