LED string with PWM

[Creative25]

Hi
I am thinking of building an LED camping light that connects to the car battery.
I am planning to use the LM317 Regulator for current regulation.
The plan is to use two or three LED's in series. Forward voltage is 3.3 volts Maximum current is 350 mA.
I am planning to make two strings of LED's in parallel, and apply about 700mA
Three in series would give 9.9 volt.
Would the current regulation still be stable or should I rather only use 2.

Is it possible to use PWM in combination with the LM317 current regulator.
What would be the be the best place to apply the pulses before or after the current regulator?
Best Regards:
Uli

[Enamul]

I am planning to use the LM317 Regulator for current regulation.

LM317 is a constant voltage regulator as you know can supply maximum 1.5A according to data sheet. So current will vary within the limit according to load requirement. But you can make current controlled version as well.

I am planning to make two strings of LED's in parallel, and apply about 700mA.Three in series would give 9.9 volt.

I am suggesting you to use two LM317 in parallel even though two strings will take 1.4 A as I have seen practically those don't give 1.5A.

Is it possible to use PWM in combination with the LM317 current regulator.
What would be the be the best place to apply the pulses before or after the current regulator?

Actually you can't give pulsed input to LM317 as this will provide poor regulation..
Definitely Output of LM317 will be the Vcc of your PWM transistor/FET..I have used FET to control 48V 12 parallel strings each with 24 LEDs. This was used as Tower light in Mobile tower...
Enamul

[Creative25]

Thanks
With a 12 Volt supply would you make strings of two or three LED's
What fet would you use?
Best Regards
Uli

[Enamul]

Hi,
You are welcome

With a 12 Volt supply would you make strings of two or three LED's

For strings voltage is not a big issue as long as number of LEDs are properly driven..I would say with a car battery (at least 100AH) you should be able to use 3strings.

What fet would you use?

I have used IRF Z44 for my case....
http://www.vishay.com/docs/91291/91291.pdf

Best Regards
Enamul

[Creative25]

Do you mean with 3 Led's
current regulation will still be stable?

[Enamul]

If you use two LM317, 3 LEDs/ string if takes 700mA..2 strings is fine..

[Creative25]

Hi Enamul,

Definitely Output of LM317 will be the Vcc of your PWM transistor/FET..I have used FET to control 48V 12 parallel strings each with 24 LEDs. This was used as Tower light in Mobile tower...
Enamul

Does this mean you connect the drain to VCC and source to the LED's
Or do you connect the drain to the LED's and the source to GND.
Do you connet the fet directly to the chip or would you use a transistor to drive the fet?

Best regards:
Uli

[Enamul]

Does this mean you connect the drain to VCC and source to the LED's
Or do you connect the drain to the LED's and the source to GND.

Positive of LED string will be connected to Vcc and negative end will be connected to drain and source will be GND..you can use the Fig I have posted

Pic12F617 PWM not working

...page 3 MOSFET_driver.gif

Do you connet the fet directly to the chip or would you use a transistor to drive the fet?

I never recommend to do that as this might cause a damage to PIC port PIN...Always better to use one signal diode + 10 K resistor from pic pin to transistor base and pull down base of transistor and drive FET from collector...
Enamul

[Creative25]

Hi Enamul
Thanks for the answer.
So you will never recommend to connect a fet to a pic with a resistor only , no matter what voltage or current you are switching, even if the voltage applied to drain would be the same as the supply voltage of the chip?

The IRFZ transisters are quite pricy compared to NXP etc.
But they are widely used, what is so special about them?
Best regards:

[Enamul]

Hi,
Sorry for the delay..I am extremely sick and can't sit to computer...

So you will never recommend to connect a fet to a pic with a resistor only , no matter what voltage or current you are switching, even if the voltage applied to drain would be the same as the supply voltage of the chip?

One thing I always maintain in any circuit design..PIC power supply is separated than the other circuits not even LCD is driven by same supply..Bcz if any problem happens to other device it causes the performance problem to PIC..

The IRFZ transisters are quite pricy compared to NXP etc.
But they are widely used, what is so special about them?

Yes you right. A bit pricey but in low and medium power application especially in UPS and IPS...this FET is used world-wide bcz of it's reliability...even in inductive switching environment...
Enamul

[dazz]

Hi
For led drivers i use these, very simple pcb required and very cost effective
http://uk.futureelectronics.com/en/tech ... .aspx?IM=0

Regards
Dazz

[Creative25]

Hi Enamul
Thank for all the info.
I just found out that the LED's have a forward voltage of 3.7 volt not 3.3 So I guess it would be better to use only strings of two.
Or should I just drop the current per LED a bit? Will this affect the colour of the LED?
I am using the XlampMX3

Hi Dazz,
I think the CAT4101 looks like a cool device.
Maybe it could even build it into a LED flashlight to get better performance.
I might use it in the future.
How difficult is it to solder these surface mount devices?
I guess you have to make a proper pc board for them.
Best Regards:
Uli

[Enamul]

Hi,

I am using the XlampMX3

If you are using the LED in the following link..
http://www.cree.com/~/media/Files/Cree/ ... ampMX3.pdf
In that case you should not use more than two in a string and maximum two string is better otherwise it will discharge the battery too much....it might happen that you can't start your car while returning from camp...

Or should I just drop the current per LED a bit? Will this affect the colour of the LED?

Actually looking at the datasheet seems that 3 LED will provide illumination but not in its maximum capability..
Enamul

[dazz]

Hi uli
quite easy to solder with a normal iron, making a pcb is a relitively simple task for a simple board like that, what country are you in ??

[Creative25]

Hi
Yes I am using this one.
And I was thinking of using strings of two.
I do not think my battery will become flat so quickly.
3 strings will draw a little more than 1 Amp
If you use it for six hours it will use 6 amp hours.
That is 10% on a 60 Amp hours battery.
That should still safe if you drive every day.
But if the battery is old then it is a problem.
I have seen cars that still start when the battery is old and has only 10% of their capacity left.
Then of course you should not go camping anyway. You might get stranded even without a camping light.
Are there some other issues to consider?
Thanks Ueli

[Enamul]

No..it's fine then..
About Z44 price..you can get from ebay not too pricey...
if you are from UK..
http://www.ebay.co.uk/itm/IRFZ44N-power ... 500wt_1156
or other than uk..
http://www.ebay.co.uk/itm/2-pcs-IRFZ44N ... 231d29bb2d
Enamul

[Creative25]

Thanks,
I actually have some of them.
It might be a bit overkill for one amp, is it still better to use a fet than a bipolar transistor.
Has anyone experience with soldering the XlampMX3
Are there special considerations if you use a normal soldering iron?

Best Regards:
Uli

[dazz]

Hi
Not those led's specifically but on the other cree's i tend to mount them on the ali star pcb's, i wet the pcb with a small amount of solder add a miniscule amount of flux to the led tags then tack one side on then solder the other and rewet the tack, but make sure you do it quickly as they dont like to be overheated, i use an antex xs25 soldering iron and with smd components i can solder 0603 packages quite happily on a good day,but due to the meds im on i shake quite a bit so use 0803 smd's more now, depending on the country your in i can send you a few random smd components to practice on if you want(no cost i overordered ).

Regards
Dazz

[Creative25]

Hi
I am trying to make a simple single sided board to solder on the LED's
Something with large copper areas where the contacts are soldered onto.
What about the heat sink of the die.
Can I just drill a hole where the heat sink will be so that it will be exposed to air?
Or do you need a proper design for it.
Best regards:
Uli

[dazz]

Hi

If you check out the cree site they have the recommended pcb gerber files to download and view, they also have a pdf detailing thermal design for the pcb's

Regards
Dazz

[Enamul]

Hi,
I have no experience to share in soldering this component.
Enamul

[Creative25]

Hi,
I am busy making a simple board to test the LED's
If you are interested I can put some pictures.
I have three strings of two and a LM317 as a current regulator for each string.
For the beginning I will test it without PWM
On the data sheets it never shows capacitors on the circuit.
What capacitors should I use?
Where would you put them?
Best Regards:
Uli

[Enamul]

Hi,
We will be glad to see your nice works...Before uploading the picture you can open the picture in paint and save as in .jpg format which will minimize size of picture..

I have three strings of two and a LM317 as a current regulator for each string.

That seems fine..

On the data sheets it never shows capacitors on the circuit. What capacitors should I use?

Are you talking about the capacitor to stabilize the voltage that drives the LED strings? I feel that the only capacitor you might need to use in circuit.

Where would you put them?

You should use 100uF/35v capacitor between 317 output and GND i.e, if you are using FET then +ve of LED string to GNDed source.
Enamul

[Creative25]

Hi
I tested it without capacitors driving it directly without fet.
But I will add one later.
I was not able to solder the Heat sinks of the XLampMX3 nicely. These package re difficult since the edge of the Heatsink of the LED is not exposed to the edges.
It got a bit hot so I changed the resistors to reduce the current to 150mA now it is fine and the Light it gives is still impressive. At 150mA it still gives 50% of light according to the datasheet.
It has a bit of a flicker, that is why I asked about capacitors.
For Vin of the circuit would you use anything?
Must it be exactly 100uF I am not sure if I find some.
Best Regards:
Uli

[Enamul]

Hi Uli,

It has a bit of a flicker, that is why I asked about capacitors.

There shouldn't be flicker unless any drop in voltage..if you are driving from car battery..
In that case LM317 is causing the problem while supplying current it's voltage gets dropped..so you need to put capacitor in it's output...higher is better..470uF.

For Vin of the circuit would you use anything?

You can use 100uF at the input of LM317..
Enamul