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Author Topic: Lost with 50w LEDs + PWM  (Read 8439 times)
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That schematic won't work, there is nothing to pull down the mosfet gates, also the transistors reduce the mosfet gate drive.

If the LED is common anode then you can use N-channel mosfets connected in a similar way to PWM the LEDs, but connect the mosfet gates to the Arduino PWM outputs through 100 ohms resistors (no transistors). Use logic-level mosfets. Also connect a 10K resistor between each of the Arduino PWM outputs and ground.
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The seller has answered, seems to be an expert about his products.

Doesn't really matter...

The day they arrive you can just connect a low voltage across them and find out which way lights them up. The only difference it will make is the type of transistor you need.

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The seller has answered, seems to be an expert about his products.

Doesn't really matter...

The day they arrive you can just connect a low voltage across them and find out which way lights them up. The only difference it will make is the type of transistor you need.

Unfortunately, it also affects the circuitry needed. Common anode LEDs needing more than 5V can be controlled form the Arduino just with a N-channel mosfet or NPN transistor. Common cathode LEDs needing more than 5V require a P-channel mosfet or PNP transistor plus a level shift circuit, unless your 36V power supply has a floating output.
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That schematic won't work, there is nothing to pull down the mosfet gates, also the transistors reduce the mosfet gate drive.

If the LED is common anode then you can use N-channel mosfets connected in a similar way to PWM the LEDs, but connect the mosfet gates to the Arduino PWM outputs through 100 ohms resistors (no transistors). Use logic-level mosfets. Also connect a 10K resistor between each of the Arduino PWM outputs and ground.

You're right, forgot about pull down the gates. I think I fixed it.

The seller has answered, seems to be an expert about his products.

Doesn't really matter...

The day they arrive you can just connect a low voltage across them and find out which way lights them up. The only difference it will make is the type of transistor you need.

Unfortunately, it also affects the circuitry needed. Common anode LEDs needing more than 5V can be controlled form the Arduino just with a N-channel mosfet or NPN transistor. Common cathode LEDs needing more than 5V require a P-channel mosfet or PNP transistor plus a level shift circuit, unless your 36V power supply has a floating output.


I didn't know that the transistor type would change depending of if the led is common anode or cathode, I thought it would be as easy as the second schematic I made.



* common anode.PNG (12.16 KB, 763x643 - viewed 93 times.)

* common cathode.PNG (12.69 KB, 762x711 - viewed 74 times.)
« Last Edit: January 21, 2013, 02:14:58 pm by Hinjeniero » Logged

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Unfortunately, it also affects the circuitry needed.

Not really. He's already using a BJT to drive the MOSFET gate. He just has to pick NPN or PNP and connect accordingly. It's not a radical change in design.

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That first schematic is OK. If each mosfet is only switching one LED (max 330mA), then you don't even need to use mosfets, you can use NPN transistors such as BC337, with a 220 to 470 ohm resistor between the base of the transistor and the Arduino output, and no pulldown resistor.

The second schematic unfortunately won't work, because it will only pass a very low voltage to the LED. Unless the supply for the LEDs has a floating output, you need an NPN transistor followed by either a PNP transistor or a P-channel mosfet.
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Ok, I understand it now. I'm not sure about the specs that I got from the seller and I guess that each LED will take much more than (330mA). I'll run my own tests to be sure and then I will decide which transistor to use.

Thanks.

PD: I found this picture and I would say that is common anode because of the colors of the clamps but, of course, it's not sure.

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That first schematic is OK.

The red LED needs about 10V less than the blue/green so it might need a resistor to absorb the extra volts.

10V@300mA = 33 Ohms, although it will need a large wattage.

Something like this should be enough: http://www.ebay.com/itm/271109598315
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> The red LED needs about 10V less than the blue/green so it might need a resistor to absorb the extra volts.

Each LED needs a resistor to limit current, but yes, the red LED needs a different value from the rest.

As for taller heatsinks, look at these...

http://www.ebay.com/itm/Heat-Sink-for-20W-30W-50W-LED-/380534842067?pt=LH_DefaultDomain_2&hash=item5899a4e2d3

http://www.ebay.com/itm/20W-30W-Led-Light-Heatsink-With-FAN-Aluminium-Cooling-For-20W-30W-Led-12V-/190785568355?pt=LH_DefaultDomain_0&hash=item2c6bb4be63
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The red LED needs about 10V less than the blue/green so it might need a resistor to absorb the extra volts.

10V@300mA = 33 Ohms, although it will need a large wattage.

Something like this should be enough: http://www.ebay.com/itm/271109598315


I'm trying to avoid the use of resistors to limit the current directly to the led because it's a waste of power. I prefer (if possible) to limit the current trough the MOSFET.

> The red LED needs about 10V less than the blue/green so it might need a resistor to absorb the extra volts.

Each LED needs a resistor to limit current, but yes, the red LED needs a different value from the rest.

As for taller heatsinks, look at these...

http://www.ebay.com/itm/Heat-Sink-for-20W-30W-50W-LED-/380534842067?pt=LH_DefaultDomain_2&hash=item5899a4e2d3

http://www.ebay.com/itm/20W-30W-Led-Light-Heatsink-With-FAN-Aluminium-Cooling-For-20W-30W-Led-12V-/190785568355?pt=LH_DefaultDomain_0&hash=item2c6bb4be63

The first one is perfect, tall but the base is small, it would fit. The second one there is no way I could fit it in the spot light.

Speaking about the voltage of the LEDs, I found something interesting.


Code:
30W:
Color: RGB
DC Forward Voltage (VF):  Red 22-24V, Green 32-34V, Blue 32-34V
DC Forward current (IF): 300MA
Out put Lumens: Red 400-500LM, Green 600-800LM, Blue 200-300LM
Wave Length : Red 620-625nm , Green 515-520nm, Blue 455-460nm
Beam Angel: 140 degrees
Life span: >50,000 hours


50W:
Color: RGB
DC Forward Voltage (VF):  Red 22-24V, Greed 32-34V, Blue32-34V
DC Forward current (IF): 300MA
Out put Lumens: Red 400-500LM, Green 600-800LM, Blue 200-300LM
Wave Length : Red 620-625nm , Green 515-520nm, Blue 455-460nm
Beam Angel: 160 degrees
Life span: >50,000 hours

The specs for 50w and 30w are the same but the layout of the array of LEDs is completly diferent!! (see the attachment)

In the 30w version , for every color, there is 1 row of 10 leds and in the 50w version you will see 2 rows of 8 led.

Since the 10w version is 3 led per row and the 20w is exactly doble and the voltages match, it easy to figure out the voltage for the 50w version.


Code:
10W:
Color: RGB
DC Forward Voltage (VF):  Red 6-8V, Greed 9-12V, Blue 9-12V
DC Forward current (IF): 300MA
Out put Lumens: Red 120-150LM, Green 200-300LM, Blue 70-100LM
Wave Length : Red 620-625nm , Green 515-520nm, Blue 455-460nm
Beam Angel: 140 degrees
Life span: >50,000 hours


20W:
Color: RGB
DC Forward Voltage (VF):  Red 13-15V, Greed 18-20V, Blue 18-20V
DC Forward current (IF): 600MA
Out put Lumens: Red 260-300LM, Green 400-600LM, Blue 150-200LM
Wave Length : Red 620-625nm , Green 515-520nm, Blue 455-460nm
Beam Angel: 140 degrees
Life span: >50,000 hours

If the 10w red is 6-8v and contains 3 LEDs then 7/3= 2.4v per LED. Therefore the 50w version,wich contains 8 led (in each row) will be 19.2v.

Applying the same for the other 2 colors we will have than the green and blue are 28v each.

I don't know any other way to find the voltage safely without destroying the LEDs.


* $T2eC16V,!y8E9s2fjtNhBQ4PnwBU8g~~60_57[1].JPG (143.2 KB, 1000x1000 - viewed 28 times.)
« Last Edit: January 22, 2013, 07:17:40 am by Hinjeniero » Logged

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> I don't know any other way to find the voltage safely without destroying the LEDs.

If you supply them with the correct (and less than maximum rated) current and enough voltage to turn them on then it doesn't matter what the exact voltage is... the LED will drop whatever voltage it does given the supplied current. Look into buying/making some constant-current drivers for the LEDs. That way you set the current (in your case <300ma) and the driver puts out that current... no load resistors, no guessing, and not much wasted power. Maxim makes some driver devices as does On-semi. A driver circuit will cost more but is simpler and more efficient.

This device is pretty neat but it's only for up to 25V...
http://www.onsemi.com/pub_link/Collateral/CAT4101-D.PDF

This one might work to regulate current but you'd still need the MOSFETs though. I just discovered it and you'd have to do some reading about it.
http://www.onsemi.com/pub_link/Collateral/NSI50350AS-D.PDF
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I'm trying to avoid the use of resistors to limit the current directly to the led because it's a waste of power. I prefer (if possible) to limit the current trough the MOSFET.

The only way to avoid wasting power when driving large LEDs is to use a switched mode constant current regulator. You can build them yourself around a switching regulator IC, inductor and a few other components, but they can be tricky to design.
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> I don't know any other way to find the voltage safely without destroying the LEDs.

If you supply them with the correct (and less than maximum rated) current and enough voltage to turn them on then it doesn't matter what the exact voltage is... the LED will drop whatever voltage it does given the supplied current. Look into buying/making some constant-current drivers for the LEDs. That way you set the current (in your case <300ma) and the driver puts out that current... no load resistors, no guessing, and not much wasted power. Maxim makes some driver devices as does On-semi. A driver circuit will cost more but is simpler and more efficient.

This device is pretty neat but it's only for up to 25V...
http://www.onsemi.com/pub_link/Collateral/CAT4101-D.PDF

This one might work to regulate current but you'd still need the MOSFETs though. I just discovered it and you'd have to do some reading about it.
http://www.onsemi.com/pub_link/Collateral/NSI50350AS-D.PDF


I've been reading about CC and CV, and CC seems to be the best way to drive LEDs, but I would have to buy/build 15 of those and the circuit will become more complicated and less reliable and efficient. I don't think it's worth to control every spot light individually so I decided that I will join the Rs, the Gs and the Bs to finally get just 3 channels.

I would say that every single color of every LED will forward like 600mA, therefore if I join the colors of the leds, the total current would be 600mA * 5 = 3A per color.



The only way to avoid wasting power when driving large LEDs is to use a switched mode constant current regulator. You can build them yourself around a switching regulator IC, inductor and a few other components, but they can be tricky to design.

I've been watching how to build one and you're right, is tricky (and much more for me).

I'm thinking to buy 3 of these, put a heatsinks, and adjust the output at 2 - 2.5A.



http://www.ebay.com/itm/DC-Converter-Constant-Current-3A-Voltage-2-30V-LED-Driver-Battery-Charger-LM2596-/270955376640?pt=LH_DefaultDomain_0&hash=item3f1632f000


Code:
Feature:

-Fixed turn lamps current,that can show charging or not
-Reverse Protection,add input reverse connect protection diode on it
-Output the counter-current diode to stop battery power back
-Use the special benchmark of IC, and high precision sampling resistance,is more stable

Specifications:

-Module Properties: Non-isolated step-down / Buck charge module;constant voltage module(CC CV) charging module
-Input Voltage:DC 7~35 V
-Output Voltage:DC 1.25~30 V ( adjustable, continuous output ) 
-Max output Current: 3A  (if output power more than 15w,you can plug heat sink)
-Constant Range:0-3A (adjustable)
-Turn lamps current:CC value*(0.1),CC value will change with Turn lamp current.
-The minimum voltage difference:  2V   
-Output Power:Natural cooling 15W,  25W with heat sink
-Convert Efficiency:  Max 90% (higher Output voltage, higher efficiency)
-Output Ripple:  20M Bandwidth(Just reference )  input 12 V output 5 V 3 A 60 mV (MAX)
-Full Load temperature rise:45℃
-No-load Current:   Typical 10mA 
-Load Regulation:   ± 1%
-Voltage Regulation rate:  ± 0.5%
-Dynamic response speed:  5% 200uS
-Potentiometer adjustment direction:  clockwise (increase), counterclockwise (decrease) ,Close to the input  potentiometer is voltage regulation(CV), close to the output potentiometer is current regulation(CC)
-Indicator light:RED(charging);GREEN(finish charging)
-Output Short-circuit Protection:  Yes, Constant current
-Input Reverse Protection:  YES
-Output prevent reflux: YES, output have internally series against a prevent the diodes.
-Operating temperature:  Industrial grade (-40 ℃ to +85 ℃) (ambient temperature exceeds 40 degrees, lower power use, or to enhance heat dissipation)
-Wiring way: welding, add pin can be weldinged directly in PCB
-Size: 49 x 23.4 x 11.4 mm ( L*W*H )not include potentiometer
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I'm thinking to buy 3 of these, put a heatsinks, and adjust the output at 2 - 2.5A.

I was thinking of suggesting those to you. Here are some warnings if you use them:

1. The input capacitor is only rated at 35V and you indicated you needed about 36V. You should be OK if you swap out the input capacitor for a 50V one, since the chip it is based on (LM2596) is rated at 40V continuous (45V abs max).

2. Don't try to PWM those by switching the supply. However, if they have left pin 5 of the IC floating, then you can apply a 5V inverted PWM signal directly to that. This will be easy to do if your LEDs are common cathode.

3. You shouldn't need heatsinks, because they are rated for 3A and you are only drawing 350mA from them.

4. There may be quite a lot of ripple in the output current, because they are designed for up to 3A and you will be using them for only about 1/10 of that current. You could reduce the ripply by changing the inductor on the board for one with a higher inductance but lower current rating.
« Last Edit: January 22, 2013, 01:55:42 pm by dc42 » Logged

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3. You shouldn't need heatsinks, because they are rated for 3A and you are only drawing 350mA from them.
He;s looking at putting the LED's in parallel and therefore drawing 2-2.5A
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