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100 watt led driving with 150w dc to dc converter. Help needed

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Mindmywork

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Hai.
I want to drive a 100 watt led from a 150 dc to dc boost converter.
I'm successfull in getting the volatge upto 33v. But not getting the desired current of 3 amp.
When tested with a multimeter at 10 amps rating it just showing .7 amps.

The input of the *150 dc dc boost* is 12 volt rail from a pc smps{300 watt rated}

My question is simple. Why im not getting 3 amps?
Is it my smsps problem or boos conveter problem.
Is there any way i can increase the current.


My 100 watt led specification is
Voltage 30-35v
Current 3000 m amps

Here are the pics the things.
 
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Les Jones

Well-Known Member
Most Helpful Member
You should drive your LED from a constant currenet source. (NOT the constant voltage source that you are using.) The reason is that only a very small voltage increase caused a very large increase in current. The voltage at the rated current also varies with temperature and between samples of the same type of LED. You can buy constant current drivers designed for LEDs.

Les.
 

Mindmywork

Member
Thanks for the response.

I have attached a cc circuit with ghe dc dc boost

Dc dc boost Positive out to lm338t v in, .43 ohm resistor(5watts) to adj and v out. And used the adj pin to led.
 

Les Jones

Well-Known Member
Most Helpful Member
You will need a larger heat sink on the LM338. The dropout voltage of the LM338T is about 2.5 volts at 3 amps so you need to dissipate at least 7.5 watts. As you say that your LED could require up to 35 volts then the need to set the voltage from the boost converter to about 38 volts to allow for 3 volts drop in the current regulator. If you post the schematic of the boost regulator it may be possible to modify that to give a constant current output. which would save having to dissipate at least 7.5 watts in the linear current regulator.

Les.
 

Mindmywork

Member
You will need a larger heat sink on the LM338. The dropout voltage of the LM338T is about 2.5 volts at 3 amps so you need to dissipate at least 7.5 watts. As you say that your LED could require up to 35 volts then the need to set the voltage from the boost converter to about 38 volts to allow for 3 volts drop in the current regulator. If you post the schematic of the boost regulator it may be possible to modify that to give a constant current output. which would save having to dissipate at least 7.5 watts in the linear current regulator.

Les.
thanks for ur responce :)
dont know the exact schematic as i bought it from ebay. i can send some pics of it,
 

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Mindmywork

Member
You will need a larger heat sink on the LM338. The dropout voltage of the LM338T is about 2.5 volts at 3 amps so you need to dissipate at least 7.5 watts. As you say that your LED could require up to 35 volts then the need to set the voltage from the boost converter to about 38 volts to allow for 3 volts drop in the current regulator. If you post the schematic of the boost regulator it may be possible to modify that to give a constant current output. which would save having to dissipate at least 7.5 watts in the linear current regulator.

Les.
these are the specs on the selling page


Specifications:
  • Input voltage: DC 10V-32V.
  • Output voltage: DC12V-35V(adjustable).
  • Output current: 10A (MAX).
  • Input current 16A (MAX) (more than 10A please strengthen heatsink).
  • Output power: natural cooling 100W (MAX), strengthening cooling 150W (MAX) real power.
  • Conversion efficiency: 94% (when Input 19V 2.5A Output 16V, Reference).
  • Output ripple: 2% (MAX).
  • Working temperature: industrial (- 40 c to +85 c) ( environmental temperature, more than 40 degrees, please
  • reduce power to use, or - enhance cooling).
  • Full-Load temperature: 45 degrees.
  • No-load current: 25mA typical.
  • Voltage regulation: 0.5%.
  • Load regulation: 0.5%.
  • Dynamic response speed: 200uS 5%.
  • Short circuit protection: No (Please installs the fuse or protection circuit at input parts).
  • Input Reverse protection: No (please comply with a reverse protection or connect a diode at input pa
 

ronsimpson

Well-Known Member
Most Helpful Member
I have attached a cc circuit with ghe dc dc boost
Back in post #3, I can not see the attached "cc".

I can not see the big picture.
You have a PC power supply that makes 12V. (What does the 12 volts measure?) Many PC power supplies can not make 100 watts at 12V when there is no load on the 5V and 3.3V.

Then there is a DC to DC boost supply. (need more information) I think you have it set to 33 volts output.

Next you have a LM338. What is the input and output voltage? How hot is it? Can you hold it in your hand, hot?
 

Mindmywork

Member
Back in post #3, I can not see the attached "cc".

I can not see the big picture.
You have a PC power supply that makes 12V. (What does the 12 volts measure?) Many PC power supplies can not make 100 watts at 12V when there is no load on the 5V and 3.3V.

Then there is a DC to DC boost supply. (need more information) I think you have it set to 33 volts output.

Next you have a LM338. What is the input and output voltage? How hot is it? Can you hold it in your hand, hot?
thank for the reply
1. i dint give any load to 5v or 3.3v rail. just used the 12v line 1. 100 watt led 2. for the heat sink fan. same 12v line for the two.
2. that is dc to dc 150 w power supply already posted the specifications . thats all what i know about boost supply
3. i havent tested the heat of lm338t. will try and post with in 2 minutes. :)
 

Les Jones

Well-Known Member
Most Helpful Member
The LM338T current regulator will not work if you cannot supply it wit at least 3 volts more than the LED requires at it's rated current. If the boost regulator is rated at 35 volts maximum output I would not use it so close to it's maximum ratings.

Les.
 

Mindmywork

Member
just tested
output at dc dc booster is 34.7 volts
output at lm338 is 32.4 volts

and lm338t is normal hot to cool.
 
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