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[SOLVED] constant current source

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yoosefheidari

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hi every body
i have a NCP6354 and i want to use it as a constant current source to drive a 6w white led at 2000ma
help me how can i do it:thinker:
 

KlausST

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Hi,

Instead of voltage feedback you may try:
connect a 300mOhms shunt from FB to GND.
Connect the LED from OUT to the shunt.

The problem is, that the shunt will dissipate 1.2W at 2A.
Not that good efficiency...
Maybe you need some capacitive feedback to ensure stability.

Klaus
 

yoosefheidari

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so can i add a current limit to prevent led burning?a 2000ma current limit
 

KlausST

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Hi
so can i add a current limit to prevent led burning?a 2000ma current limit

I´m confused.
of course you can. But Why?
You´ve built a constant current source, it already limits the current. ... why do you need another current limiter?

Klaus
 

crutschow

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You could use a shunt feedback resistor as KlausST suggested, but add a single-supply type opamp (such as an LM324, LM358) with non-inverting gain to reduce the value of the shunt resistance and its dissipation.
For example a gain of +6 will reduce the value of the needed shunt resistor to 50mΩ and its dissipation to 0.2W for a 2A constant-current output.
You likely will have to add some compensation capacitance across the opamp feedback resistor for stability.
 

yoosefheidari

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You could use a shunt feedback resistor as KlausST suggested, but add a single-supply type opamp (such as an LM324, LM358) with non-inverting gain to reduce the value of the shunt resistance and its dissipation.
For example a gain of +6 will reduce the value of the needed shunt resistor to 50mΩ and its dissipation to 0.2W for a 2A constant-current output.
You likely will have to add some compensation capacitance across the opamp feedback resistor for stability.
thank you and KlausST
i dont understand that op-amp
can you show me by schematic shunt resistor and op-amp?
 

crutschow

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.....i dont understand that op-amp
can you show me by schematic shunt resistor and op-amp?
R1 is the shunt resistor.
The op amp has a gain of six, which will give an output current of 2A through the LED D1 using a 50mΩ shunt.

Capture.PNG
 
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yoosefheidari

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R1 is the shunt resistor.
The op amp has a gain of six, which will give an output current of 2A through the LED D1 using a 50mΩ shunt.

View attachment 148121

thank you

but i have a question
what is the difference between original schematic that we adjust output voltage and then specific current go across the led with this schematic that we adjust voltage between led and resistor and again same current go across led?
 

KlausST

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Hi,

but i have a question
what is the difference between original schematic that we adjust output voltage and then specific current go across the led with this schematic that we adjust voltage between led and resistor and again same current go across led?

The diffference:
* Datasheet = constant voltage source
* this solution = constant current source (as requested)

Behaviour:
Datasheet: change the load resistance: voltage will remain constant, current will change
this solution: change the load resistance: current will remain constant, voltage will change

Klaus
 

yoosefheidari

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can i set op-amp gain higher around 12 and use a 25mΩ resistor instead 50 ?
 

crutschow

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...........
what is the difference between original schematic that we adjust output voltage and then specific current go across the led with this schematic that we adjust voltage between led and resistor and again same current go across led?
Because if you use a voltage, you need a resistor in series with the LED to regulate the current, and that wastes power.
can i set op-amp gain higher around 12 and use a 25mΩ resistor instead 50 ?
Yes.
 
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