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How to make constant current source

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j2bichon

New Member
I am now trying to design constant temperatuer current souce about 2A, 12V to drive 40 LIGHT Emiting diode, To drive LED light contstatnly, there's lots of problem, I found, such as consumption current variation following temperature variation etc...,
Could anybody give me a swolution for this?
 

Phasor

Member
constant temperatuer current souce about 2A, 12V
Can you clarify this a little - do you want constant current, constant voltage, or constant light output? They are all very different things, requiring very different circuitry.
 

j2bichon

New Member
I need constant current supply to keep constant light

Now, I intend to make a constant current supplier to control constant luminosity of LED. if current was decreased by temperature variation or other reason, the luminosity of LED is decreasd. so I need constant current source independent to temperature variation.
 

bogdanfirst

New Member
i think you don't have such a great variation of temperature so that the leds loose the intensity.
could you use a LM317 voltage regulator? it soulds limke the simplest solution for me.
and about modifying there brightness, you could use a PWM, wich will have a better efficency.
 

Phasor

Member
j2bichon has not mentioned whether efficiency is an issue, so assuming it is not, then this circuit should work adequately. Vin should be at least 3V higher than the highest output voltage required.
 

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pebe

Member
Phasor, I think your circuit would give problems. LEDs should never be driven in parallel without limiting resistors. The voltage/current relationship between indvidual LEDs when lit will differ, so that at a given voltage their relative brightnesses will be different. Indeed, some may not light at all.

If I read the data sheet for the LM350 correctly, there is 1.2v between output and the control pin when it is working correctly. That means the current through the 0.6ohm resistor (and the LEDs) will be 2A. I think some of the LEDs may pop!
 

Phasor

Member
When I drew the LEDs, I just meant to draw a general load - I didn't intend hooking them up specifically that way :wink:
 

john1

Active Member
Hi,

ive used leds in parallel many times with a common resistor,
i know that they may differ individually, and i always look
out for it, but i have had no trouble like that, even with
different size leds.
I have used seven segment displays with a common resistor,
and yes, there has been a slight difference in brightness
from the one (two segments) to the eight (all seven), but
not enough to worry me.

John :)
 

j2bichon

New Member
I think there's problem on your solution

8) I need 10V, 2A to control cross connected paralleled LED strings(10 x 4), I will use this for automotive filed to control Turn and stop signal
and by using this constant currrent source, i should detect how many LED are fail.
 

pebe

Member
Re: I think there's problem on your solution

j2bichon said:
8) I need 10V, 2A to control cross connected paralleled LED strings(10 x 4), I will use this for automotive filed to control Turn and stop signal
and by using this constant currrent source, i should detect how many LED are fail.
I cannot understand how a constant current source can help you decide whether LEDs fail. Assuming the current in each LED to be 20mA, then if you have 4 strings of 10 LEDs each or 10 strings of 4 LEDs each, the maximum current will be 80mA or 200mA. So you will not reach the limit of a 2A, so your source will be a constant voltage one.

So what have I missed?
 

j2bichon

New Member
LED consume 150mA/each

My Led consum Max 150mA, normaly 100mA and if I use constant vltage source, the currunt consumtion is changed denpend on LED series resistor.
 

pebe

Member
Re: LED consume 150mA/each

j2bichon said:
My Led consum Max 150mA, normaly 100mA and if I use constant vltage source, the currunt consumtion is changed denpend on LED series resistor.
Correct. So why do you need a constant current source? What are you trying to measure to decide if an LED has failed?
 

kwc

New Member
constant LED brightness but variable input voltages?

Hi all,

i am new here and I would like to require some help.

Could anyone tell me how to design a simple constant LED brightness but have variable input voltage (DC voltage range of 0 to 60 Volts).

My problem is that whenever I varies the input voltage, my LED light brightness varies with the voltage. But I do not want this. I would like it to be constant all the time. The LED purpose is just to indicate the power is on.

Thanks, :?
ckw
 
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