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how is a constant current supply made

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Gaston

Member
I need to make a constant current power supply. I am thinking that i could use an adjustable voltage regulator and read the voltage drop across a resistor to measure the curent and use that to modulate the voltage regulator. does that sound right?
 

audioguru

Well-Known Member
Most Helpful Member
An LM317 adjustable regulator plus one sensing resistor makes a regulated current supply. Two parts.
 

solis365

New Member
like audioguru said, the lm317 will do it in a different configuration than what is used to make it a voltage reg. the current source schematic might be in the applications notes on the data sheet, or there are lots of websites that will have it. bit harder to find than the voltage setup, but its around.
 

dknguyen

Well-Known Member
Most Helpful Member
I need to make a constant current power supply. I am thinking that i could use an adjustable voltage regulator and read the voltage drop across a resistor to measure the curent and use that to modulate the voltage regulator. does that sound right?
Yes, but it does require that the regulator be built the right way...like the LM317. You can also do it with an op-amp (similar to how you can build a regulator with an op-amp). YOu can look at the schematic of how a LM317 works and if you understand it you can do the same thing with an op-amp.
 
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MikeMl

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317 or any adjustable regulator like it. However, be cognizant of the power dissipation taking place in the regulator, and that the minimum voltage that can be across the regulator is the dropout voltage of the regulator (~2V) plus the drop across the current sampling resistor (1.2V). So to get a CC supply that works from 0-10V, you would have to start with a minimum input voltage of 13.2V. If this is a problem, there are ways to do it so that the minimum voltage drop across the current regulator is less than a few mV. Also, if you need, say 1A, depending on the drop across the regulator it could be dissipating 10 or more W, which requires a large heatsink.
 
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Leftyretro

New Member
I need to make a constant current power supply. I am thinking that i could use an adjustable voltage regulator and read the voltage drop across a resistor to measure the curent and use that to modulate the voltage regulator. does that sound right?
Yes, it's basic feedback theory.

Lefty
 

Mr RB

Well-Known Member
Yes, but it does require that the regulator be built the right way...like the LM317. ...
Nah you can use any 3pin voltage regulator.

I've used 7805's for driving Luxeon 1watt white power leds from 12v, you just have to allow for the 7-10mA of the gnd pin's quiescent current, and of course the current sense resistor will be run at 5v not at 1.2v like with a LM317.

7805 can be bought in bags of 100 for a few bucks these days (firms selling off non-ROHS stocks) and with 5v on the sense resistor the regulator chip dissipates less power.
 
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Hero999

Banned
LM7805s are hugely wasteful though - you loose up 7.5V!
 

Mr RB

Well-Known Member
Yep, but in the case I mentioned that is actually a good thing. Running a high power led from 12v, (say 13.8v) the led is about 3.3v, the resistor drops 5v and the regulator IC drops 5.5v.

So the regulator runs almost twice as cool as a LM317 with the same parts count (no heatsinking needed), and you can just run a slightly larger resistor.
 

colin55

Well-Known Member
You mention a: CONSTANT CURRENT POWER SUPPLY.

What do you really want?

Do you really want a supply that has current limiting?

In other words do you want a 12v supply that is limited to say 300mA so that it will not damage the device you are testing.
 

Boncuk

New Member
Yes, but it does require that the regulator be built the right way...like the LM317. You can also do it with an op-amp (similar to how you can build a regulator with an op-amp). YOu can look at the schematic of how a LM317 works and if you understand it you can do the same thing with an op-amp.
What can I say? (Postscript)
 

dknguyen

Well-Known Member
Most Helpful Member
Shouldn't you be using active current-limited PWM then? Instead of an inefficient linaer constant current source?
 

rikilshah

Member
can you suggest something about this??
i found so many circuits but I found everything so much complex....
How it will be if I use microcontroller for same??

Like in active PWM supplies the simple logic is like..take current feedback and change PWM according to that(I may be skipping some point...correct me here...)...For feedback I can use Internal ADC and for PWM internal PWM channels...so the circuit will be easier and cheaper..

Now I think there will not be any complexity If I 'll do it by uc...
Eagerly waiting for replies...
 
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