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what is constant current source, please help

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sidharth

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if the resistance increases,current decreases according to the Ohms law & it's a law so
how current source can be constant?
:confused:

tks
sid
 
Because they're not linear devices. They're active devices that take control of the current, and can keep the current constant regardless of the load conditions (resistance or impedance of the load) or supply voltage. (Within certain limits, of course.)

If the constant-current source senses that the current through the load wants to increase, let's say due to decreased resistance of the load, then it adjusts the current downward to maintain it at a constant level. Think of it as an active, variable current-limiting device. A resistor limits current too, but it cannot maintain current at a constant level (because of Ohm's law, as you point out).
 
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if the resistance increases,current decreases according to the Ohms law & it's a law so
how current source can be constant?
:confused:

tks
sid
Ohms law contains three variable terms. You have only mentioned two of them.

Keep the value of I constant, and that still allows V to vary in response to a change in R

EDIT, so your original quote of ohms law is incomplete and should be something like:
IF VOLTAGE REMAINS CONSTANT, then if the resistance increases,current decreases (according to the Ohms law)
 
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it means to keep I constant these devices controll's the R ?
No, not really. It will control the "V".

I think R would be considered to be something in your circuit, but outside of the constant current source.

A constant current source will adjust its output voltage to maintain constant current (possibly by adjusting its own internal resistance). Whatever it needs to do to keep the output current constant.

There will always be a limit to how much adjustment range a constant current source can provide depending on its design.
 
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If it helps, here's an actual constant-current source I've built that works (I use it to test LEDs using a 9-volt battery):

**broken link removed**

The transistor adjusts its gain, based on the voltage drop sensed across Rset (which increases as current increases) to keep the current constant. It works very well, within a certain voltage range.
 
If it helps, here's an actual constant-current source I've built that works (I use it to test LEDs using a 9-volt battery):

**broken link removed**

The transistor adjusts its gain, based on the voltage drop sensed across Rset (which increases as current increases) to keep the current constant. It works very well, within a certain voltage range.
To be picky, the transistor doesn't adjust the gain which a constant for any particular transistor. The transistor gain causes the transistor to adjust its current to maintain a constant 0.7V across Rset, thus generating a constant current.
 
A constant current source will adjust its output voltage to maintain constant current (possibly by adjusting its own internal resistance). Whatever it needs to do to keep the output current constant.

There will always be a limit to how much adjustment range a constant current source can provide depending on its design.
It should be noted that the feedback is such as to make the output impedance of the constant current appear very high (infinity for the ideal source). Thus any changes in the output voltage due to a change in load resistance have no effect on the output current.
 
To be picky, the transistor doesn't adjust the gain which a constant for any particular transistor. The transistor gain causes the transistor to adjust its current to maintain a constant 0.7V across Rset, thus generating a constant current.

Not picky at all. Thanks for the correction. These things matter.
 
A constant current source will adjust its output voltage to maintain constant current (possibly by adjusting its own internal resistance). Whatever it needs to do to keep the output current constant.


so to keep the I constant V-out is controlled in a certain range by Manipulating internal resistance?
 
so to keep the I constant V-out is controlled in a certain range by Manipulating internal resistance?
Hmm... no, not really.

Sorry, my post was misleading and I will edit it.... please delete the bit of my statement in brackets. crutschow explained it much better.....
 
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