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Constant Current Source

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graemec

New Member
I apologise for what may seem an obvious question.

Is a constant current source one that is constant regardless of its load?

G
 

JimB

Super Moderator
Most Helpful Member
Yes, within limits.

A constant current source will fall over if the load resistance is too high, as it will not be able to provide enough voltage to drive the desired current through the resistance.
Good old Ohms law again.

Similarly a constant voltage source will give a constant voltage regardless of the load, within limits.
It cant feed the required voltage into a very low resistance, because it cannot provide enough current.

JimB
 

Roff

Well-Known Member
graemec said:


ok.

Is this a constant current source?

g
No, because the current through Rb will vary as the load resistance varies. It will be fairly high impedance, but you can improve it by moving RL to the collector of TR1.
It will be temperature sensitive. The current will vary about -0.3% per degree C.
 

graemec

New Member
Sorry so let me understand.

If I move Rl to the collector it will be constant current source?

Does anyone have any links to designs for constant current sources?

Thanks

Graeme
 

Hero999

Banned
Yes, look on the datasheet for the LM317 which is easy to find from Google.
 

Roff

Well-Known Member
graemec said:
Sorry so let me understand.

If I move Rl to the collector it will be constant current source?

Does anyone have any links to designs for constant current sources?

Thanks

Graeme
Do you still need up to 300mA, with a 5V supply?
 

graemec

New Member
Yeah I do.

But more importantly I want to improve my understanding. I've drawing a number of circuits which deliver a constant current irrespective of the input voltage but just realised that the current is highly dependent on the load which is really not what I want. I want a circuit that will deliver 300ma do any reasnoble load. If 300ma is not possible I want the most I can get.

Id like to be able to construct the circuit using off the shelf components rather than buy devices.

thanks for all the help so far.

G
 

Roff

Well-Known Member
graemec said:
Yeah I do.

But more importantly I want to improve my understanding. I've drawing a number of circuits which deliver a constant current irrespective of the input voltage but just realised that the current is highly dependent on the load which is really not what I want. I want a circuit that will deliver 300ma do any reasnoble load. If 300ma is not possible I want the most I can get.

Id like to be able to construct the circuit using off the shelf components rather than buy devices.

thanks for all the help so far.

G
I tried to find a tutorial on the Internet, without much success. I have designed and built a LOT of current sources in my life, and I could describe some techniques, but I don't know what your level of understanding is.
Do you know Ohms law? Do you understand op amps and feedback theory? Do you understand how transistors work? If you don't understand these, you won't be able to understand how current sources work.
 

Hero999

Banned
Last edited:

Roff

Well-Known Member
graemec said:
I understand all these concepts.

any help would be great.

Graeme
OK, here's what is probably the most common voltage-controlled current source configuration, with several options for output current boosters (you can make low-current sources with just an op amp and some resistors).
There are many other ways to make voltage-controlled current sources.
 

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