# Current Regulator

Discussion in 'General Electronics Discussion' started by Omran65, Apr 10, 2017.

1. ### Omran65

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Apr 10, 2017
hi
I have a question :
I want to make current regulator
LM317 can't work because the voltage of my circuit have be same in output and input
Vi 14v
Vo 14v
Io 0.1A
can you help me?

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3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Current regulators require something to sense the current. Most methods of doing this will incur some voltage drop, even if it is only 0.1V.

4. ### Omran65

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Apr 10, 2017
you guess when my output current is 0.1A my voltage is be lower than 13v?

5. ### Omran65

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Apr 10, 2017
I have output voltage of my circuit is be higher than 12.5v anyhow
can you proposal simple circuit to me?

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Take a look in the LED resource. There is a simple constant current source that can drop less than 1V. For lower drops you need more exotic circuits.

7. ### Alec_t

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Jul 7, 2015
By 'current regulator' do you mean a controllable constant-current source or a current-limiter?

8. ### Omran65

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Apr 10, 2017
l have to use this circuit for very slow charging solid battery

9. ### Alec_t

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Jul 7, 2015
What battery chemistry? Lithium batteries are very fussy about their charging regime. A dedicated battery charge control IC might be better/safer than just a constant-current source.

10. ### BobK

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Jan 5, 2010
Your question, as stated is asking for something impossible. You cannot have a current regulator that has an output of 14V and 0.1A. To regulate current, the voltage must vary depending on the load. Perhaps you really mean that the output voltage must be capable of going all the way up to the the supply voltage if necessary. This is called the compliance voltage.

As others have said, you cannot even measure the current without some loss of voltage.

If what you want is 14V in and and current regulator all the way up to 14V, you will probably need to boost the voltage first, then run it through a current regulator.

Bob

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14. ### Bluejets

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Oct 5, 2014
I've used a standard plug pack (transformer type, not switchmode) 12v 500ma supply via an adjustable LM317 current regulator for different applications for years.

The supply outputs more than 12v with varying loads naturally, but just trying it now on a fully charged (via another auto charger yesterday)12v sealed lead acid 7Ah battery, it will still deliver 40mA at maximum adjustment on the pot.

So I think your 1.3Ah battery should be ok with something along these lines.

15. ### Omran65

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Apr 10, 2017
This circuit should be OK?

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16. ### duke37

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Jan 9, 2011
You have shown a voltage regulator, I thought you needed a current regulator.

17. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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This will limit the current:

Assuming the regulator variable with a nominal 1.25V output:

Vout at 0A is 1.25 * (R1 + R2)/R2
Iout (short circuit) is 1.25/R3

There is a linear relationship between these points.

If you're planning to charge a 12V lead acid battery, I'd set Vout as 14V and Iout as maybe 1A

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