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    Current Limter Design, constant voltage

    I need to set-up a circuit that can protect a load from currents above 200 mA. I use 10 Volts VDC from the power supply and I need all 10 volts to go across the load and stay constant. I will be adjusting the load resistance from between 750 - 100k ohms and the voltage drop needs to stay at 10 volts. Some slight variation will work but we are going to be trimming potentiometers, so the voltage drop needs to be constant 10 volts with any change in resistance of the load.

    If anyone has any ideas on how i can design this circuit, feel free to post, it would be much appreciated.
    Also if anyone knows if I am better off using FETS or BJT's. With BJT's I have used, the voltage drop due to Vbe doesn't fit my specifications.

    Thanks in Advance

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    Re: Current Limter Design, constant voltage

    You must specify what’s the maximum limit for the drop voltage, otherwise may need to use some step-up DC-DC converter combined with a current limiter to follow.



    see some IC used for this application:

    Advantages of IC-Based Hot-Swap Circuit Protection - Maxim
    Last edited by mister_rf; 18th July 2011 at 20:19.



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    Re: Current Limter Design, constant voltage

    Thank you for responding.

    When we trim the pots they are trimmed within a certain percentage of the base resistance so for 1000 ohms its trimmed between 800-1100 or for 100k its trimmed between 80,000 and 120,000. Within those limits, the voltage across the load cant move more than roughly .4 mV. This is why I am having so much trouble with this design.



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    Re: Current Limter Design, constant voltage

    In this moment still not clear for me without some schematics, how changing in the load current may occur. At 10V for 200mA we need a 50 ohms resistor as a load.



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    Re: Current Limter Design, constant voltage

    The load resistor is a potentiometer that can range from 700 - 100k ohms. I need a constant 10V drop across it for trimming purposes. If by any chance the pot shorts out, (because of wires touching, etc.) the current will burn the pot, so i need to limit the current to 200mA. When 200mA or above goes through the circuit i need the circuit to shut off and save the pot. 200mA is the max current



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    Re: Current Limter Design, constant voltage

    And you can’t do all required changes in the power supply circuit?



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    Re: Current Limter Design, constant voltage

    Basically i have a normal power supply and i would like to add this circuit after the power supply so i can hook up a different pot to the load every time and the circuit will limit the current no matter what size pot i use.



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    Re: Current Limter Design, constant voltage

    You can use LM317 ic.In datasheet this aplication is described.
    Make R2 variable eg replace with 5k variable pot, adjust to 10V.
    Modify Rs to limit output current.
    There is input/ output voltage differene.
    Last edited by ALERTLINKS; 18th July 2011 at 21:37.



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    Re: Current Limter Design, constant voltage

    Thank you, I am reading through the datasheet now

    ---------- Post added at 16:27 ---------- Previous post was at 16:20 ----------

    Using the LM317 on page 11 of the Datasheet PDF (LM317 Datasheet pdf - 3-Terminal Adjustable Regulator - National Semiconductor), Is that the one you would reccomment? Otherwise i see other current limits that use the LM117 that seem applicable.

    Thanks agian i appreciate the reply



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    Re: Current Limter Design, constant voltage

    Ok i see the circuit now, the only thing thats a little confusing is, where is the load resistor in the circuit, i dont know if they use different symbols than what i have used in the past but it looks like the load is on the right.

    ---------- Post added at 16:39 ---------- Previous post was at 16:37 ----------

    oh sorry i just saw your information now, Is the power supply on the right the same as the Vin going into the LC317?



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    Re: Current Limter Design, constant voltage

    So, in this case if you can adapt free the power supply, problems quickly solved.



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    Re: Current Limter Design, constant voltage

    Replace battery with load. Input is to the left (IN) and -ve is common.



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    Re: Current Limter Design, constant voltage

    I'm sorry i dont understand what you meant in the last post, so Vin and the power supply symbol on the right side of the circuit are the same then? they are both 10 volts?

    ---------- Post added at 16:46 ---------- Previous post was at 16:44 ----------

    ok now i get what you are saying, so the battery on the right of the circuit is the load and i will put the power supply (10V) into the Vin on the LC317. Adjust the Rs to limit the current i wish. Thank you for all your help, i will order the parts and try them in the next couple of days.



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    Re: Current Limter Design, constant voltage

    For the current application using LM117 and LM317 no differences.



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    Re: Current Limter Design, constant voltage

    Thank you both of you, i will get back to this post after i have tested the circuit. Thanks



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    Re: Current Limter Design, constant voltage

    Quote Originally Posted by ALERTLINKS View Post
    You can use LM317 ic.In datasheet this aplication is described.
    Make R2 variable eg replace with 5k variable pot, adjust to 10V.
    Modify Rs to limit output current.

    post #8
    I am afraid that Rs here is meant the possible parasitic resistance between the LM317 output and load and when R1 is connected at the side of the load.

    So when the load current inccreases the load voltage decreases as if it were a higher series resistance between the regulator and the load according to the formula:

    Rs(effective) = Rs * (1 + R1 /R2)

    Knowing that (after neglecting the term Iadj*R2 for being releatively very small) :
    Vout = Vref * (1 + R1 /R2)

    We get:
    Rs(effective) = Rs * Vout / Vref
    where
    Vref = 1.25 V

    So if Vout = 10 V
    Rs(effective) = Rs * 10 / 1.25 = 8*Rs
    That is the drop voltage at the load would be 8 times more than if R1 is connected close to LM317 output pin.

    Kerim
    Last edited by KerimF; 18th July 2011 at 22:06.



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    Re: Current Limter Design, constant voltage

    There is one point i overlooked in your post.It needs for good voltage regulation, input voltage shoud be 2-3V HIGHER THAN REQUIRED OUTPUT.If your supply is exactly 10v it wont work at 10v output. There are low dropout regulators whitch still drops. In that case you dont have to regulate 10 volts either. You need current dependent circuit breaker?



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    Re: Current Limter Design, constant voltage

    so if i were to decrease the load resistance, the the voltage drop across the load would decrease with this circuit?



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    Re: Current Limter Design, constant voltage

    Yes ,If your supply is exactly 10 volts then it can regulte to 7 volts.It will be stable and it will not drop. There is option ,if you put 3 dry cells in series making 4.5 volts btween input supply and lm317 input ,it will make 14.5 volts and it will work. Or just use mobile chager inseries to increse few volts.
    Last edited by ALERTLINKS; 18th July 2011 at 22:28.



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    Re: Current Limter Design, constant voltage

    If i were to supply the system with 14.5 will 10 volts drop appear across the load, and it will not drop?



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