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Voltage Regulator Based Current Source Circuit

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lkari

New Member
Hi

I am trying to build a 0-1 A variable current source circuit. I built it using LM317 as directed in the following link

LM317 / LM338 / LM350 CURRENT REGULATOR CALCULATOR | Circuit Project Electronic

I used a decade resistance box in place of the variable resistor. the load is around 10 ohms and the current does not go higher than 350mA. The whole circuit heats up and the current changes very drastically. I was hoping that I could find help rectifying the circuit or building a constant current source in any other possible way.

I have approaching deadlines for this project. I would appreciate any help in this regard.

Thank You
Lakshmi
 

Arkham00

Member
The circuit of your link is correct.
If you want to send 1A to your 10Ω load, you need a 1.2 resistor and the resistor will dissipate 1.2W.
With a 1A current, the load will have 10V at its terminals plus 1.2V across the resistor plus at least 2V between input and output of the LM317 so your Vin should be at least 15V (DC power).
With this Vin, your LM317 will dissipate about 4W.
 

MikeMl

Well-Known Member
Most Helpful Member
ArkHam is right. You also need a large finned heatsink to mount the LM317 on. A 317 has a self-protecting heat sensing circuit inside it. If you let it get too hot, it will shut down to protect itself. If you can dissipate the heat by conducting it to a heatsink of sufficient size, that will keep the innards of the 317 below the temperature at which it shuts down.

Typical heatsink
 

lkari

New Member
Thank you guys. I tried changing my input to like 15V and then 20 V. The circuit works fine for like few minutes and then goes bad. I also noticed something. the voltage between the adj and out pin is like 0.45V while it had to be 1.25V. do u think the regulator is shutting off because of the temp rise? Also, I noticed that the circuit when working well initially, works for the resistance between out and ADJ pin in a range greater than 10 ohms(load). I also noticed that I was using 1W dissipating resistors whilei needed 10 W dissapating resistors. I changed them to 10 watts. the resistor is soo huge. the problem persists though
 

MikeMl

Well-Known Member
Most Helpful Member
Read the spec sheet for the exact version of the 317 you are using. It has both over-temperature shutdown, as well as max current limiting.
 

lkari

New Member
I did check it it has both over temperature and over current shut down. was just wondering if my circuit is effected by both of these or what was happening.

So guys can anyone suggest any ideas for a 50-800mA regulated current supply for a 10 ohm resistor, 10 volts output at max?
 

MikeMl

Well-Known Member
Most Helpful Member
...can anyone suggest any ideas for a 50-800mA regulated current supply for a 10 ohm resistor, 10 volts output at max?
Here is a current regulator which has a very low drop out voltage. Its output current is proportional to a control voltage; 1V produces 1.0A in the 10 Ohm load. In the LTSpice simulation, the independent variable (plotted along the X axis is the Control Voltage.

Note the peak dissipation in the FET, and in the Sense Resistor. The minimum input voltage is only 11V!
 

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lkari

New Member
I appreciate that, but my load has to be grounded. it cannot be a floating load. in your design the load is between the supply and transistor. can i use a pfet instead on nfet?
 

MikeMl

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Most Helpful Member
I appreciate that, but my load has to be grounded. it cannot be a floating load. in your design the load is between the supply and transistor. can i use a pfet instead on nfet?
Yes, but the Control voltage is referenced to the positive supply. How about this?
 

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crutschow

Well-Known Member
Most Helpful Member
I did check it it has both over temperature and over current shut down. was just wondering if my circuit is effected by both of these or what was happening............................
If you don't have the LM317 on a heat sink then it will limit from over-temperature.
 

audioguru

Well-Known Member
Most Helpful Member
here i have the problem with same circuit LM317 / LM338 / LM350 CURRENT REGULATOR CALCULATOR | Circuit Project Electronic
I need to adjust in the range of 1mA to 500mA with the precise of 1mA difference.
The output voltage should be 12V.

Am trying to adjsut with 10 K digital potentiometer near 10 ohms resistor
  1. the precise current must 1mA difference
Please suggest any ideas or theoritical expressions for the circuit.
Your link shows an ordinary LM317 voltage regulator, not a current regulator. It does not have a 10 ohms resistor.
A digital pot can be used on a different current regulator circuit.
You cannot regulate voltage and current at the same time. The current regulator will adjust the voltage to the load.
 

MikeMl

Well-Known Member
Most Helpful Member
Hello,

here i have the problem with same circuit LM317 / LM338 / LM350 CURRENT REGULATOR CALCULATOR | Circuit Project Electronic
I need to adjust in the range of 1mA to 500mA with the precise of 1mA difference.
The output voltage should be 12V.

  1. Am trying to adjsut with 10 K digital potentiometer near 10 ohms resistor
  2. the precise current must 1mA difference
Please suggest any ideas or theoritical expressions for the circuit.

Best Regards.
The circuit you linked is not a Current Regulator circuit, it is a Voltage Regulator circuit. There is a big difference between the two.

In a voltage regulator, you turn a pot to adjust the output voltage E, and the impedance of the load R (what the supply is connected to) determines what the current I is (Ohms Law: I=E/R, the supply sets E, R determines the current I).

In a current regulator, you turn a pot to adjust the output current I, and the impedance of the loadR (what the supply is connected to) determines what the voltage is (Ohms Law: E=IR, the supply sets I, R determines the voltage).

I think you need a pot adjustable current regulator that starts with an unregulated input voltage of 12Vdc (wall-wart plug-in power supply or battery?), creates an adjustable constant-currentbetween 1mA and 500mA with a resolution of 1mA. Such a regulator would put out nearly 12V if the output is left open.

If set to 500mA , this regulator could keep the current constant as the load resistance varies from 0 Ohms (a short) up to R=12/0.5= 24 Ohms.

The circuit in post#9 is how a theoretical current regulator would work. Make the sense resistor 2 Ohms instead of 1. The control voltage source can be replaced with a reference voltage and a digital pot.
 
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