Current Limiter Circuit for Variable Voltage Supply using LM317

[basit701]

Hello Guys!

I have got a semester project in which I have to make 0-20 V DC power supply using LM317. Besides this the other requirement is to limit the current to 500mA(current should not exceed this limit) whatever the output voltage is chosen by adjusting the variable resistor.

Up till now I have created the voltage regulation part (0-20 V); now to set the current limit I decided to use LM317 in precision current limiter mode and chose the limiting resistance to 2.5 ohms so (1.25/2.5= 500mA). The limiter works like a charm as long as Vin(output voltage from voltage regulator) - Vout(output from current regulator) is > 5~6V (in my case if Vin > 6V). If the voltage falls below 6V the limits starts to decrease e.g. at 3.77V the maximum limit falls to 360mA.

I have searched a lot for other limiting circuits on the internet but no avail. Any suggestion for current limiting circuit?

***For Voltage Regulator I chose R1=620 ohm and R2 =10kohm variable*****

[RoboTechEd]

I could be wrong, but could you just put the current limiter on the first half of your circuit (drive the voltage side witht he current)? That way the current supply will get a steady voltage

[Zero999]

Yes, it's the wrong way round. The Current limiter should be come first.

This is a pretty rubbish way to do this as the drop out voltage will be very high.

[basit701]

I swapped the position of crrent limiter and voltage regulator. The circuit is working perfectly but Current Regulator heat sink gets heat up.

[Zero999]

That's to be expected as the current limiter will drop a few volts in normal operation. Put it on a decent heat sink and it should be fine.#

You'll probably also notice the LM317 won't go down to 0V. To do that you need a negative power supply and a 1.25V reference or an op-amp differential amplifier to invert the LM317's own 1.25V reference.

[basit701]

I am using LM337 to generate constant -1.25 volt ouput and have connected this to LM317 so its starts from 0.

[basit701]

Another issue exists now. When i will connect load the ouput voltage will change according to load. But i want to display voltage selected on Voltmeter on the casing. E.g. i choose 15V output and after connecting load it fell to 14.2 volts. Since voltmeter will be connected in parallel it will read 14.2 volt not 15. I want to show 15 volts. What to do?

[Kalvin]

Another issue exists now. When i will connect load the ouput voltage will change according to load. But i want to display voltage selected on Voltmeter on the casing. E.g. i choose 15V output and after connecting load it fell to 14.2 volts. Since voltmeter will be connected in parallel it will read 14.2 volt not 15. I want to show 15 volts. What to do?

Read the LM317 data sheet carefully. There is said something about minimum load. Then google for a constant current sink which can be made using a led and two cheap transistors. I addition to better output regulation, you will also get nice indicator for active output voltage.  

[basit701]

Whats the function of current sink? Can you explain what it does?

[Kalvin]

Whats the function of current sink? Can you explain what it does?

Constant current sink is constant current load. A constant current sink will draw constant current no matter the voltage applied to it (within reasonable limits of course). LM317 requires a minimum load current before it requlates its output properly. You will find this information in the data sheet.

Here is one practical application for a constant current sink / load. Let's say you want a led indicator for your power supply output which will be lit when the power supply is turned on. As you know, a led needs a series resistor which will limit the current through the led at the given voltage. At 3V the resistor may be around 70 ohms. Now your led is lit brightly when your power supply is set to 3V output voltage. The problem starts when you increase the output voltage: as you increase the output voltage, the led will be brighter and brighter as current through the led increases until it will "burn" at 5V as too much current is flowing through it. So, you would like to have a constant current sink of 10mA (or 20mA depending of your led) connected in series with your led. At 3V the current through the led will be 10mA, at 5V the current will be 10mA and even at 15V the current through the led will still be 10mA. The led will be happy and the brightness of the led will not change as you change the output voltage from 3V to 15V. And when you read the LM317 data sheet, you might kind of like this constant current sink and the led idea.

[basit701]

So basically you are trying to say that current sink will draw current = minimum load current for IC. So it will perform regulation and voltage will not drop when load will be applied. Is that right?

[Kalvin]

So basically you are try to say that current sink will draw current = minimum load current for IC. So it will perform regulation and voltage will not drop when load will be applied. Is that right?

Yes, correct. You should of course verify this with a resistor before you build the constant current sink. Let's say the minimum load is N mA, so adjust the voltage output to 15V, calculate the resistor value needed for N mA at 15V, and connect the resistor to the output. The output voltage may droop to 14V as you noticed before. Keep the resistor in place and adjust the voltage back to 15V. Now the minimum required current is flowing through LM317 and its output should be stable and regulated. Add some more load to it. Does it droop any more? If you are now happy with the output regulation, you can build a simple constant current sink of N mA so that the required minimum output current will drawn at all output voltages (and the led will be lit at constant intensity).

Edit: You may want to verify this also with, let's say 5V output. Disconnect the load, adjust the output to 5V and reduce the resistor value accordingly so that N mA flows through the resistor at 5V. Connect the load. Is the voltage regulation still good enough?

[Kalvin]

One more thing to check: It is also stated in the LM317 datasheet, that the regulator needs a minimum voltage difference between its input and output in order to work and regulate the output properly. You may want to look that up from the LM317 data sheet and verify that at 15V output voltage the LM317's input voltage is more than the minimum voltage difference stated in the data sheet (ie. Vin >= Vout + Vdiffmin). If the voltage difference is less than minimum voltage difference stated in the data sheet, the output voltage regulation will be poor and the output voltage will droop as more current is drawn from the regulator.

[basit701]

My DC output from Rectifier circuit is 33 Volts and in any case Vin -Vout is in between 3 and 40 volts.

I adjusted the voltage to 12 volts. In order to draw 11mA (currenr sink equivalent) i connected two 2.2K resistors in parallel. The voltage dropped a bit but i increased it again to 12 volts. Then i connected a DC fan (found in computer PSU) of rating 12 V 0.21 Amp and the voltage fell from 12 volts to 11.4. I can't figure out why the IC is not regulating properly.

[Vgkid]

How hot is your lm317 getting?
What is your peak current draw?

[Kalvin]

My DC output from Rectifier circuit is 33 Volts and in any case Vin -Vout is in between 3 and 40 volts.

I adjusted the voltage to 12 volts. In order to draw 11mA (currenr sink equivalent) i connected two 2.2K resistors in parallel. The voltage dropped a bit but i increased it again to 12 volts. Then i connected a DC fan (found in computer PSU) of rating 12 V 0.21 Amp and the voltage fell from 12 volts to 11.4. I can't figure out why the IC is not regulating properly.

Strange. What kind of transformer you have? What is its current rating? What kind of rectifier you have? What kind of capacitors you have after the rectifier? What kind of heat sink you have?

You should also measure these voltages when the fan is not connected and when the fan is connected:
- Voltage at the input of the LM317
- Voltage at the rectifier
These measurements should tell something what is going on.

Pls see Vgkid's post above.

[basit701]

How hot is your lm317 getting?
What is your peak current draw?

Without heat sink it does get hot. But with heatsink no issues (i am using it with heat sink).

Maximum Current drawn is 500mA.

[basit701]

Transformer max output current is 2Amp.

[Kalvin]

How hot is your lm317 getting?
What is your peak current draw?

Without heat sink it does get hot. But with heatsink no issues (i am using it with heat sink).

Maximum Current drawn is 500mA.

If the peak current drawn by the fan is 500mA, then the current limiter will reduce (limit)  the input voltage at the LM317's input. When that happens, the LM317's output voltage will of course drop. You may want to measure the voltage at the LM317's input and output when the fan is connected, and verify that the voltage difference is at least 5V  between LM317's input and output. You should also check the voltage at the rectifier output. This should help you figuring out what is going on in your circuit and why the output voltage drops.

[Zero999]

R1 should be 120R, to avoid poor regulation at low currents.

If you set R1 to 120R then R2 would need to be scaled to keep the output voltage the same, try 2k.

Using a potentiometer to set the voltage like this will mean the maximum voltage will have a tolerance of 20%.

[macboy]

The LM317 generally has excellent load regulation, which means very little variation in output voltage when the load current changes. There is obviously something wrong with your circuit.

Start by measuring the voltage at the input of the (second, voltage-controlling) LM317. What is it, with no load and with load? With 12 V out, it must be at least 15 V or so.

Measure the voltage at the input and output of the first (current limiting) LM317. The drop from input to output should be small, roughly 2 V or less (the dropout voltage of the LM317). When the current limit of 500 mA is exceeded, then this voltage drop will suddenly increase ... this is how it limits the current, by reducing the voltage at the load.

What kind of output capacitor are you using? I wonder if the LM317 is oscillating. Try a different size capacitor. Try adding a resistor of a few ohms in series with the capacitor. Increasing the ESR like that can improve stability when a regulator tends to oscillate due to the capacitive load.  It is also possible that the first LM317 is oscillating, so try the same tricks there. If you have access to an oscilloscope, use it to look at the voltages (use AC coupling) to see if there is oscillation. You will need a 10 MHz or better scope.

You definitely need a minimum load for the LM317, of 10 mA or more. Setting R1 to 120 Ohm satisfies this even when no other load is connected, since the Vref of 1.25 V will appear across this resistor: 1.25/120 ~= 10 mA.