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LM317: 220VAC to 0-30V DC LM317 CIRCUIT

Part Number: LM317

Hello,

I want to design a circuit that can take 220V mains voltage and can be adjusted with a potentiometer and outputs 1-27V. For this, I first converted the mains voltage to 24V AC voltage with the help of transformer. Then, I rectified the 24VAC voltage with the help of a bridge diode and obtained a DC voltage of about 33V. I gave this DC voltage to 1uF ceramic capacitance and then to lm317. Finally, by connecting a 1uF electrolyte capacitance and a potentiometer to the output of the lm317, I have obtained a circuit that gives a voltage of about 1 to 28V. There was no problem in the tests I made without connecting my circuit to the load, but when I connected it to the load, I realized that I could not draw even 0.1A.

What are your solutions for this problem?

Kind regards
Can

  • Hi Can,

    Thanks for reaching out to us about this. To help you solve your problem we must first understand where the solution is breaking down. There are two main places where I can see your circuit failing. The first would be in the transformer we would need to check and make sure that the transformer is capable of sourcing enough current support the power supply output. The second place I can see being an issue is the LM317 especially on the lower output voltages you will be trying to dissipated 30+ Watts of power in the LDO when you run it at max current, so I would fully expect it to be in thermal shutdown or thermal limit at that point. Can you help us to measure the voltage at certain points in your circuit to see where it is breaking down? The key spots would be the 24V side of the transformer, the input to the LDO, and the output to the LDO.

    Regards,

    Mark

  • Hi Mark,

    We have an electronic load.

    When we connect the system to it and draw a constant 0.1A, there is a great voltage drop.

    The output voltage I get from the 24V side of the transformer is 25.58.

    The input voltage to the LDO is 28V without connecting it to the load, and the output voltage is at the values I want, but when I connect it to the load, I observe a big drop in the input voltage. Because of this, I cannot give the required voltage to the lm317 and therefore I cannot get a proper output.

  • Hi Can,

    My recommendation would be to check and make sure that the transformer can supply the desired output power then. It sounds like you are not using a strong enough transformer to support the desired output current.

    Regards,

    Mark

  • Hi Mark,

    I solved the problem by increasing the value of the input capacitance and using a 35W transformer, but the same problem persisted when I used a 10W transformer. Do you think this power difference in the transformer is a factor?

    King Regards,

    Can

  • Hi Can,

    It has been a while since i have worked on any AC applications, but my guess is that your power factor is bad for the circuit, so even though you have a theoretical 10W of apparent power if you have a power factor of 1, you are probably "burning" most of that power a reactive (imaginary) power that is being lost due to poor power factor correction. Stepping up to the bigger transformer will allow you to overcome that just due to the fact that if the PF remains the same it will still be capable of delivering more power to the output in terms of apparent power.

    Hope this helps!

    Mark