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Background

I'm a beginner trying to make a linear regulator and made some decent progress on a breadboard powering from a 20V laptop charger. After battling off the typical opamp instability issues I seemed to get a pretty acceptable adjustable CC/CV output with no ripple so I decided try it out with a more realistic input source, a mains transformer.

enter image description here

The transformer has dual isolated 12VAC windings and the circuit above shows the setup I'm using. The maximum output voltage of the supply should be ~20V at 2A and so C1 and C2 are sized appropriately to give enough headroom for the output pass transistor. The diode and C3 into the linear regulator is for powering the control loop circuitry, which draws < 50mA and must stay at 28V. I can provide a diagram for the control circuit if necessary, It looks something like the circuit in another question I asked here

Problem

When the mains transformer is used all of my circuit is plagued with 100Hz ripple -- it seems to be everywhere. The output ripple is at least 50mV and increases with load up to 300mV and beyond. The ripple is not being introduced by the voltage references since they are being powered off a stable DC supply to make sure. Here's a scope trace of the AC coupled output: https://i.imgur.com/DvnZAIW.jpg

The ripple at the LM317's input is just 50mV yet the ripple at the output is 20mV, completely unexpected based on the PSRR of a chip like this. Dropout is not the issue here either. I don't think it's ripple on the supply of the op amps either which are decoupled and see a pretty smooth DC voltage. I thought that it might have something to do with bad grounding, since I saw the ripple even between different grounds which were physically far from one another. After making more of an effort to ground properly this ripple between grounds is almost gone, but not at all from the output of my circuit.

I'm not sure where to go from here as I don't know enough, I definitely don't understand where the problem lies. I'm almost certain that if powered from a smooth DC source the problems would go but that's not really the point in a regulator. I would have thought that a couple of volts of input ripple would be no problem for a circuit like this to regulate, especially with such a headroom. The problem must be from the input supply, is a better input filter (like a Pi filter) needed? Is it because I'm putting two rectified outputs in series but only smoothing the 0-36V rail? Worth noting that the 18V rail looks like a completely unfiltered rectifier output.

Any help would be greatly appreciated.

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    \$\begingroup\$ Have you really got the transformer primaries in series? That could be a significant problem but unrelated to your question. Also, your schematic doesn't appear to be complete - where are the op-amps you refer to and what does the ADJ pin connect to? \$\endgroup\$
    – Andy aka
    Aug 19 '18 at 18:57
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    \$\begingroup\$ That 36V input might be a little too low for the 28V output. \$\endgroup\$ Aug 19 '18 at 19:06
  • \$\begingroup\$ @Andyaka The transformer is single primary dual secondary I just couldn't find the appropriate symbol. Here is a picture of the control circuit: i.imgur.com/VLILrYP.png. Here V2 and VCC correspond to the power FET drain input voltage and the control circuit supply respectively. The LM317 is setup correctly (I hope) here's a diagram: i.imgur.com/kzY9IFK.png I just left out the adjust pin in the post to save time. \$\endgroup\$
    – Damo
    Aug 19 '18 at 19:22
  • \$\begingroup\$ @RobertEndl The LM317 specifies min input/output differential to be 3V so I should have plenty of room \$\endgroup\$
    – Damo
    Aug 19 '18 at 19:23
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    \$\begingroup\$ Are C1 and C2 2.2 microfarads, or 2.2 millifarads? If microfarads, they are much too small. \$\endgroup\$ Aug 19 '18 at 20:01
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1) In general you would not use 2 bridge rectifiers to make 2 x rectified 12 VAC (that's 17 V DC peak) and connect those in series. It is more common to connect the transformer windings in series (so you get 24 V AC) and feed that to the bridge rectifier.

2) I hope you're not really using a 1N4148 at the input of the LM317, a 1N4148 is a signal diode not a power diode. It cannot handle more than 100 mA DC and also will drop quite some voltage when you put 100 mA through it. Replace it with a 1N4001 and keep the current below 1 A. For more than 1 A you will need a different diode and also more than 1 A will be pushing the LM317 too hard, then you should consider a switched regulator solution.

3) the 100 Hz ripple might also be related to grounding issues. Do make sure you follow the "star ground" principle, read about that here.

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    \$\begingroup\$ Thank you for your answer. I was aware that the typical method would be to connect the windings in series but I used two rectifiers in case I wanted access to a lower voltage later on. Don't worry I'm not using that diode it was just the default on that schematic editor, sorry for any confusion. The LM317 is only outputting around 50mA so power dissipation is not an issue. As for the grounding, I've used one large continuous ground bus on perfboard which all sub-circuits are connected to, I'm not sure if that is sufficient. I could make PCB but it's a big commitment for an uncertain problem. \$\endgroup\$
    – Damo
    Aug 19 '18 at 20:09

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