The problem starts with my 3D printer which is powered by a 12V SMPS. When it drives a heavy load such as heated base and hot end, it causes a ripple on the 12V rail. Also it affects the 5V rail with the same ripple as the image shows.

Note: My printer uses Ramp 1.4 with Arduino Mega2560


Chanel A is 5V from Arduino Mega, Chanel B is 12V from PSU.

For ramp1.4 , For Arduino Mega 2560

What I want to know is:

  1. How does this happen? As I know, voltage regulator should regulate all above 5V+(dropout voltage), so it should be a flat 5V line.
  2. How to fix or prevent this? (Do I need to add some component filter or change regulator ICs?)
  • 3
    \$\begingroup\$ We'll probably need at least a schematic and links to the part specifications to get started. \$\endgroup\$ Oct 19, 2020 at 19:39
  • 1
    \$\begingroup\$ If the 5V rail is the same ... they have the ground rail in common. Measure voltages at all ends of the ground wires. If some of them have this issue too, learn how to star ground a system. \$\endgroup\$
    – user16324
    Oct 19, 2020 at 19:44
  • \$\begingroup\$ @BrianDrummond It just Arduino board and Ramp shield with 1 power pair from PSU what I gonna do? \$\endgroup\$
    – M lab
    Oct 19, 2020 at 19:47
  • \$\begingroup\$ @CristobolPolychronopolis I added link to schematic. \$\endgroup\$
    – M lab
    Oct 19, 2020 at 19:48
  • 2
    \$\begingroup\$ You need to be extremely careful when probing noise and ripple as the unwanted signal may be on the ground because of where you are probing, not on the supply rail itself. \$\endgroup\$ Oct 19, 2020 at 20:04

3 Answers 3

  1. In the datasheet of a linear regulator there should be a parameter called "power supply rejection ratio" (PSRR), which describes how much the ripples on the input voltage get suppressed. In fact this parameter is frequency dependent, wherefore there should be a graph about the course of PSRR over the frequency of the input noise.
    Example: Linear regulator with 5 V output voltage and 60 dB PSRR @ 1 kHz. Input voltage around 7V but there is an oscillation of 1 Vp-p and 1 kHz frequency on the input signal. The output voltage will be around 5 V with the oscillation suppressed by 60 dB, so Vp-p of the oscillation would be around 1 mV.

  2. Without further information about your circuit and components I can't really say much about that. A simple point would be to get a linear regulator with a higher PSRR, but I don't know if that is possible for your application.

  • \$\begingroup\$ As shown in Arduibo Mega 2560. It use LD1117 5.0V LDO regulator. As far as I know, LDO is good for ripple rejection. but as scope result some thing not just right. \$\endgroup\$
    – M lab
    Oct 20, 2020 at 17:23

You need to be extremely careful when probing noise and ripple as the unwanted signal may be on the ground because of where you are probing, not on the supply rail itself.

It is preferable to use the probe in the x1 setting not x10. And select a ground point very close to the supply you are measuring. The spring ground attachment you probably got with your probe is one way of helping that.

Is it causing a problem? If not it may not be real.

The scope trace shows about 200mV p-p ripple. That shouldn't affect the digital circuitry.

  • \$\begingroup\$ It cleary see on LCD light which blinking accordingly to ripple. I'm quite confident this is real. \$\endgroup\$
    – M lab
    Oct 19, 2020 at 20:17
  • 1
    \$\begingroup\$ Where is the LED driven from? Is the ripple causing any incorrect operation? \$\endgroup\$ Oct 19, 2020 at 20:36
  • \$\begingroup\$ Just connect with 5V via resistor. Seem like It randomly reset the MCU when low voltage occur. \$\endgroup\$
    – M lab
    Oct 19, 2020 at 20:45
  • 1
    \$\begingroup\$ Can you get a scope trace with DC coupling to see the actual voltage excursion of the 5v supply. It shouldn't reset as a result of 200mV ripple. \$\endgroup\$ Oct 19, 2020 at 21:07
  • \$\begingroup\$ Yes it not but my assumption is when heavy load occur, the ripple cause voltage to go down below cut off point. So I manage to get rid of it first. \$\endgroup\$
    – M lab
    Oct 19, 2020 at 21:28

Let us talk heating, of the silicon die/package/PCB.

Thermal time constant of cubic meter of silicon is 11,400 seconds.

Thermal time constant of 0.1 meter cube is 100X faster, at 114 seconds.

Thermal time constant of 1cm cube is 100X faster, at 1.14 seconds.

Thermal time constant of 1mm cube (about the size of a silicon die) is 100X faster, at 0.0114 seconds, or 11.4 milliseconds.

If you have a 2mm size die, you get 4X the time constant or 45.6 milliSeconds.

Or if you examine the mounting flag/paddle inside the plastic package, you'll have about that size.

Notably, copper has nearly the same ( 9,600 versus 11,400) seconds/meter^3 as silicon.

  • \$\begingroup\$ I totally don't understand what you talking about. can you explain a little more? \$\endgroup\$
    – M lab
    Oct 20, 2020 at 17:20

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