Below is flow diagram of my pcb power side, the issue is when relay turn on a 12V 30 watt heater element the voltage drops (or swing) and stay down until load disables. This drop cause issue on my amplifier circuit and I want to know why other regulator dont regulate the voltage back to normal. (I have tried external AC/DC converter with Rigol DL3000 DC load and voltage drops too)

The most interesting is this does happen some time and some time by just turning on and off unit it goes away but mostly is over there. Note: 5V buck regulator below is typo enter image description here

Below graph shows' the input supply when load turns on and off enter image description here

Below graph is 12V buck boost regulator (Different recording) enter image description here

External Supply : RACM230-24SG Output 24V 9.58A 80 ~ 264 VAC, 120 ~ 370 VDC Input

Let me know if I you need more info

  • \$\begingroup\$ So, what currents are all your supplies designed to supply? That's the one most important info here, and I think you forgot to tell us. \$\endgroup\$
    – mmmm
    Commented Feb 26, 2021 at 1:23
  • \$\begingroup\$ Thanks for the suggestion, I added the external AC dc convertor info \$\endgroup\$
    – Shahreza
    Commented Feb 26, 2021 at 1:26
  • \$\begingroup\$ The data sheet only guarantees +/- 1.0% output accuracy, and load regulation of 0.5%. 0.08V is 0.3% of 24V. \$\endgroup\$
    – nobody
    Commented Feb 26, 2021 at 1:27
  • 1
    \$\begingroup\$ Why are you using a buck-boost regulator for your 5V supply if the input is already regulated 12 V? buck-boost regulators are less efficient than buck regulators and the boost function doesn't seem to serve any purpose here. \$\endgroup\$
    – Hearth
    Commented Feb 26, 2021 at 1:27
  • \$\begingroup\$ @Hearth my mistake is just buck regulator \$\endgroup\$
    – Shahreza
    Commented Feb 26, 2021 at 1:28

1 Answer 1


Your first graph shows a ~70 mV change on a 24 V supply. That's 0.3% variation. You don't have a vertical scale on your scope plot of the 12V regulator, but if the magnitude of the changes is similar, there's nothing to worry about. Neither your voltage reference nor your amplifier should be bothered by small changes in the supply voltage. Is this a noise issue?

  1. Do you have capacitors on the voltage reference and amplifier's supply rails?
  2. Are you able to power the 5V regulator directly from the 24V supply? The heater probably draws a lot of current, and switching that will add noise to your power rails.
  3. What are the supply currents?
  • \$\begingroup\$ I agree 0.3% change on 24v is okay my problem this will effect my voltage reference voltage which connect to virtual gnd and cause swing in opamp output. I have capacitor on both input and output of each regulator , voltage reference, and opamp (I even tried 10uf on voltage reference) \$\endgroup\$
    – Shahreza
    Commented Feb 26, 2021 at 1:56
  • \$\begingroup\$ I forgot to add the external supply is rate for 24V 9A \$\endgroup\$
    – Shahreza
    Commented Feb 26, 2021 at 2:04
  • \$\begingroup\$ We're not asking for the current rating, we're asking how much current they're actually drawing. What's the 12V regulator current when the heater is on and when it's off? \$\endgroup\$
    – Adam Haun
    Commented Feb 26, 2021 at 5:00
  • \$\begingroup\$ Oh, when is on 2.5A ar 12v \$\endgroup\$
    – Shahreza
    Commented Feb 26, 2021 at 5:01
  • \$\begingroup\$ Honestly, it sounds like the main problem is that you're running the 5V regulator off of the 12V regulator's output. If you do 24V -> 5V, then you'll have a good ground, right? \$\endgroup\$
    – Adam Haun
    Commented Feb 26, 2021 at 5:02

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