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How can input voltage fluctuations impact the output of a linear power supply?

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Hatsuwr:
Some background - I was charging some 18650s with a cheap linear power supply. Its displayed voltage isn't the most accurate, so I set the voltage limit with my multimeter. The output was steady to 1 mV. During the charging there was a brief power outage, no more than a few seconds. Looks like some nearby houses might still be without power though. When checking on the power supply, I noticed it was slightly above the limit I set.

Checking the output voltage with my multimeter and the line voltage coming in to my house showed fluctuations on both that seem to be correlated.

Hooked up my oscilloscope to the power supply to see this. Sorry for the picture of the screen.



My basic understanding of how these regulate voltage is that there is a voltage reference of some sort that is modulated by a potentiometer, and the output feeds into an op amp that drives a transistor to provide the resistance needed to achieve the desired output voltage. I can't figure out what in this chain would be sensitive to some minor fluctuations in the incoming line voltage.

aliarifat794:
Use your oscilloscope to monitor the AC input voltage to see the extent of the fluctuations. If the fluctuations are significant, you may need  an uninterruptible power supply (UPS) to stabilize the input voltage.

mikerj:
Regulators have two important performance characteristics, line regulation and load regulation.  Line regulation is how much the output voltage changes when the input voltage changes, load regulation is how much the output voltage changes with changes in output current.  Many things could cause poor line regulation, an obvious one would be a mediocre voltage reference such as a basic zener being driven by a resistor from the unregulated voltage rail.

tooki:

--- Quote from: Hatsuwr on May 18, 2024, 07:35:47 pm ---Some background - I was charging some 18650s with a cheap linear power supply.

--- End quote ---
Don’t do this. It’s unsafe, and it’s also easy to fry your power supply.

As for the rest: please provide more info on the kind of power supply (model, etc) it is and how you have it set up.

Hatsuwr:

--- Quote from: mikerj on May 19, 2024, 10:15:11 am ---Regulators have two important performance characteristics, line regulation and load regulation.  Line regulation is how much the output voltage changes when the input voltage changes, load regulation is how much the output voltage changes with changes in output current.  Many things could cause poor line regulation, an obvious one would be a mediocre voltage reference such as a basic zener being driven by a resistor from the unregulated voltage rail.

--- End quote ---

My initial thoughts were toward the voltage reference as well. Been working on trying to trace things out, but this thing is confusing me... There is a quad op amp with two of its outputs connected to the transistor driving the BJTs that drop the output voltage. I'm guessing those two outputs are one for the voltage limit and one for the current limit, but still trying to figure it out. Excuse the terrible tracing, made it before thinking about sharing it:




--- Quote from: tooki on May 19, 2024, 11:39:00 am ---Don’t do this. It’s unsafe, and it’s also easy to fry your power supply.
As for the rest: please provide more info on the kind of power supply (model, etc) it is and how you have it set up.

--- End quote ---

I'm only doing 500 mA and 4.1 V, setup with each of those as limits and terminating at 50 mA. What are the safety and equipment concerns?
The power supply is a Lavolta BPS-305, which is a rebranded Longwei PS-303D-M that is sold under various other names as far as I can tell.


--- Quote from: aliarifat794 on May 19, 2024, 08:18:27 am ---Use your oscilloscope to monitor the AC input voltage to see the extent of the fluctuations. If the fluctuations are significant, you may need  an uninterruptible power supply (UPS) to stabilize the input voltage.

--- End quote ---

Ok, I have a couple screenshots here. The upper one is for about 20 minutes and captures at least one very noisy event that happens occasionally. I haven't been able to determine a pattern to these, or if they are at all consistent with one another. The lower is <2 minutes and doesn't have one of these events.

RMS variation seems to be around 1.6 V. I'm not sure if this is the right way to calculate it, but 1.6 V * 4.1 V / 123 V = 53 mV, which is just a bit higher than the output voltage variation I observed when I had the supply set to 4.1 V.

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