317 Power Supply DIY Kit voltage drop-a newbie question.

Thread Starter

Joseph Trinh

Joined Feb 9, 2019
3
Hello all, I'm new in the field. I am familiar with the basic of Ohm's law and Kirchoff's laws but most knowledge I've gathered up until now is very basic. I want to build this Power Supply kit to practice soldering and perhaps get something practically useful out of it. I've been watching the reviews and build videos on youtube and one thing pops out. It's the (significant) voltage drops at certain current drawn. I tried to reason the cause behind the phenomenon but to my best effort, I can only note it down as due to an internal resistance of the power supply. But at different current ouputs, the internal resistance seems to vary. Could anyone please help me with this question?
Here are the two videos that I've watched that mention the voltage drops:


For the second video, the voltage drops are mentioned at around 3:00-3:05. The first video maker said that by removing the IC and some other components, the power supply has a higher current output and also, in the comment section, there are suggestions for an upgrade of transformer and diodes for better result output. Do you think it's a good idea as a project for a beginner like me?
Thank you for your time and answers. I hope this is not a silly question.
 

sghioto

Joined Dec 31, 2017
1,039
After watching the second video it would seem that the power supply is using the wrong transformer or a transformer only capable of delivering a few hundred millamps. What is important is the supply voltage at the input of the LM317 regulator. With the correct components it should work reliably. From the photo the heatsink on the regulator looks small which may confirm the low current. Actually after reading customer questions apparently the supply is only rated at 200 ma.
SG
 
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Thread Starter

Joseph Trinh

Joined Feb 9, 2019
3
After watching the second video it would seem that the power supply is using the wrong transformer or a transformer only capable of delivering a few hundred millamps. What is important is the supply voltage at the input of the LM317 regulator. With the correct components it should work reliably. From the photo the heatsink on the regulator looks small which may confirm the low current. Actually after reading customer questions apparently the supply is only rated at 200 ma.
SG
Thank you sghioto. In the 2nd video, the youtuber said that he managed to go up to 300 mA but the voltage dropped from 5V to 3.2V. But, say, in general, if we treat the whole thing as black box, is it right to say the internal resistance of any PSU is the cause for this kind of drop when the current output exceeds the rated one? Also, in this particular case and according to the 2nd video's test, it appears to me that whatever causes the voltage drop is not constant but changes proportionally with the current output.

I manage to find the schematic of this PSU.
'https://img.banggood.com/file/products/20170616021631SKU215283.pdf
 

sghioto

Joined Dec 31, 2017
1,039
I think the one problem with this power supply is the power transformer. To maintain voltage regulation the supply voltage must be at least 3 volts higher then the output voltage of the regulator. I suspect this voltage is dropping when the load increases. In the videos no one is measuring the input voltage to the regulator to confirm.
Another problem, the LM317 is not genuine. In this case no telling what the regulated output truly is.
SG
 

Thread Starter

Joseph Trinh

Joined Feb 9, 2019
3
I see. It's the problem with these kits, it's hard to find the correct datasheet so in my limited experience when experimenting with no name transistors, I had to measure a lot of stuff just to know the vital information to compare with the closest datasheet (which I suspect the no name tries to model after/copy). SG, thank you once again for your responses. I'm fairly new in this field and started at rather late age, at 30, I aim to start electronics as a hobby and hopefully I could be able to design things later on. So far, the experience is pleasurable yet confusing at times as I often found myself wanting a direction of where to go, what to learn and how to do it.
 
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