USB Power Supply Transformer Inquiry

Thread Starter

zyf444e668

Joined May 5, 2018
3
I bought this cheap 3 USB Adapter on Amazon. Many folks online have reviewed the item and found the Transformer to be dodgy.
After a while i smelled the transformer burning and it soon was outputting 15V on the 3 usb ports!

I bought these one usb black adapters that are UL Listed and safe. Only thing is it can only take up 0.5A which the dodgy one took a bit more than that.
I looked at the circuit boards and found similar layouts between the two transformers. The outputs are the same too between the two. The corresponding output pin on one is negative like the other transformer.
With the pictures of both circuit boards A & B are the outputs of the transformer. A is negative, B is positive. So both are the same. They both have the same two input pins. The only thing different i see are the two middle pins on the input of both of the transformers.
Would swapping the transformer work or should i find a different transformer? I understand if i use the transformer from the non-dodgy one in the dodgy one it won't be able to handle a lot of Amps.
 

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Yaakov

Joined Jan 27, 2019
3,464
You keep saying "transformer". It wold be helpful to see the component side of the PCB to understand what you are talking about. Transformers are AC devices and don't have positive and negative terminals.
 

Thread Starter

zyf444e668

Joined May 5, 2018
3
You keep saying "transformer". It wold be helpful to see the component side of the PCB to understand what you are talking about. Transformers are AC devices and don't have positive and negative terminals.
The transformer on both are provided below. On the dodgy one it's the red component.
On the other it's the greenish blue component.
 

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Yaakov

Joined Jan 27, 2019
3,464
The transformer on both are provided below. On the dodgy one it's the red component.
On the other it's the greenish blue component.
Hate to make you do it, but could you retake those, with better light and less movement? (Better light will probably fixed the motion blur by reducing shutter time). I just can't see things very well with those.
 

Thread Starter

zyf444e668

Joined May 5, 2018
3
Hate to make you do it, but could you retake those, with better light and less movement? (Better light will probably fixed the motion blur by reducing shutter time). I just can't see things very well with those.
I am trying and did try to get better light on the pictures. Kinda hard when the camera i am using is a Kodak EasyShare C138. Only camera i can use. I tried multiple times to get a better picture of the boards. If they are not helpful then i am unsure what to try.
 

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Yaakov

Joined Jan 27, 2019
3,464
Those appear to be switching supplies and the large components inductors. If you desolder the inductors and measure the resistance of the coils and have a match, they might work interchangeably. You don't have an LCR meter, do you?

Also, you are dealing with mains voltage and really, really need to be careful. Both those boards look really poorly made and don't have anti tracking slots for isolation. It is possible that even 100% "working" from the factory they are poorly isolated.

It might not be worth messing with them given the potential risk, but if you do, make sure you never com in contact with the mains supply and don't power them up without the case on.
 

DickCappels

Joined Aug 21, 2008
7,690
Often these transformers don't have sufficient insulation between the primary and secondary, relying on the enamel insulation of the wires to keep them separate.

The number of people electrocuted while using their cell phones in this part of the world was disturbing (I was going to say shocking) often the chargers are made in a place where the safety marks are fake and there is no enforcement. My working rule is to never touch any conductive part of anything connected to the output of the chargers while it is plugged in. I also check leakage current to ground with a 1500 ohm resistor as the shunt.
 

Delta Prime

Joined Nov 15, 2019
1,147
I also check leakage current to ground with a 1500 ohm resistor as the shunt
To elaborate on this good safety practice and wish to apply it in your next project, let me break it down for you .The ANSI, UL, CSA and IEC circuits are identical all four give the same measured value.Resistor tolerance creates almost the same percentage error in the measured value.
Different standards specify different values for the current-sampling resistor in the current-measuring circuit for electric shock current and leakage current
Current-sampling resistor
500 ohms
Standard ,UL 1270, Paragraph 19.1

1000 ohms UL 544 Paragraph 27.13
1500 ohms UL 478 Paragraph 28A.6
2000 ohms UL 1459 Paragraph 48.6
Again, the ANSI,UL,CSA and IEC circuits are identical.
leakage current.png
For kicks and giggles we are measuring 0.5 milliampere of leakage current from a 120-volt product. To have leakage current we must have a circuit consisting of a voltage source, a series impedance, the current-sampling resistor (1500 ohms), and a return path (ground)
We know E (120 volts) and I (0.5 mA). Using Ohm’s law, the total resistance in the circuit, including the 1500-ohm current-sampling resistor is:
R=E÷I
R=120÷0.0005
R=240,000 ohms
Now we have to subtract the 1500-ohm current-sampling resistor, giving us a source resistance of 238.5 kilohms. Using this value, we can calculate the current when using other values of a current-sampling resistor and we can repeat the calculations for a 240-volt source
and, we can repeat the calculations for 3.5 milliamperes and 5.0 milliamperes leakage current because the current is nearly independent of the load which, in this case, is the current-sampling resistor.
 
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Delta Prime

Joined Nov 15, 2019
1,147
Both are shoddy designs and there is no way it would pass real UL or be safe. Regular chinese junk.
Agree with you 95% ,5% is good junk when testing anything I always meet or exceed documented standards for my safety purposes so I keep one hand in my pocket and a welding glove on the other. :p
 
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MisterBill2

Joined Jan 23, 2018
9,235
I looked at the circuit board patterns and I do not see any similarities. Probably both are actually low current devices but one has a much bigger LIE about it's ratings. While the one does seem to have a wide band between the line side and the LV side, I would classify both of them as junk.
 
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