Author Topic: How to lower the maximum current output of usb charger?  (Read 290 times)

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Offline Fadsa36

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How to lower the maximum current output of usb charger?
« on: June 19, 2019, 04:07:37 pm »
Good day to you, experts!
This USB charge gives a maximum output of 0.8 A. I want to lower its maximum current to 0.3 A (short circuit current) without using resistors or regulators on the USB output. Tell me what to change?
On the photo: a diode is hidden under the transformer, and a choke is between two capacitors

P.S. The question why: because the spots on the Sun. the question of security: burn the barn, burn the house. the question what I will connect: a nuclear reactor. the question of meaning: the meaning of meaninglessness
« Last Edit: June 19, 2019, 04:12:58 pm by Fadsa36 »
 

Offline LateLesley

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Re: How to lower the maximum current output of usb charger?
« Reply #1 on: June 20, 2019, 01:34:09 pm »
Unless you can find the datasheet for IC1, it's going to be very difficult. The easiest way would be to have a 2 transistor current limiter circuit added to the output, but you've stated you don't want to regulate the output. If the IC1 has a current limiting option on it, then it may be possible to mod the input side to limit the current further, but looking at it, I doubt it. It's a VERY cheap PSU, and not a very safely designed one from what I can see.
 

Offline Ian.M

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Re: How to lower the maximum current output of usb charger?
« Reply #2 on: June 20, 2019, 02:34:19 pm »
I agree with LateLesley that it looks like a fairly crappy SMPSU,  but at least there appears to be reasonable creepage and clearance distances between primary and secondary sides.  However its behaviour when overloaded may well be unsafe so relying on its current limiting and modding that to a lower limit is probably unwise.

With the above disclaimer out of the way, try replacing SMD resistor R7 (1.74 ohms) with a 4.7 ohm one.  Either it will lower the limit or if you are unlucky it may let the magic smoke out!

Caution: This part of the circuit is mains live.

Reasoning:  It appears to be between the MOSFET drain source and the negative of the primary side DC bus, and has a track going to a dedicated pin on the controller chip IC1, so is almost certainly a current sense resistor to limit the peak current in the switching transformer primary.   There doesn't appear to be any method of directly sensing the load current, so its existing current limit behaviour is probably determined by the input power.

Edit: Corrected suspected MOSFET pin.  The center one which is probably the drain goes to the transformer. b The other end one, most likely the gate, goes directly and only to IC1.
« Last Edit: June 20, 2019, 10:51:01 pm by Ian.M »
 
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Offline shakalnokturn

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Re: How to lower the maximum current output of usb charger?
« Reply #3 on: June 20, 2019, 10:34:50 pm »
I really like the way they have drawn what look like PCB spark gaps between "Live" and GND/5V... on what I'm guessing is an un-earthed device.
 


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