This 5V power supply is giving me a headache - Page 1
Forum section: electronics / beginners

author: nyo
author_rank: Contributor
timestamp: July 09, 2011, 11:56:21 pm
content: Hi,Can anyone help me make sense of this circuit?It's a 120V to 5V usb charger. I'm trying to understand how this circuit with does not use a rectifier or regulator can use such a small transformer and be able to change from 120V to 5V.What's the theory behind this and would anyone be able to draw me a simple schematic of it?Help would be really appreciated, my head hurts trying to figure this out.Here are some pics

author: Unknown
author_rank: Unknown
timestamp: July 10, 2011, 12:46:24 am
content: I don't see the issue. D2 appears to be a half-wave rectifier for the mains, D3(?) behind the transformer the rectifier for the secondary side. The zener on the secondary side provides the regulation, and controls the transistors on the primary side via the opto-coupler. These probably oscillate at a frequency that depends on the current through the opto. Looking up flyback converter might give you the basic idea.

author: nyo
author_rank: Contributor
timestamp: July 10, 2011, 03:28:53 am
content: Interesting, that helps, it's all on how you look at it and I was looking from the wrong angle.ThanksI plan on posting my final conclusions here when I understand the circuit 100%.Here is a schematic I did of it...

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2011, 09:44:54 pm
content: Alm is correct.It will be configured so the circuit self oscillates and the opto-coupler either stops or reduces the duty cycle of the oscillator.The circuit doesn't look quite right to me. I suggest you check it.

author: nyo
author_rank: Contributor
timestamp: July 11, 2011, 09:52:35 pm
content: I know, something is off here. Suggestions?

author: Zero999
author_rank: Super Contributor
timestamp: July 14, 2011, 04:41:17 pm
content: Yes, that you look at the PCB and component datasheets again.

author: Zero999
author_rank: Super Contributor
timestamp: July 15, 2011, 07:35:29 pm
content: No problem.I mean seriously the circuit obviously won't work: look at the 9014, notice anything?Some of the component values also look wrong, again check the resistor colour codes or unsolder one of the legs and measure with a meter.

author: nyo
author_rank: Contributor
timestamp: July 15, 2011, 09:45:59 pm
content: Well Hero999, thank you for not giving up on me. I will recheck the datasheets. I believe the component values are correct, I actually de-soldered all the components and measure all the resistors with lcr meter. And I was also able to read all the values for diodes and caps. The transformer is still unknown, no markings. I did apply an ac voltage to it to figure out the primary coil and I checked the impedance. That's why this circuit is giving me a headache, looks simple but can't make sense of it. I even probed it all with oscilloscope very carefully not to ground the high voltages with my probe gnd. Anyone else here sees anything wrong? Or suggestions?

author: Zero999
author_rank: Super Contributor
timestamp: July 16, 2011, 08:48:26 am
content: Well you've obviously not checked it very thoroughly, to point out the obvious, the S9014 is connected upside down.

author: nyo
author_rank: Contributor
timestamp: July 23, 2011, 04:36:55 pm
content: Hero999, you make it sound like I'm an idiot, the circuit is obviously populated with cheap Chinese components, why don't you point me to the correct datasheet?For example, look at the 2 different 13001 datasheets

author: Unknown
author_rank: Unknown
timestamp: July 23, 2011, 05:53:50 pm
content: Figuring out the pinout of a transistor should be trivial if you've desoldered the component and it's not released its magic smoke, just use the diode check to measure from 'base' to collector and emitter, both should behave like a diode, with the base-emitter forward voltage drop slightly higher than the base-collector drop.

author: Zero999
author_rank: Super Contributor
timestamp: July 24, 2011, 06:18:54 pm
content: Quote from: nyo on July 23, 2011, 04:36:55 pmYou're right, it's full of cheap Chinese components which will be hard to get datasheets for.No you're not stupid, you just have more learning to do. The NPN transistor is obviousness the wrong way round because the emitter is connected to the positive, rather than the negative. It could also be a PNP transistor.