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Forum Index : Microcontroller and PC projects : USB Charger
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palcal Guru  Joined: 12/10/2011 Location: AustraliaPosts: 1873 |
Hi all,  To save me breaking it open, does anyone know what electronics are inside. Are they safe to use with a Micromite or Picomite ? Edited 2023-10-09 07:19 by palcal "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6798 |
Your guess is as good as mine. If it's a decent make then it's probably ok. If it's a super-cheap Chinese thing then I'd be very wary. The circuit is a switching converter. The possible danger arises in the distance between the tracks on the PCB and in the quality of the HF isolating transformer used. Some are fine, others are next to lethal. Try plugging it in and measuring the voltage between the USB shell and mains earth. Try it with and without a shell=earth load of about 1K. If you get any voltage in the latter case then there's almost certainly too much leakage to consider it safe. You can get away with a tiny bit of leakage for a charger but not if you are going to keep putting your fingers on it. You could try linking negative output to mains earth but that might stress the transformer. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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stanleyella Guru Joined: 25/06/2022 Location: United KingdomPosts: 2129 |
The answer will be here somewhere. https://www.youtube.com/@bigclivedotcom |
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| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 1873 |
USB shell to ground measures 100 volts, with the 1K resistor I get zero. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
Almost all 2 pin mains devices have some deliberate leakage between mains and output as a safety measure. It is usually via a high value resistor though some use a low value capacitor as well. The intention is to blead away any electrostatic charge accumulating on the output (which can be many thousands of volts) that could cause the transformer insulation to break down. If correctly designed they must accept one output terminal being tied to earth. |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6798 |
I wouldn't get too upset if there's no voltage with a 1K resistor. You might like to connect the negative output to mains earth just to be safe. The 100V that you see has virtually no current capability. It's not easy to say whether these things are "safe" without ripping them to bits. They don't work at all then. :) Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 1873 |
I have used them myself with no problems for years but I have made something to give to someone else and I just want to be sure. If I fry myself,OK, someone else a slight problem. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6099 |
A lot of no-name mains equipment doesn't have very good clearances. I would rather see a brand-name charger, especially for someone else. Jim VK7JH MMedit MMBasic Help |
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| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 1873 |
Thanks Jim, I have ordered one. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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| DaveJacko Regular Member  Joined: 25/07/2019 Location: United KingdomPosts: 76 |
I've PAT tested a million devices in my life, or at least it seems like it. part of this involves, if my memory serves, putting 1kv between live+neutral and earth. would it be useful to put 1kv between the mains side and the 5v side ? (and see what pops) It's probably best to just buy a branded part, for plenty of other reasons.. |
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| phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
The 1000V DC test is for 415V equipment. For devices rated at 250V or less the test voltage is 500V. The logic for not using higher test voltages is they may initiate processes that eventually lead to insulation breakdown, making the device unsafe. |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6798 |
1kV might be pushing it if there's a little cap in there, but the transformer should be ok up to over 400V. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4243 |
Seeing the picture, it is a 2 pin mains connection. This means it does not rely on safety ground pin in the socket. This makes it a class II device. For 250Vac class the isolation should (between mains and 5V side) be tested at 4240V (4.2kV) according to CE regulations. Whether this device meets that test voltage, I cannot tell. But Australian regulations may be different from European CE. Volhout P.S. if it had a safety ground connection (3 pin mains connector) the isolation test voltage would be 2120V (2kV). The 500V phil is referring to is the dielectric test voltage. This is the AC voltage that is used to measure the isolation resistance (in most cases more than 10 meg ohm, unless there is a waiver for this circuit). the 2120V (dc) or 4240V (dc) @ 1 minute, is to test potential flash over of the isolation barrier. PicomiteVGA PETSCII ROBOTS |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6798 |
I think that regardless of Class I or Class II isolation, the test would still be to mains earth on the LV side. The difference is that in Class II there should be an additional insulating barrier in case the primary one should fail. The Class II charger shown *should* have basic (wire insulation) and reinforced (additional insulation layer) insulation on the transformer with an opto-isolator or additional insulated winding for feedback. Additionally the PCB should be able to withstand the isolation voltage. UL regulations state this as twice the working voltage + 1000 volts for 1 second. The UK says that the resistance should be greater than 1M when measured using 500V AC between mains input and LV output. Edited 2023-10-10 18:42 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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