Cheap Power Supplies - Page 1
Forum section: electronics / general

author: Bryan
author_rank: Frequent Contributor
timestamp: October 09, 2016, 09:59:06 am
content: Shocking to say the least.https://youtu.be/g-NQSNoWlZ4

author: bktemp
author_rank: Super Contributor
timestamp: October 09, 2016, 11:10:17 am
content: It is not only cheap power supplies. Many notebook power supplies have this problem.Probably hundreds or even thousands of Arduinos are killed by connection some grounded cables into Arduinos connected to notebooks using floating power supplies.Using notebooks with floating power supplies are a pain for working with microcontrollers, because you can't connect/disconnect anything while the power supply is plugged in without risking damage unless you keep at least one ground connection.Even if you connect GND first, the spark generated has enough high frequency energy to crash the USB connection of most programmers/debuggers.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 09, 2016, 11:35:28 am
content: Having a floating voltage of about half the mains voltage is normal - there has to be a capacitive coupling (usually a class Y capacitor something like 5 nF). So if they are not using the PE pin for this the second best you can get is half the voltage.However, having the heat-sink right across the isolation barrier is a major safety violation. So this particular model is really unsafe and should be recalled and all scraped. So not using it is right. At least in Europe you should be able to get a refund from where you bought it.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: October 09, 2016, 12:35:27 pm
content: So lets see, the center connector on the heat sink was somehow not referencing the same plain as the other heat sink connections.Why not buzz this out and see where the center heat sink pin was different than the two outer pins?I don't see why crossing over the isolation barrier is the problem. There is a design problem but what was it?nico

author: nctnico
author_rank: Super Contributor
timestamp: October 09, 2016, 12:46:23 pm
content: Quote from: Kleinstein on October 09, 2016, 11:35:28 amI strongly doubt that. I've seen lots of power supplies made in a similar way. As long as the heatsink is isolated from the mains parts there is not problem at all.There is nothing really dangerous about this power supply. The person who made the video just discovered the mains input filter caps! Woohooo!!

author: bktemp
author_rank: Super Contributor
timestamp: October 09, 2016, 01:14:34 pm
content: Quote from: nctnico on October 09, 2016, 12:46:23 pmI rewatched the whole video, and it looks like the heatsink is connected both at primary side and secondary side the the negative voltage, so it is connected directly to mains and the output!Maybe they left the heatsink deliberately floating and some guy thought it would be a good idea to mount it to the copper to make it more rugged?It is hard to believe seeing such a stupid mistake in an otherwise fine power supply.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 09, 2016, 01:16:11 pm
content: Quote from: nctnico on October 09, 2016, 12:46:23 pmI'd bet the schematic uses the same ground symbol for both sides of the supply. Seems if the metal plate had an unconnected via on the 12V side all would be well.If they are using a CAD system as opposed to a hand drawn layout, it would want to connect the pins to the same net if it uses the same symbol.

author: dannyf
author_rank: Super Contributor
timestamp: October 09, 2016, 03:18:47 pm
content: QuoteWhat else do you expect?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 09, 2016, 03:50:32 pm
content: Even a cheap supply should be safe. This unit has more than the typical RF suppression cap causing a few µA of leakage. In Europe this supply should get an entry in the rapex list, if they try to bring it here. I guess the US and Canadian authorities should have a similar list of unsafe items.They had the heat sink connected to the output side. So the only isolation was that between the heat sink and power device on the input side. A pice of mica could work at 110 V, but it is not reliable and thus not safe. In this particular unit it has failed. Removing the link on the output side just added another such unreliable barrier - possibly weaker than the one that had just failed. As removing the link apparently fixed the isolation failure the current path is relatively clear.For me the only way to have a common heat sink for both sides would be having the heat sink isolated on both sides and connected to PE. Even than the isolation on the primary need some extra care - not just any cheap ass mica or pad. Also distances could get a little close at the screw, at least for 230 V operation, even if only class I.

author: dannyf
author_rank: Super Contributor
timestamp: October 09, 2016, 03:53:33 pm
content: QuoteI should be filthy rich and you should have billions in your bank accounts...Unfortunately, reality is different.Quotewhy?Darwinism works far more efficiently and effectively.

author: vodka
author_rank: Frequent Contributor
timestamp: October 09, 2016, 05:25:22 pm
content: Quote from: bktemp on October 09, 2016, 01:14:34 pmThat is hard to believe but certain and i discovered this for the bads(i discharged a capacitor of 400v by the arm) with the original laptop charged(plugged-off).And with a PC power supply(plugged-on) i touched the heatsink with the oscilloscope tip and it jumped the circuit breaker from my home.So , i have by costum i never don't touch the heatsink(booby traps chinese).

author: nctnico
author_rank: Super Contributor
timestamp: October 09, 2016, 05:35:54 pm
content: Quote from: AcHmed99 on October 09, 2016, 01:20:56 pmI have watched parts of the video more closely and I have to take some back. It seems to me the heatsink connects the mains part to the output part which ofcourse is dangerous (but doesn't get noticed by the person who made the video).

author: DimitriP
author_rank: Super Contributor
timestamp: October 09, 2016, 08:39:07 pm
content: For about ten bucks, anyone can try this at home:https://www.amazon.com/SUPERNIGHT-100-240V-5-5x2-1mm-Converter-Adapter/dp/B00K6ARFTS/ref=cm_cr_arp_d_product_top?ie=UTF8

author: Kleinstein
author_rank: Super Contributor
timestamp: October 09, 2016, 09:16:34 pm
content: I rewatched the video - it's even more shocking than I thought: The heat sink has soldered contacts on both the primary and secondary sides. So absolutely no isolation - not by failure of a weak isolation but by design. You should even be able to check this with the diode test - there is not much more than the input filter and a diode in between the mains side and the output.How could quality control ever miss this ? There is not even a chance to have it plugged in so that only neutral is connected to the output.So the OP can be lucky to have only 110 Vand should invest in a working RCD.