/ News & Press / Video / Teardown of a cheap UK style USB power supply.
Teardown of a cheap UK style USB power supply.
WEBVTT Kind: captions Language: en
00:00:00.890 this is a typical Chinese Apple knockoff 00:00:05.86000:00:05.870 USB charger it's the scrip in version 00:00:09.29000:00:09.300 for the UK and it came from a company 00:00:11.72000:00:11.730 called banggood online who are just an 00:00:13.91000:00:13.920 absolute delight for a Chinese stuff now 00:00:17.33000:00:17.340 this one cost a staggering $2 seriously 00:00:21.17000:00:21.180 two dollars for a USB supply and and I 00:00:25.22000:00:25.230 got one just to take two bits now it 00:00:28.04000:00:28.050 arrived very quickly arrived within 10 00:00:29.87000:00:29.880 days and it came so I could fit it into 00:00:32.38900:00:32.399 our square pin sockets it came with a 00:00:35.17000:00:35.180 square pin adapter no I'm not sure why 00:00:41.03000:00:41.040 they did that but anyway they did which 00:00:43.52000:00:43.530 was very nice of them so this thing is 00:00:45.83000:00:45.840 rated 5 volts at 1 amp and I've well I 00:00:49.58000:00:49.590 mean it's theoretically rated 5 volts at 00:00:52.06900:00:52.079 1 amp and I tried testing it I tried 00:00:54.50000:00:54.510 test it with a very rough setup of this 00:00:57.56000:00:57.570 and you know first problem this is a 00:01:01.72900:01:01.739 digital readout in the front so I 00:01:03.31900:01:03.329 thought like most USB supplies they 00:01:06.23000:01:06.240 usually do point in the same direction 00:01:08.21000:01:08.220 so I rammed at him and it appeared to be 00:01:12.28900:01:12.299 dead I didn't actually realize that this 00:01:14.09000:01:14.100 one makes the display face back to front 00:01:17.35900:01:17.369 but it still went in and just folded the 00:01:19.67000:01:19.680 inside up in officer direction so full 00:01:22.70000:01:22.710 start there but anyway once I've sorted 00:01:24.44000:01:24.450 that out I plugged it in and I plugged 00:01:26.92900:01:26.939 in this USB splitter and it this hub and 00:01:31.78000:01:31.790 I plugged in four of these lights which 00:01:35.24000:01:35.250 take roughly 150 milliamps each but I 00:01:38.03000:01:38.040 have to say I'm not overly sure about 00:01:41.17900:01:41.189 the accuracy of these units and I'm 00:01:44.21000:01:44.220 pretty sure they drop a small amount of 00:01:46.42900:01:46.439 voltage across these bully must to sense 00:01:48.67900:01:48.689 of course and this also seemed to be 00:01:52.49000:01:52.500 limiting so I wasn't able to really a 00:01:54.56000:01:54.570 hundred-percent good accurate test the 00:01:56.87000:01:56.880 voltage and this started off at five 00:01:58.42900:01:58.439 point three volts and it went down a bit 00:02:00.13900:02:00.149 and however that's not what we wanted we 00:02:03.56000:02:03.570 want to open it so let's open it and see 00:02:05.92900:02:05.939 what it's like inside particularly the 00:02:07.91000:02:07.920 isolation from the mains 00:02:09.40000:02:09.410 so I'm 00:02:11.16000:02:11.170 basically to own this this yes that's 00:02:15.21000:02:15.220 the best way to open it 00:02:16.14000:02:16.150 it is held together quite strong they 00:02:18.05900:02:18.069 have to see now first surprise the 00:02:21.63000:02:21.640 circuit board is single sided and there 00:02:25.32000:02:25.330 are really there's no chips or anything 00:02:27.93000:02:27.940 on other than this out twice later and 00:02:30.77000:02:30.780 the secretary is really simple I mean 00:02:33.69000:02:33.700 there's two electrolytic to smoke 00:02:36.60000:02:36.610 pastors there are four resistors a Zener 00:02:40.29000:02:40.300 and two transistors and coupla diodes 00:02:42.69000:02:42.700 that's it so am i reversed engineered it 00:02:47.90000:02:47.910 and it looks like this 00:02:51.71000:02:51.720 the mains comes in through a diode 00:02:55.05000:02:55.060 charges up this capacitor the means 00:02:57.44900:02:57.459 mythic pastor on the main side - it's to 00:03:00.09000:03:00.100 point to make fire for under volts so it 00:03:01.77000:03:01.780 goes up to about 330 volts DC on our 00:03:04.53000:03:04.540 main supply here in the UK which is 230 00:03:07.05000:03:07.060 240 volts then the primary of the little 00:03:11.55000:03:11.560 transformer here is switched through the 00:03:15.44900:03:15.459 I was going to say power transistor but 00:03:17.46000:03:17.470 you know s all I'll say about this 00:03:19.50000:03:19.510 transistor is it's a high voltage 00:03:20.69900:03:20.709 transistor it's still not till 92 00:03:22.62000:03:22.630 package it's a tiny little thing so the 00:03:25.25900:03:25.269 primary is switched from the positive 00:03:28.11000:03:28.120 300 volts ID through this transistor to 00:03:30.78000:03:30.790 the negative side with this in line 12 00:03:34.14000:03:34.150 ohm resistor and I'll come back to that 00:03:35.97000:03:35.980 later 00:03:36.71000:03:36.720 the feedback for that it's a very simple 00:03:39.33000:03:39.340 feedback circuit the transformer has 00:03:41.64000:03:41.650 three windings it's got the primary the 00:03:43.38000:03:43.390 secondary for the five volts and a 00:03:44.67000:03:44.680 feedback winding down here the feedback 00:03:47.00900:03:47.019 winding and feeds back via this 00:03:51.27000:03:51.280 capacitor through the 680 ohm resistor 00:03:54.87000:03:54.880 to the base of the transistor and to 00:03:57.36000:03:57.370 start it going there's a very high value 00:03:58.89000:03:58.900 3.6 mega ohm resistor that in a powerup 00:04:02.46000:04:02.470 state this will just trickle enough 00:04:05.72900:04:05.739 current to start turning this turns as 00:04:07.25900:04:07.269 drawn as soon as it starts turning on 00:04:08.67000:04:08.680 and current starts flowing through this 00:04:10.71000:04:10.720 it starts an avalanche effect and it 00:04:13.19900:04:13.209 starts driving itself from this feedback 00:04:15.80900:04:15.819 circuit now the capacitor there I'm 00:04:19.25900:04:19.269 guessing that's just to limit the 00:04:22.26000:04:22.270 the on-time I guess it means they can 00:04:24.70000:04:24.710 fine-tune the oscillation frequency are 00:04:29.80000:04:29.810 not in a percent sure percent way the 00:04:33.37000:04:33.380 reason for this resistor here is current 00:04:36.40000:04:36.410 regulation circuitry whether it's due to 00:04:40.09000:04:40.100 the output being short circuited or 00:04:41.95000:04:41.960 which I haven't tested or the maybe it 00:04:46.33000:04:46.340 even comes into operation in each half 00:04:48.19000:04:48.200 on each on each cycle of the oscillation 00:04:51.25000:04:51.260 but if the current through this whole 00:04:53.41000:04:53.420 circuit exceeds about 50 milliamps 00:04:55.33000:04:55.340 you'll get roughly 0.6 volts across this 00:04:57.82000:04:57.830 12 ohm resistor which will then turn 00:04:59.71000:04:59.720 this transistor on and that will shunt 00:05:02.47000:05:02.480 the power spectrum transistor down to 00:05:05.98000:05:05.990 ground and alternate off the other way 00:05:09.13000:05:09.140 that transistor can be turned off is 00:05:10.69000:05:10.700 with this opto isolator transistor which 00:05:13.87000:05:13.880 is coupled to this LED over here so 00:05:16.09000:05:16.100 let's look at the second secondary so 00:05:18.28000:05:18.290 now the secretary winding is rectified 00:05:20.23000:05:20.240 through this diode and charges as 220 00:05:23.44000:05:23.450 Meg fired 10 volt pasta and that puts 00:05:27.31000:05:27.320 the five volts out to the USB port the 00:05:31.48000:05:31.490 LED in the opto isolator is connected in 00:05:34.15000:05:34.160 parallel with a 470 ohm resistor and I'm 00:05:36.55000:05:36.560 guessing that's just for stability for 00:05:38.20000:05:38.210 the Zener here and when the Zener 00:05:41.50000:05:41.510 voltage will be rated M for five volts 00:05:44.65000:05:44.660 minus the forward voltage of this OP 00:05:46.78000:05:46.790 twice little isolator LED so as soon as 00:05:50.11000:05:50.120 about five volts is exceeded this Zener 00:05:52.65900:05:52.669 starts conducting this LED light which 00:05:55.03000:05:55.040 then couples over to this transistor at 00:05:57.04000:05:57.050 the primary side and it just shunts the 00:05:59.98000:05:59.990 base to the emitter of this transistor 00:06:02.23000:06:02.240 and transit often stops oscillating so 00:06:04.63000:06:04.640 it's very simple circuit 00:06:06.15900:06:06.169 it's really quite neat actually there 00:06:08.71000:06:08.720 there's a decoupling coupling capacitor 00:06:12.67000:06:12.680 here for RF interference reasons that 00:06:15.04000:06:15.050 couples the low voltage side to the main 00:06:17.56000:06:17.570 site in what was a bit way of these but 00:06:19.21000:06:19.220 then I'm also wary of the insulation 00:06:21.01000:06:21.020 windings and also but we of the 00:06:26.13000:06:26.140 isolation circuit board now it's not bad 00:06:29.11000:06:29.120 in this unit it does at least kind of 00:06:31.90000:06:31.910 identify a track here 00:06:35.08000:06:35.090 now I've got a picture of us somewhere 00:06:39.42000:06:39.430 here we are which I was using to 00:06:42.28000:06:42.290 decipher the whole thing and and it's 00:06:46.54000:06:46.550 got this isolation root here which is 00:06:51.68900:06:51.699 barely more than a millimeter it's quite 00:06:54.87900:06:54.889 small and and that's not really 100% 00:06:59.58000:06:59.590 compliant with electrical standards 00:07:01.45000:07:01.460 because if you consider that these pads 00:07:04.57000:07:04.580 here are the USB shell and which would 00:07:08.29000:07:08.300 often be connected to the negative which 00:07:10.39000:07:10.400 it is here if you consider that then 00:07:13.84000:07:13.850 this track comes very close but within 00:07:17.35000:07:17.360 just barely over a millimeter from the 00:07:19.71900:07:19.729 main side via a diode so you know it's 00:07:22.90000:07:22.910 not really good enough isolation it 00:07:25.60000:07:25.610 makes you think of that instance in 00:07:27.10000:07:27.110 Japan where the bureau hostess was a 00:07:30.01000:07:30.020 electrocuted by her phone and that's a 00:07:32.02000:07:32.030 thing that you know it could have been 00:07:33.62900:07:33.639 moisture getting into it it could have 00:07:35.86000:07:35.870 just been tracking across it could have 00:07:37.87000:07:37.880 been a solder splash or it could have 00:07:40.27000:07:40.280 been the isolation these transformers 00:07:43.06000:07:43.070 because the I I'm not have a sense sure 00:07:45.67000:07:45.680 how good that is looking at at the 00:07:48.52000:07:48.530 moment it looks as though the secondary 00:07:51.19000:07:51.200 is wound possibly on the outside a lot 00:07:57.37000:07:57.380 of censure those stripping that 00:07:58.81000:07:58.820 transformative bits but another hand is 00:08:02.71000:08:02.720 it's quite nice that they've included 00:08:04.83900:08:04.849 this opto isolator there is proper 00:08:07.54000:08:07.550 feedback and under loading the voltage 00:08:10.93000:08:10.940 it started off round about five point 00:08:12.79000:08:12.800 three volts and it did drop but not a 00:08:15.01000:08:15.020 huge amount I would have guessed that 00:08:17.32000:08:17.330 500 milliamps is what I'd be comfortable 00:08:19.30000:08:19.310 running a power supply like this ad but 00:08:22.02900:08:22.039 yeah you know what for two dollars it's 00:08:24.90900:08:24.919 surprising oh and here's another thing 00:08:26.89000:08:26.900 the two transistors an aren't some 00:08:28.69000:08:28.700 special anonymous transistor okay the q1 00:08:32.14000:08:32.150 the one that's actually switching the 00:08:33.64000:08:33.650 primary is a one three zero zero one and 00:08:36.51900:08:36.529 I looked it up and it's a readily 00:08:38.11000:08:38.120 available transistor and the little 00:08:40.82900:08:40.839 current regulation transistor is a s 00:08:45.40000:08:45.410 nano 1/4 which is again 00:08:47.92000:08:47.930 just a generic NPN transistor this was 00:08:51.40000:08:51.410 rated from 400 volts this one is rated 00:08:53.47000:08:53.480 50 volts it's all just common components 00:08:56.71000:08:56.720 and you know no surface mode just all on 00:08:59.88900:08:59.899 the surface stuff and as designs go it's 00:09:04.30000:09:04.310 you're quite surprising it's actually 00:09:06.57900:09:06.589 better than I was expecting not sure if 00:09:09.46000:09:09.470 I'd be confident in trusting my life on 00:09:11.41000:09:11.420 it but for things like a USB power bank 00:09:14.76900:09:14.779 where most of the things you know you 00:09:16.66000:09:16.670 can treat everything metal as all being 00:09:19.09000:09:19.100 live it's not bad it would work I 00:09:22.32900:09:22.339 wouldn't really want to plug any 00:09:24.46000:09:24.470 expensive la la u.s. be charged 00:09:28.03000:09:28.040 appliance int it but you know it's not 00:09:31.03000:09:31.040 bad it's not bad at all
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