Keweisi KWS-10VA (no good) - Page 1
Forum section: electronics / testgear

author: jitter
author_rank: Frequent Contributor
timestamp: March 19, 2016, 11:50:37 am
content: A while ago I bought one of those cheap Keweisi USB volt/current testers off Banggood for very little money (this one).[Short version]:Cheap, so it won't be particularly accurate, and specs reflect that:Voltage 3-9 V, +/- 1%Current 0-3 A, +/- 1%.At first it seemed to work OK, but the measured values looked a bit dodgy. I decided to have a closer look if the accuracy claimed was actually met in reality.Turns out, they don't even come close. In reality, they vary quite a lot, up to tens of percents, depending on the values measured. It's absolutely no good at measuring low current, which IMHO, you might actually want to do with a device like this.If you're looking for a USB tester like this, don't buy this one, try your chances with one of the numerous others.[Extended version:]Here's what I did:I took a 5 V / 1 A USB charger and hooked it up to a Maynuo M9812 electronic DC-load through the Keweisi KWS-10VA at no load, 500 mA and 1 A.No load:Keweisi: 5.18 V / 0.00 AMaynuo: 5.095 V / 0.0000 ADifference: 0.085 V / 0.00 AWith no current, there's no voltage drop across the USB wire between USB-tester and DC-load.According to specs the Keweisi should read within 1% (max. 5.15 V in this setup), which it clearly doesn't. At least the current doesn't show any offset (but it turned out there is one of about 60 mA).500 mA:Keweisi: 5.04 V / 0.45 AMaynuo: 4.881 V + 46 mV (= 4.927 V) / 0.4998 ADifference: 0.113 V / 0.050 AThe voltage drop of 46 mV across the wire is what the DC-load will read too little w.r.t. the Keweisi side of the wire. Obviously, the current is the same everywhere in the loop.The voltage measurement error has increased a bit, but the error in the current measurement is a whopping 10%!1 A:Keweisi: 5.01 V / 0.96 AMaynuo: 4.781 V + 92 mV (= 4.873 V) / 1.0001 ADifference: 0.137 V / 0.040 AAgain correction for the voltage drop across the wire, now of 92 mV.The voltage measurement error has increased even more, but the error in the current measurement has now become a "mere" 4%, a lot better than at 500 mA.I started to suspect that the current measurement has some sort of offset which causes a shift in the measurements that always stays the same.Because of this I repeated the test with a lowish load of 100 mA. The Keweisi then displays only 0.04 A or 60% below actual value!Slowly increasing the load in steps of 10 mA from 0 A leads to the Keweisi starting to display 0.01 A at about 60 mA. This confirms that there's likely a fixed offset that's quite large (~40-60 mA). At low currents this obviously has a huge impact on the measured values which gradually becomes less, relatively, as current goes up.Verification:The measurements made by the Maynuo M9812 DC-load were double checked with a calibrated Agilent U1232A and they were bang on. The voltage drop across the wire was measured with the same DMM.The lower voltages at higher loads may be a result of poor regulation of the USB charger and the contact resistance of the USB socket and plug.

author: encore2097
author_rank: Contributor
timestamp: November 12, 2016, 05:35:35 am
content: Thanks for your post. I was thinking about buying a couple of these and was wondering about the accuracy, you saved me a lot of time!Any ideas on similar products which are more accurate? What about remote logging? It would be great to be able to log the V and A over time via USB, Serial, GPIO or something..

author: jitter
author_rank: Frequent Contributor
timestamp: November 12, 2016, 04:16:02 pm
content: You're welcome.As it is, it's good enough for rough checking of devices, e.g. if and what charging current a device takes from a USB adapter, but no precise measurements.As for better USB volt/current testers... your guess is as good as mine...

author: encore2097
author_rank: Contributor
timestamp: November 13, 2016, 12:51:36 am
content: I came across these posts:https://www.indiegogo.com/projects/portpilot-inline-usb-power-analyzer#/the Pro version has a separate USB for data logging but I'm unable to find where to buy it.andhttp://goughlui.com/2016/08/20/review-teardown-keweisi-kws-v20-usb-tester/http://goughlui.com/2016/08/21/weekend-probing-keweisi-kws-v20-usb-tester-ic-connections/http://goughlui.com/2016/08/22/weekend-probing-pt-2-keweisi-kws-v20-usb-tester-ic-connections/#comment-15167the V20 - seems to be inaccurate as well: ~1% on V and ~3% on A with some spikes at certain points.It might be possible to calibrate in software with a bench power supply

author: Matty
author_rank: Contributor
timestamp: January 04, 2018, 01:40:19 am
content: Hello - sorry for bringing up an old topic. But I have just got one of these devices and it is wildly inaccurate.Does anyone know what themicrocontrolleris?.... because I want to adjust the amps reading. The volts reading can be corrected by changing a resistor, but if I can do both in the software... that would be nice. (assuming there is no firmware lockout)I know its not worth the effort, but I'm curious.

author: keng
author_rank: Newbie
timestamp: February 25, 2021, 01:42:41 am
content: The attached pdf shows modifications I made to the KWSO-10VA to remove the negative zero offset and correct for a span error.Hope someone finds this useful.

author: EngelP
author_rank: Newbie
timestamp: August 29, 2021, 01:57:55 am
content: Hi, colleagues!Can someone tell me what kind of transistor is U1 (HT30)? I think it is a Si-PNP Mid Power Transistor, what is the 3-pin arrangement (BCE)?Thanks!

author: keng
author_rank: Newbie
timestamp: August 29, 2021, 05:30:09 am
content: HT30 is a 3 terminal voltage regulator, 5V input to 3.3V output.Package marking on my unit is U4RC.

author: EngelP
author_rank: Newbie
timestamp: August 29, 2021, 10:10:57 pm
content: Quote from: keng on August 29, 2021, 05:30:09 amOK, I did not think about that type of regulator (I do not detail the silkscreen of board, the IC U1), if the IC number of mine is HT30, surely it is a 3.0 VDC regulator, I imagine that the tullo is 3.3 VDC. If they use the same IC U2, then it can be compatible those used in 3.3 VDC in the motherboards PC. Can work:A) AMS1117-3.3V, 1A, SOT-223 Voltage Regulator IC (https://pdf1.alldatasheet.com/datasheet-pdf/view/205691/ADMOS/AMS1117-3.3.html)B) LM3940_3.3 VDC, 1A_Voltage Regulator IC_Texas InstrumentsC) AMS1117-3.0V, 1A, SOT-223 Voltage Regulator IC (this is Yours)If I feed with an external 3.3 VDC source of U2, and the displays turn on both, but the voltage measurement is wrong and there is no current reading (mAmp.), That means that the IC U2 is damaged, this I suppose is a programmed EPROM (guess you don't know what kind of IC (U2) this is?), that wants say that it is impossible to recover or acquire it, right? I suspect that we have to add a 9 VDC./1W Zener diode at the input of this line, so as not to commit future accidents again. If I have to buy a new one, which one do you recommend for me in 2021?Damm .... Thanks bro!

author: keng
author_rank: Newbie
timestamp: August 30, 2021, 09:36:44 pm
content: My apologies, yes you are correct.The regulator is 3.00V and is used for the measurements reference voltage.With a 3.3V reference the voltage measurements would be incorrect as you found.As I had found earlier the current has to exceed about 85mA before the meter starts reading current.Fitting R10, R11 and R12 reduced this offset to about 0mA and the current readings are about right.Incidentally have you an IC package number decoding chart i.e. to decode U4RC? I have not been able to find a reliable chart.