Review, Teardown: Two Popular USB Charger Doctor/Detector/Current Meters

There seem to be many circumstances where checking the current and voltage to a USB device could come in very handy. For example, maybe you are troubleshooting a slow charging device – maybe it’s cable related and the voltage is dropping, or maybe it’s a charger compatibility issue causing the device not to kick into high speed charging. Maybe it’s an old charger and it’s on its last legs.

Other times, you might be running peripherals and devices such as the Raspberry Pi, and you might be interested in finding out its current consumption or that of USB Wi-Fi adapters and USB storage devices so as to plan your power sources accordingly.

I’ve had a desire to do this many times in the past, and the solution was a moderately painful one of buying a USB extension lead, and splicing it apart to put some multimeters in line for voltage and current detection. The burden resistance of many multimeters at low currents is fairly high, and can skew the results by reducing the voltage to the end product, and other times, the meters just don’t have the right ranges (e.g. DT830B clones with a 200mA or 10A range) to get a good reading.

But alas, this problem has been solved. Enter a product commonly known as a USB Charger Doctor. Sometimes these are known as USB Detector or a USB current meter. Basically these devices feature a pass-through USB connection to connect your source port and device, and a seven-segment display which is driven by ICs powered by the USB bus power to indicate the voltage and current.

This is such a simple idea, but it can be remarkably useful. Since I thought it would be nice to have one, I couldn’t resist ordering the two more common designs and giving them a quick look-over.

Blue-Coloured Charger Doctor


This is a pretty basic charger doctor unit. This one features a USB A-M for plugging into the port, and a USB A-F for plugging your device into. It features a single 4-digit 7-segment red LED display, and claims to be able to meter between 3.5-7.0V and current from 0A-3A. This one was selling for a bargain price of AU$1.83 a piece.

Having tried it out for basic function, it was found that only three digits of precision are available, with the last digit used to indicate volts (U) or amps (A). This is slightly disappointing as they could have just made that indication using a simple LED. Readings seem to be refreshed at around 3Hz, with the display automatically alternating between current and voltage every 4 seconds or so. This, unfortunately, doesn’t allow you to simultaneously monitor voltage and current which could make diagnosing voltage-drop-due-to-current faults a little more tricky, but it does save on a display.

A quick check seems to show this unit consuming about 20-30mA, with the display easily displaying current levels of 0.01A upwards. The accuracy of voltage and current seems to be within 2 to 3 counts (i.e. 0.02-0.03V/A). This corresponds to about 1-2% error which is enough for diagnostic purposes, but is not a precision instrument.

Due to its size, use with certain ports may run into obstructions, and may cause obstruction of adjacent ports as well. While it’s commendable that they’ve avoided cables which may increase resistance, it’s also a potential problem for some, so maybe a quality USB extension cable is advisable.


While it comes in a translucent blue casing, there really isn’t much to be seen from both sides. The PCB seems to be fully silkscreened on the bottom to make it appear white.



The unit itself is of a moderate size, and the casing itself is held together with plastic clips.


Taking the casing off, we can see the 7-segment display still has its protective film on. The first thing I did was peel it off!


Things are much the same with the case off, but you can see we will have to go to great lengths to find out what’s “inside”. So I fired up my trusty iron to try and desolder the 7-segment display.

What an arduous process that turned out to be. The lead-free solder didn’t want to cleanly remove, the small solder holes and pads didn’t help either. I tried a few braids, applied fresh solder over and over … even a solder sucker didn’t help much. You really don’t want to pry if the legs aren’t completely clear, because you’ll probably peel off a few pads. But it’s under AU$2, so maybe it’s worth it.

Eventually, I decided to get the hot air rework out, and turned the temperature up to an obscene level that managed to heat it up till the whole board was “liquid” thus, freeing the display.


We can see what appears to be a microcontroller as the heart of the solution. It’s marked 00F0105, which seems to be a production marking over an unmarked chip. The burden resistor in this model is 0.05 ohms, which is the R050 surface mount resistor (likely a 2W model). With this burden resistor, a draw of 3A would result in a loss of 0.15v across the burden resistor, in which, an indication of 4.85v would be expected even though the source is 5v. This is a common pitfall of current measurements! The ultimate accuracy of the unit is dependent, in part, to the resistor tolerance as well.

It’s quite good to see how the pins are directly connected where possible, using larger traces over a very short distance to stop additional resistance causing issues with reading accuracy.

The unit’s voltage ranges is likely related to the voltage which the 5v microcontroller operates correctly – it’s quite common for them to run from 3.3v to an absolute maximum of 7v.


All that heating, desoldering braid and solder has left a lot of burnt-flux residue on the board. It looks ugly, but it still works! A little disappointing we weren’t able to find out more from this adventure.

USB Detector (KW203)


This one is another popular unit, featuring two three-digit 7-segment displays for simultaneous voltage and current readout. This was a more expensive unit, costing AU$6.70 a piece and features a short USB pigtail to connect it to a port. This makes this unit easier to fit, but also introduces an additional source of voltage drop, as the cable isn’t thick and connects to a connector on the PCB. In a quick test, it was showing 4.80v at 0.6A draw for a source whereas the blue unit was still showing 5.00v. This means the cable and connector is adding about 0.33 ohms to the link, and thus the voltage would be inaccurate by about 1 volt at 3A! This is a bit excessive.

The unit itself draws about 20-30mA, similarly to the unit above, but it seems not to indicate values below 0.05A, so thus it features some “zero blanking”. The unit itself updates the displays at about a 3Hz rate, simultaneously, and thus for “spiky” switching loads, the values do fluctuate quite a bit as it takes “samples”. In general, its accuracy seems to be comparable with the above unit, usually being within 2-3 counts (1-2%) roughly – good enough for diagnostics.

The unit itself has one major functionality difference over the other units – it’s possible to use it as a crude USB charger splitter. It has two ports – one port is passthrough with the D+ and D- lines, passing the data along. The other port has the D+ and D- lines shorted as a dedicated USB charger. Thus you can probably use this with a single port output and check the current is within range all at the same time, but the resistance in the short pigtail might work against you and cause voltage drops to show up even though the source is holding stable. The measured current is the sum of both ports.


The unit itself is labeled KW203, and claims to run from 3.2v to 10v and a current range of 0-3.00A. This is slightly wider in voltage range than the above unit, but it’s unlikely you would want to plug anything into a USB port that’s giving out more than about 5.5v anyway!

Intriguingly, this label suggests that there are several different configurations for the unit – I have no idea what they are though. I presume this unit is just a basic VA unit. Maybe the +C units can do cumulative energy, and +H can do hourly energy? No idea.


The unit is held together with an end clip as well as a single screw hidden underneath the label.


Again, the LCD displays seem to have their protective films in place. I wonder why – does it cost too much time to peel them off? Another observation is that the traces for the USB power connectors on the top look a bit small, as do the connector for the USB pigtail wiring.


The 7-segment display units seem to be model number RLD2381AHB-22.


The unit itself seems to have tinned traces, although not with significant amounts of metal, to try and reduce the resistance contribution. It also seems to have different SMD resistor pads for the second port to configure how it identifies (i.e. you can use resistor pull-up and pull-down to simulate Apple charging protocol, or as per this unit, use a zero-ohm resistor to tie D+ and D- together for USB dedicated charging protocol).

The burden resistor in this unit is 0.01 ohm, which is 5 times less than the unit above. This would reduce the voltage drop at full load, but with the potential price of increasing the noise in the measurement. The voltage burden at 3A is only 30mV, thus reading 4.93v when the source is 5v. The unit is run by another unidentified IC.


These are two of the more popular USB charger doctors on the market, and they’re very handy for quick indicative measurements of voltage and current. By using these units, you can check if a given charger works in “high current” charge modes with devices, diagnose failing switching supplies which put out too low of a voltage causing charging to fail, see the impact of USB extension cables on the voltage at the end under load and even check the power consumption of USB peripherals and devices like the Raspberry Pi.

It’s a very inexpensive device, and while it is not a precision device as such, it seems to be “accurate enough” for quick diagnostics, with readings within about 3-counts or roughly 2% in my experience. The fact I don’t have to splice my own connectors and find multimeters is a big bonus, although the fact that the unit consumes about 20-30mA of current may skew the voltage slightly, as will the fact that there is a small burden resistance of 0.01 or 0.05 ohms in series with the supply.

Each of these devices has its own advantages and disadvantages, but they are definitely handy to have around.

About lui_gough

I'm a bit of a nut for electronics, computing, photography, radio, satellite and other technical hobbies. Click for more about me!
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18 Responses to Review, Teardown: Two Popular USB Charger Doctor/Detector/Current Meters

  1. sediriw says:

    Hey Gough, where do you generally buy your electronics from?

    eBay has it for $1.61 including shipping to Australia from China (with 4 weeks shipping). Do you use AliExpress/Alibaba for these kinds of things? I find them not as easy as eBay+PayPal, but maybe you have some tips..

    • lui_gough says:

      Good question. I bought mine from eBay, and it seems the price changes as often as the weather. Sometimes it goes up dramatically when the demand starts to rise because they feel the popularity leads to a situation where price-gouging is warranted.

      Normally, I search for listings worldwide which gives a bit more flexibility for pricing but do watch out for those not priced in AU$ as conversion fees on PayPal don’t line up with the eBay estimates, and they might come out slightly more pricey. But sometimes you will strike it lucky going this way and you’ll get it maybe 20c cheaper … but expect a 1 month wait as normal for items drop-shipped from Chinese warehouses.

      I used to buy from DealXtreme as well, but their slowness and inconsistent quality along with less competitive pricing in recent times have led me to write them off. I’m not quite ready to jump into AliExpress yet, but some people I know do use them.

      – Gough

  2. Pingback: Reverse Engineering: The USB Charger Doctor | Gough's Tech Zone

  3. Pingback: Testing: Charger Doctor and USB Detector Accuracy | Gough's Tech Zone

  4. ginbot86 says:

    Excellent teardown and analysis; I’ve often wondered what was inside these little USB power meters. Perhaps the ICs are just super-cheap mask ROM-based microcontrollers that the manufacturer didn’t bother with laser-marking?

    I had a set of mains sockets that integrated a pair of USB charging ports, and I used a Texas Instruments lithium-ion fuel gauge (the bq27541 to be specific) to measure the voltage and current. As it turns out, TI not only mentions it in their presentations, but they even use it themselves as a form of portable instrumentation!

    I wrote a blog post about it (, and it’s seen a lot of traffic up in Canada where these are still being sold in Costco, a (very) large wholesale store; most of the comments I’ve received are of people no longer wanting to buy said charger or people that are returning the product because it failed to meet their expectations.

  5. Most of us who read your blog are techy, but not as techy as you, AFAIK. So I use the lowest cost electronic gadgetry, including digital multimeters.
    Could you test the reliability of these low cost devices, easily available in local shops & on eBay? I have in mind:
    “Universal Mobile Cell Phone Battery Travel Charger”, $aud 1.69. I bought two of these. They work ok, but tend to keep charging way past 100% battery fill. This charges batteries of all physical sizes AFAIK, but is troublesome to ensure the battery stays in contact with the two conductive points.

    “Universal Travel Wall Dock Docking Battery Charger USB”. Mine cost $aud 2.29. As above, but both charge faster via direct AC-connection, rather than USB-5v connection. Also, it cannot work with my physically larger batteries.

    “Cell Mobile Phone Camera Universal Battery Charger” $aud 2.69. This is much more fiddle than the above two, so I don’t use it.

    “AC/DC Digital Multimeter Electronic Tester Clamp Meter”. $aud 8.79, seems ok for my purposes. Exact accuracy is not important to me, but consistency over the years of use is, after I know how off-target it is in my first calibration.

    “DIGITAL VOLTMETER AMMETER OHM MULTIMETER DT830 2014”, $aud 4.47. Again, as with the previous multimeter. I mainly test voltages & simple resistances.

    I might upgrade my amateurish test gear to:
    “Pocket Digital Voltag Multimeters Uni-t UT-10A Meters”, $aud 17.99.
    “ARM NANO DSO201 Digital Storage Oscilloscope Handheld”, $aud 66.99.

    For amateurish tinkerers like myself, do you think this is ok?

    Yes, I’m in Australia: Canberra & Sydney, but retired via old-age. But I’m still learning & tinkering.

    • lui_gough says:

      Hi Greg,

      Nothing wrong with being techy, and nothing wrong with not being as techy as me! I’m a bit of a crazy person, I think that much is for sure!

      Unfortunately, time is always limited and my need for gear (especially lower end) is extremely limited. As a result, the time I spend on testing them isn’t likely to be very worthwhile. From my experiences, however, I can at least share some “wisdoms”:
      – Cheap isn’t necessarily bad. I’ve got a boatload of “19-range” DT830B meters. And you know what? I use them a lot. Most of them have a simple potentiometer to calibrate them, and they’re within a few digits when it’s new. It can drift slightly over time, but very often if you’re doing basic troubleshooting, you really only need to know whether your gear is getting 5v, or 4.8v, or 12v, or 0v and for those indicative purposes it’s just fine. I wouldn’t go setting my end-points for lithium ion batteries with one, or doing any “precision” work with it.
      – Cheap AC line powered gear can be nasty and dangerous. Some of it hasn’t been designed with proper isolation or clearances between mains and secondary. In case of transient surge or insulation failure, electrocution can result. Many of them also run hot and can emit audible noise, as they use very basic circuits without efficiency in mind. Others may have been designed for 220v China domestic usage, and fail early – I’ve had a few cheap chargers catch fire, and glue gun explosions.
      – Unitrend gear isn’t particularly “high end”. I wouldn’t say they’re significant upgrades to a DT830B, although large digits and auto-ranging might be nice.
      – I have a DSO Nano v2, and most of them are really only good for low-frequency (200khz or below) work. So maybe for audio work, that’s okay. Maybe even very basic IR signals, is also okay. But don’t expect to scope out digital data signals, or deal with higher frequency video signals. They can be a bit finnicky to get used to in terms of operation, but if the budget stretches to ~$400, there are some nice Rigol benchtop scopes with a much wider bandwidth and storage capability, as well as frequency analysis (FFT) features.

      – Gough

      • sediriw says:

        I’m not the original commentor you just replied to, but it sucks that there apparently isn’t cheap but quality version for AC power measurement. They seem to be called but I’m not sure if the low end versions that should exist are cheap. I totally want to test all the gaming consoles and other high power appliances in my house (TV, fridge, network-attached storage etc).

  6. Greg Zeng says:

    Back on topic to your original post: I bought the device you tested. Generally my gadgets have more than one rechargable battery. So this slow-charger is ok. I found that having this digital amp-volt-meter too close to my “empty” battery is not working, so I have a cable distancing this meter from the “empty” battery.

    It suits my purposes, telling me when & if the battery being USB-charged is fully charged or not. If fully charged, little current flows, compared to the starting current.

  7. CaptainKremmen says:

    Great writeup- shame I took so long in stumbling on to it! I pulled a “blue one” apart a while ago, and was slightly surprised to find that it was current sensing on the ‘ground’ side. But this makes it easier to sense the current with the ADC, as I guess this is ground referenced too, so no need for a differential measurement. It also looked like the USB screen and USB 0V cable were connected together at each end, and both broken by the sense resistor. Is that the same with yours

  8. Benjamin E. says:

    I might make one of these some time, just for fun.
    And, it’s a useful tool to have as well.
    And, it’s something that I could use to demonstrate my knowledge, if anyone ever wanted to know what things I can do.

    I’d probably use some sort of a more advanced display, and have more features. Maybe like counting milliamp-hours, watt-hours, logging of the voltage and current over time, etc… 😀
    I’d also probably use at least a 10mOhm shunt on the 5V rail, or use some sort of hall-effect current sensor (more expensive, but less loss :D). If I were to use a shunt resistor, I’d also use something like a ZXCT1082 high-side current sensor or the like, which would amplify the low voltage signal, to get better range on the ADC.
    Again – more expensive, but better performance 🙂

    • lui_gough says:

      There are some units which do cumulative mAh/mWh style displays as well, although I suspect they are hardly accurate. Likewise, the shunt resistances can impact on the behaviour of the attached USB device (i.e. charging slower than usual due to voltage drop) so I suppose it’s a good idea provided you don’t go so low as to have noise impinge on your measurements. I guess a higher sample-rate might be nice too.

      One thing to watch out, as I’m increasingly beginning to experience this, is the problem of USB connector dimensional fit. Lots of cheap cables and connectors out there but they just don’t seem to be quite as tightly made as they could be. As a result, the contact resistance (and charge rate) is a function of how far I plug the cable in, if I’m applying lateral pressure to the connector or not. Ultimately, I did bend the shells of some connectors to they “push” against the tongue of the connector with more pressure, and that temporarily fixes it, but I suspect highly used ports/cables could become loose in this way and affect readings.

      – Gough

  9. jayvl says:

    How does a 0.02A accuracy error correspond to 2%? If I have a device pulling 0.01A and it tells me a value of 0.03A, that is a 200% error!

    Also I noticed this device shows 0A with a simple USB audio controller (while playing music on headphones). I wonder if music really draws less than 10 milliamps, or if this is just due to the inaccuracy you mentioned.

    • lui_gough says:

      I said “with readings within about 3-counts or roughly 2% in my experience” in the article. All digital meters are problematic when you’re at the bottom end of the scale – three counts would correspond to 0.03A minimum error, or around 2% of the maximum value (as meter accuracy is generally represented as % of range +/- digits/counts of error – just refer to any reputable multimeter datasheet and you will see).

      It really makes no sense to talk about percentages for readings, especially less than 10% of full scale because of the quantization of the display.

      It’s likely that there is error because the “music” part of the equation results in varying AC content in the current, which for a low-sample-rate meter, may not pick up the transient peaks in the current consumption. It may be the case that your particular unit also zero-blanks low readings below a given threshold.

      • Juan Herrero says:

        Just for curiosity, if you still have the second one, could you solder a nice short USB lead and measure with that? See how much of a difference it makes empirically.

        I have just today received two I ordered from eBay:
        The first one looks the same as your first one but on a transparent case with a nice bright display but a slow refresh of ~9 seconds on each measurement.
        The second one looks more like a conventional USB drive, with the M and F plugs on the opposite lengthwise ends, refreshes at a more reasonable 5s and is provided in a blue transparent casing, which mutes the segment display quite badly, and there was a dusty fingerprint on the screen protector sticker to make it even worse.
        After trying them out for a bit if I was in the market for a new one I’d just buy the more fanciful and slightly expensive one with the dual readout LCD screen and not give these a second look. Waiting for the readout to switch, even in the faster one quickly gets tiresome. You do get what you pay for with them. On the other hand they will make nice gifts/lenders, and do work well enough for testings wall chargers and powerbanks.

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