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.