Comparing Three USB Testers Before Characterizing the FNIRSI DPS-150

Why I Did This Test

I recently added a FNIRSI FNB58 USB tester to the bench with the intention of using it during my characterization of the FNIRSI DPS-150. Before I started trusting the FNB58 numbers, however, I wanted to answer a more basic question: how does it compare with the USB testers I already own, and what effect does each tester have on the circuit it is measuring?

That turned what could have been a quick comparison into a useful little measurement exercise. Alongside the FNB58 I tested a Tenma 72-13540 and a low-cost KEWEISI USB tester. Rather than simply plugging each one into a charger and comparing the numbers on their displays, I built a small USB breakout fixture so I could measure the voltage at defined points before and after the tester.

The goal here is not to perform a traceable calibration or declare that one inexpensive bench instrument represents absolute truth. The goal is to characterize the measurement chain well enough that I know what each tester is telling me before I use one of them in the DPS-150 project.

Figure 1. The characterization bench with the HM310T supply, Multicomp MP730026, and Rev A USB breakout fixture.

Building a Fixture So I Could Measure the Same Thing Twice

The white Rev A fixture became more important to this experiment than I originally expected. It gives me two clearly defined measurement locations: SOURCE IN and TEST OUTPUT. The source side tells me what voltage is arriving at the USB path, while the output side lets me see what remains after the inline tester and its connectors.

Electrically, the SOURCE IN and TEST OUTPUT banana pairs are separate. SOURCE IN feeds the fixed USB-A female connector, while TEST OUTPUT is connected to the flexible USB-A male lead. Inserting the USB tester or device between those two USB connections completes the measurement path.

This matters because it is very easy to put one meter on the source side, read another meter downstream, see a difference, and accidentally call that difference an accuracy error. In reality, part of the difference may simply be voltage drop through cables, connectors, PCB traces, shunts, or the tester itself.

With no USB tester or external load installed, I measured approximately 4.989 to 4.990 V at the source and approximately 4.991 V at the test output during the baseline checks. That was close enough for the purpose of this comparison to treat the fixture as effectively transparent at no load.

First, I Needed a Working Voltage Reference

For the comparison I used my Multicomp Pro MP730026 as the working DC voltage reference. That does not mean I am claiming the Multicomp is a calibrated laboratory standard. It means I wanted one meter, one set of leads, and one measurement method to remain consistent throughout the experiment.

Before comparing the USB testers, I checked the Multicomp against the HM310T bench supply at several voltage settings. This was useful because it showed that the disagreement between the HM310T display and the Multicomp was fairly consistent through the most important part of the range.

Figure 2. The Multicomp directly monitoring the HM310T during the 5 V reference check.

Figure 3. Selecting the appropriate Multicomp range restored the extra displayed digit at the 5 V test point.

The 1 V, 2.5 V, and 5 V measurements are especially interesting because the difference stays around 9 to 10 mV. At 10 V the Multicomp changed range and the display lost a digit, so I do not want to over-interpret the apparent 20 mV difference there.

Figure 4. Reference comparison at the 1 V setting.

Figure 5. Reference comparison at the 2.5 V setting.

Figure 6. The 10 V point, where the Multicomp changed range and displayed less resolution.

The Three USB Testers

Although all three devices can be called USB testers, this experiment reinforced that they are not really aimed at the same job.

The FNB58 also goes well beyond the voltage/current comparison performed here. Its feature set includes USB fast-charge protocol detection and triggering, PD monitoring, D+/D- measurements, cable resistance functions, E-Marker support, accumulated capacity and energy, waveform functions, and PC software support. Those features deserve their own deeper review later. For this test I deliberately kept the question narrow: can I understand and trust its basic voltage and current behavior well enough to use it in the next project?

No-Load Test: The Meter Is Part of the Circuit

The first result that really changed how I looked at these testers was their own current consumption. With nothing connected to the downstream USB output, the HM310T still showed current being drawn.

That approximately 60 mA seen by the bench supply when the FNB58 is operating is not 60 mA of DUT current. It is largely the FNB58 powering itself. The color display, processor, ADC and USB/protocol circuitry all need power. The FNB58's own downstream current channel was essentially at zero with no external load.

The Tenma was particularly impressive here. Its approximately 4 mA upstream draw was dramatically lower than the other two testers.

Figure 7. KEWEISI no-load baseline. The HM310T sees approximately 29 mA of total current.

Figure 8. Tenma no-load baseline. The HM310T sees only approximately 4 mA.

Figure 9. FNB58 no-load baseline. The supply sees roughly 60 mA while the downstream current measurement remains essentially zero.

Voltage Accuracy: The Measurement Point Matters

The early FNB58 measurements produced an apparent disagreement that initially looked much larger than expected. The Multicomp was measuring the source input while the FNB58 was reporting its own internal/downstream voltage. Those are not necessarily the same electrical point.

Once I moved the Multicomp to a comparable downstream point and repeated the installation, the result changed significantly. In one simultaneous snapshot the Multicomp was around 4.971 V while the FNB58 showed 4.97064 V, a difference of only about 0.36 mV at that instant. That demonstrates very close indication agreement at the same electrical node under this particular test condition; it does not establish 0.36 mV absolute accuracy or constitute a calibration of the FNB58.

I also observed an earlier FNB58 downstream measurement that eventually settled around 4.915 V. Because a controlled reinstall and repeat produced approximately 4.971 to 4.975 V instead, I am keeping the earlier result as an anomalous startup/connection-state observation rather than treating it as the normal insertion loss of the FNB58.

The Small USB LED Became the Most Useful Test

For a repeatable low-current load I used a small flexible USB LED light. It turned out to be an excellent way to expose the practical difference between these meters.

With the KEWEISI installed, the HM310T total current increased from approximately 29 mA to approximately 46 to 47 mA. The light was clearly operating, but the KEWEISI continued to display 0.00 A.

With the Tenma installed, the HM310T increased from approximately 4 mA to approximately 21 to 22 mA. Again, the LED operated normally, but the Tenma displayed 0.00 A.

Both of those supply-side changes suggest that the LED itself was drawing roughly 17 to 18 mA.

The FNB58 directly resolved the load. With the LED connected, it displayed approximately 0.01681 A and 0.08353 W at roughly 4.969 V. That is approximately 16.8 mA, which agrees very well with the current inferred from the supply-side subtraction.

Figure 10. The FNB58 directly resolving the small LED load at approximately 16.8 mA and 83.5 mW.

For me, this was the clearest result of the entire comparison. The KEWEISI and Tenma were not necessarily failing to pass the current correctly; their displays simply did not provide useful information at this current level. The FNB58 did.

What About Voltage Drop Through the Testers?

Because the fixture exposes both sides of the USB path, I could also get a feel for how much the measurement setup itself was disturbing the circuit.

At the controlled no-external-load points, the Tenma showed roughly 1 mV of source-to-output difference while drawing about 4 mA, the KEWEISI roughly 7 mV at about 29 mA, and the controlled FNB58 repeat roughly 13 mV at about 60 mA.

With the KEWEISI and LED operating, I measured approximately 4.988 V at SOURCE IN and approximately 4.968 to 4.971 V downstream, giving roughly 17 to 20 mV of total observed drop in that loaded arrangement.

I do not want to turn those numbers into a claimed shunt resistance because the HM310T current includes the tester's own electronics and the exact internal current path is not identical between products. What the test does demonstrate is that an inline USB meter is not electrically invisible.

So Which Tester Would I Actually Use?

After doing the measurements, I do not think there is one universal winner. There is a clear winner for the DPS-150 characterization, but each tester still makes sense for a different job.

Personally, I really like the Tenma as the quick everyday meter. The LCD is bright and easy to read, the unit is uncomplicated, and its own current draw was the lowest of the three by a large margin.

The inexpensive KEWEISI still has a place. If I only want to know whether a USB cable has power or whether a charger is somewhere in the right neighbourhood, I do not need a color-screen analyzer with protocol decoding.

For the characterization work performed here, however, the FNB58 is the preferred inline tester of these three. The small LED test demonstrated why: a roughly 17 mA load disappeared below the useful current display resolution of the simpler meters, while the FNB58 measured it directly. This selection is based on the conditions tested so far and should not be read as a claim that the FNB58 is universally accurate or calibrated.

The Measurement Chain I Will Use for the DPS-150

This comparison was really a prerequisite for the next project. I now have a much better idea of how I want to divide the measurement jobs when I start characterizing the FNIRSI DPS-150.

For loaded measurements I will record the Multicomp voltage at the defined source or downstream point, the FNB58 voltage/current/power, and the source supply's total current. The important rule is that I will not compare two voltage readings as an accuracy test unless they are measuring the same physical node under the same operating condition.

I will also keep the FNB58's roughly 60 mA upstream self-consumption in mind. That current is part of what the source sees, but it is not automatically part of the downstream DUT current reported by the FNB58.

What This Test Does Not Claim

• This is not a traceable calibration. I did not use a calibrated voltage or current standard.

• The current work is a baseline confidence characterization, not a complete FNB58 calibration. Future testing should include controlled load points around 0.1 A, 0.5 A, 1 A, 2 A and higher where the equipment safely allows, broader voltage-range checks, insertion loss, repeatability, resolution/noise, warm-up and thermal drift, and operating-range dependence.

• I have not measured long-term temperature drift or repeatability over many hours.

• Advanced FNB58 functions remain a later characterization phase. These include charging-protocol detection/triggering, PD monitoring, E-Marker functions, cable resistance, waveform tools, Bluetooth/PC connectivity and long-term logging.

• The exact KEWEISI model/revision matters. Similar-looking low-cost testers can use different internal hardware and published specifications.

A Vintage-Meter Detour for Another Project

I also briefly brought an older Micronta autoranging digital multimeter into the reference checks. It was interesting enough that I decided not to mix it into the USB-tester conclusion.

At approximately 1 V the Micronta indicated 0.997 V while the Multicomp indicated 0.991 V. At 2.5 V it indicated about 2.502 V versus 2.491 V on the Multicomp, and near 5 V it indicated roughly 5.04 V versus 4.990 V. The changing difference deserves a proper characterization of its own.

That may become a future GAM3T3CH project: take a collection of older digital and analog meters, compare them against modern bench equipment, and see how well they have survived the decades.

Figure 11. Optional sidebar image from the Micronta comparison near 1 V.

Figure 12. Optional sidebar image from the Micronta comparison near 5V.

Final Thoughts

The biggest thing I learned from this exercise was not simply that the FNB58 has more digits or more features. It was that every measurement instrument becomes part of the system being measured.

The KEWEISI is still useful as a cheap sanity checker. The Tenma turned out to be a very nice everyday USB tester and had remarkably low self-consumption in this test. The FNB58 is the one I will carry forward into the DPS-150 characterization because its resolution and feature set make it much more useful when I actually need to quantify what is happening.

Most importantly, I now know how to interpret the numbers it gives me. I know that the source sees current used by the FNB58 itself. I know that source-side and downstream voltage are not automatically interchangeable. And I know that a meter showing more decimal places still needs to be checked against a sensible reference before those digits mean anything.

At this stage I consider the FNB58 validated as the preferred inline USB tester for the next DPS-150 measurements within the conditions explored here. That is a practical confidence decision, not a declaration of universal accuracy. The broader characterization remains open and will continue as controlled loads, voltage range, thermal behavior and the FNB58's advanced functions are tested.

That was the whole point of building the fixture and doing this comparison before moving on to the DPS-150: establish the measurement chain first, then characterize the device.

References / Product Information

Tenma 72-13540 product information: 72-13540 TENMA, Voltage Tester, LCD, USB | Newark Electronics

FNIRSI FNB58 product information: FNIRSI product documentation and software-download information.

KEWEISI background: product-specific information varies by revision; published KWS-series teardown/characterization material can be used as supporting context rather than as a specification for every KEWEISI-branded unit. There is a bunch of reviews on the one I used in this out there I think its labeled as the KEWEISI KWS-10Va.  here is another test result of this unit from eevblog user of a similar unit not sure if 100% same model or not.