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For those who only occasionally need to measure a capacitor or a coil, this LC measuring module costing only € 12.85 seems like a bargain. However, our test shows that this little meter is completely unusable. |
Introduction to the LC100A from Junteks
Manufacturer, suppliers and prices
This module is sold under the brand name 'Junteks', the manufacturer is the Chinese 'Hangzhou Junce Instrument Co., Ltd.'. The LC100A is a very popular module that you can order from all known internet channels for quite different prices. The cheapest seller we found on AliExpress charges, at the time of writing this article, just € 9.02 for this LC meter excluding € 3.83 shipping costs.
The scope of delivery
The PCB is delivered, sloppily packed, with two short measuring leads with alligator clips and a USB-A to mini-USB lead for powering the module from a standard 5 V power supply. A manual is missing, but you can find it on the manufacturer's site.
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| What you get for just over ten euros. (© 2024 Jos Verstraten) |
The appearance of the LC100A
The LC meter is housed on a PCB measuring 80 mm by 50 mm. Located on this PCB is the famous LCD1602 LCD display, with which the LC100A communicates with its user. On the right side is a slide switch used to switch the power supply on and off. Not visible in the photo below are the two connectors on the back of the PCB that allow you to power the module from a 5.0 Vdc source. One is a mini-USB connector, the other a standard 2.1 mm x 5.5 mm power connector.
Below the display are four push buttons on the PCB to select the measuring range and, before a measurement, zero the reading. Next to the PCB screw terminal in which you attach the two measuring leads is another small red pushbutton 'Func'. This sets the measurement frequency on the display.
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| The appearance of the LC100A. (© 2024 Jos Verstraten) |
The technical specifications of the LC100A
According to the manufacturer, the LC100A has the following specifications:
- Measuring range Cx: 0.01 pF ~ 10 μF
- Resolution range Cx: 0.01 pF
- Measuring range Hi.C: 1 μF ~ 100 mF
- Resolution range Hi.C: 0.01 μF
- Measuring range Lx: 0.001 μH ~ 100 mH
- Resolution range Lx: 0.001 μH
- Measuring range Hi.L: 0.001 mH ~ 100 H
- Resolution range Hi.L: 0.001 mH
- Accuracy of capacitance measurement: ±1.0 % ~ ±5.0 % (> 1 μF)
- Inductance measurement accuracy: ±1.0 % ~ ±5.0 % (> 1 H)
- Test frequency: 500 Hz ~ 500 kHz
- Supply voltage: 5.0 Vdc
- Dimensions: 80 mm x 50 mm x 28 mm
- Weight: 198 g
If you take a quiet look at these specifications, it immediately becomes clear that this manufacturer is very fond of poetic exaggerations. A measuring range of 0.01 pF for a device that costs less than fifteen euros? And that without using four-wire measurement technology? Surely that must be a miraculous circuit! Why are there manufacturers publishing such unreal specifications? What do they want to gain by doing so?
The manual
We have added the English version of the manual to our account on Archive.org, you can download it here:
The electronics in the LC100A
The main PCB
Most of the electronics are under the display PCB. After removing four screws, you can remove this PCB from the connector and the main PCB will reveal its secrets. The manufacturer makes no effort to hide the identity of the chips used. So it only takes a quick look around under the magnifying glass to discover the ICs below:
- U1: STM8S903 microcontroller
- U2: LM311 open-collector comparator
- U3: LM393 double comparator
- U4: TL431 programmable voltage reference
Besides the modern SMD components, three through-hole components stand out:
- a toroid coil L1 of 50 μH
- a capacitor C14 of 1 nF (yellow component)
- a capacitor C12 of 100 nF (blue component)
It is these three components that are responsible for measuring the unknown capacitor or coil.
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| The main PCB of the LC100A. (© 2024 Jos Verstraten) |
The schematic of the LC100A
Thanks to the diligent work of our Russian colleague 'Незнайка21', the schematic of the LC100A is publicly available. We reproduce it, with many thanks, here. Please note that this is the schematic of version 4.7 of the device, as usual with Chinese products there are several versions circulating under identical names.
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| The schematic of the LC100A. (© 2020 Незнайка21) |
The operating principles
The LC100A operates on two principles:
- Measuring ranges 'Cx', 'Lx' and 'Hi.L':
Embedding the component to be measured in an LC resonator. Measuring the oscillator frequency and deriving the value of the component to be measured from this via the software. - Measuring range 'Hi.C':
Charging and discharging the capacitor with a known current and calculating the value of the capacitor from the duration of these events.
We will discuss this in detail.
Range 'Cx'
In this range, see figure below, the capacitor Cx to be measured is connected in series with C12. That series circuit in turn is in parallel with L1 and C14. For open inputs, the capacitance of the parallel circuit is 1 nF, for shorted inputs 101 nF. Indeed, if the values of L1, C12 and C14 are accurately known, the software can accurately derive the value of the capacitor to be measured from the resonance frequency.
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| The measuring principle in the range 'Cx'. (© 2024 Jos Verstraten) |
Range 'Lx'
In this measuring range, the inductor Lx to be measured is connected in series with L1. Together with C14, these components form a parallel resonant circuit, from which the microcontroller in turn measures the frequency and calculates the value of the inductor to be measured from this.
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| The measuring principle in the range 'Lx'. (© 2024 Jos Verstraten) |
Range 'Hi.L'
The only difference is that now capacitor C12 is switched in parallel with C14.
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| The measuring principle in the 'Hi.L' range. (© 2024 Jos Verstraten) |
Range 'Hi.C'
A completely different technique is used when measuring electrolytic capacitors. The electrolytic capacitor Cx to be measured is charged and discharged via 100 Ω resistors. That process is controlled from the microcontroller and carried out by two MOSFETs in series with the charging and discharging resistors. The two comparators in the LM393 determine the time it takes for the capacitor to charge from a certain threshold voltage to a second threshold voltage. From that time duration and the knowledge of the two voltage thresholds, the software can calculate the value of the electrolytic capacitor. However, to do this with the specified accuracy, a number of parameters must be stable. The two threshold voltages are derived from the output voltage of the stabiliser TL431, so that's fine. However, the unknown capacitor is charged directly from the 5 V supply voltage and its exact value and stability are unknown factors.
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| The measuring principle in the 'Hi.C' range. (© 2024 Jos Verstraten) |
Working with Junteks' LC100A
MEASURE Cx
With all switches in the unpressed position, the LC100A starts up in 'Cx' mode. The illustration below shows the display in this mode and the device is ready to measure capacitors up to 10 μF. Before each measurement, with the test leads open, press the red pushbutton 'Zero'. The text 'CALCULATING...OK' appears on the second line of the display and a little later '<DATA SAVED>'. The LC100A is now apparently calibrated and you can measure capacitors up to 10 μF.
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| The display in the 'Cx' range. (© 2024 Jos Verstraten) |
MEASURE Hi.C
Press the white 'Hi.C' key if you need to measure capacities higher than 10 μF. Again, before each measurement, go through the calibration procedure.
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| The display in the 'Hi.C' range. (© 2024 Jos Verstraten) |
MEASURE Lx
If you need to measure small coils, press the yellow pushbutton 'L/C' from the starting position. After calibration (red button, however now after shorting the two measuring leads), you can measure inductances up to 100 mH.
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| The display in the 'Lx' range. (© 2024 Jos Verstraten) |
MEASURE Hi.L
To measure coils with a value higher than 100 mH, first press the yellow key and then the blue one. Again, before each measurement, remember to calibrate briefly with inputs shorted.
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The display in the 'Hi.L' range. (© 2024 Jos Verstraten) |
The measurement frequency on the display
If for some reason you want to know with which frequency is being measured you have to press the small red button 'Func'. You will then see the frequency appear on the bottom line of the display.
Testing the Junteks LC100A
Preliminary note
The accuracy of the measurement methods used depends on the accuracy of a number of components on the PCB and on the accuracy of the supply voltage. In particular, this concerns the components L1, C14, C12, R9 and R11. However, the accuracy of these components is extremely questionable. The coil and the two capacitors are the cheapest versions available and they are not precision components with a tolerance of, say, ±1 %. R9 and R11 are ordinary tiny SMD resistors. So we are very curious how this module can measure with the specified accuracy of ±1 %!
To test this, we have at our disposal a few capacitors with a tolerance of ±1 %. In addition, thanks to one of our donors, we have recently acquired a pair of standard capacitors with a tolerance of ±0.05 %!
We do not have accurate coils, for that we have to rely on the readout on our laboratory RLC meter ET4401 from East Tester. This has, according to the specifications, a basic accuracy of ±0.1 % and is thus good enough for assessing the manufacturer's claims of the LC100A.
To exclude influence of the measurement results by poor power supplies, the tests below were performed with the LC100A powered from a 5.0 V power bank.
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| Measuring our standard capacitor of 100 nF ±0.05 %. (© 2024 Jos Verstraten) |
Testing the measuring range 'Cx'
The table below summarises the results of our measurements of our ±0.05 % and ±1 % capacitors using both the LC100A and the ET4401. These measurements show that our ET4401 measures much more accurately than its specifications promise. To calculate the percentage error on the measurements of the LC100A, we therefore used the measurement with the ET4402 as a 100 % reference.
The results are startlingly bad! The percentage errors on the measurements of the LC100A always exceed 10 % with outliers as high as no less than 40 %!
In such a case, a defective module is naturally the first thing to be thought of. However, several other tests of the LC100A that you can find on the internet prove that these poor results are not due to an accidentally delivered defective module, but that such large errors are systematic for the LC100A.
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Testing the 'Cx' measuring range. (© 2024 Jos Verstraten) |
Testing the measuring range 'Hi.C'
For these measurements we use our 1 μF ±0.05 % standard capacitor as the lowest value and for the rest cheap Chinese electrolytics from our stock. Although the ET4401 also measures very large deviations from the values printed on the electrolytics, we trust that this measuring device works reliably even in the electrolytic mode. We therefore again take its measurements as 100 % reference for calculating the percentage deviation of the LC100A.
The results are slightly more accurate than for 'Cx', but obviously still absolutely unacceptable for a module whose measurement accuracy is claimed by the manufacturer to be ±5.0 %.
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Testing the 'Hi.C' measuring range. (© 2024 Jos Verstraten) |
A test where the LC100A completely falls through the basket
If you measure several capacitors individually and then connect them in parallel, the measured parallel value should be the same as the sum of the individual measurements. We tested this with three 100 nF polyester film capacitors.
- Capacitor C1: 61.51 nF
- Capacitor C2: 64.74 nF
- Capacitor C3: 65.63 nF
That the LC100A measures far too low values is to be expected, given previous tests. But now let's see what the device measures when we connect those capacitors in parallel.
- Capacitors C1 + C2 in parallel:
Expected value: 61.51 nF + 64.74 nF = 126.25 nF
Measured value: 106.6 nF
- Capacitors C1 + C2 + C3 in parallel:
Expected value: 61.51 nF + 64.74 nF + 65.63 nF = 191.88 nF
Measured value: 133,7 nF
To make sure we are not making a logical error, we repeat this test with our ET4401:
- Capacitor C1: 106.45 nF
- Capacitor C2: 104,22 nF
- Capacitor C3: 105.52 nF
And now parallel:
- Capacitors C1 + C2 in parallel:
Expected value: 106.45 nF + 104.22 nF = 210.67 nF
Measured value: 210,77 nF
- Capacitors C1 + C2 + C3 in parallel:
Expected value: 106.45 nF + 104.22 nF + 105.52 nF = 316.19 nF
Measured value: 316.47 nF
So, even if we were to assume that the observed large measurement errors of the LC100A are caused by a accidental bad value of C14 or C12 present in this specimen, these sum errors should absolutely not occur! There is something fundamentally wrong in the design of this little meter!
Testing the measuring range 'Lx'
For this, we use a number of coils in the μH and mH range from Fastron's L-07HCP series. These coils have a tolerance of ±10 %. The measurement results of the ET4401 are again taken as absolute for calculating the percentage error of the LC-100A.
Again, the LC100A is found to be completely unusable for measuring coil inductance values. While there is no line to be drawn in the percentage error when measuring capacitors, there is at least some logic in this: the smaller the coil, the larger the deviation.
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Testing the 'Lx' measuring range. (© 2024 Jos Verstraten) |
We do not have coils with an inductance value in the henry range. Our ET4401 does indicate such values when we measure the primary winding of a power transformer. But the measured value is very dependent on the frequency at which it is measured. Hence we first measure with the LC100A, retrieve the measurement frequency and then measure with the ET4401 at a frequency as identical as possible.
TRANSFORMER 1
- Measured with LC100A: 5.675 H at 224 Hz
- Measured with ET4401: 5.560 H at 200 Hz
TRANSFORMER 2
- Measured with LC100A: 23.18 H at 118 Hz
- Measured with ET4401: 33.504 H at 120 Hz
Our opinion on Junteks' LC100A
It will be clear that we do not have a good word to say about this misfit of a measurement module. It is completely unusable for the purpose for which it is sold. Any suggestion that we accidentally received a defective one is contradicted by other tests you can find on the internet that come to exactly the same conclusion. The LC100A measures capacitors and coils with such large percentage errors that the measurement results are completely unreliable.
Admittedly, it is quite easy to improve the accuracy of this measurement module by experimenting with the value of components L1, C14 and C12. But... you need a reliable LC meter for that! So that doesn't really get anywhere.
In short, if you need to measure capacitors and coils you will have to spend a bit more money.
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