LCR Tweezers test and analysis (HP-990, DT71, ST5S...)

[Shannon]

Many years ago, I found the smart tweezers st5s was interesting measurement equipment, it's convenient and accurate, and I think it could help me find out the issues on my PCB without tearing down those components.

But now, st5s is still expensive to me, and I still wonder if this equipment deserves 300 USD, and then I found LCR Research Pro1 which could support 100kHz, which could have higher resolution when DUT is an inductor, and the cost is about 10% higher. The cost of Lite 1 is much acceptable, but it's not so accurate.

So I try to understand how does it work.
The First step is buying the cheapest tweezers and do some test.
HP-990B is a nice option. More exactly, this one is not LCR tweezers, the official name is small multimeter, that is specially used to measuring SMD(Surface Mounting Device), there are resistors, capacitors, Diode, Zener and LED. When I went through the manual, I think it's a good tool based on the price, many tested cases have been considered, such as relative measurement.
Electrical Specification:

The dynamic count is 4000, similar to a common used 3 1/2 handy DMM, there are 2 pieces of spare test pin and special test leads: electric rating 500V 10A, it's a fantastic design, add two probes could make it support more situations.
Note: It doesn't mean this meter could stand for 500V, please don't use it to measure high voltage. Max Input 36V DC/AC has been marked on the enclosure.

This is what it looks like, not bad. And I use my 0.1% calibration board to exam it, PS. the resistors are 0.1%, the other parts are stand components, it's too expensive to get 0.1% capacitor and inductors, and the size of those parts would be huge. Short is used for zero-ohm calibration, and open is used for zero Farad capacitor calibration.

Resistance spec is +/-(2.5% + 5 digits), and all of the resistor test results are within the scope of spec, as for capacitance, Relative function is very useful for the low capacitors, and the capacitor accuracy on my calibration board is about 5%, and meter spec is +/-(2.5% + 5 digits) or +/-(3% + 5 digits). But HP-990B cannot support 100uF, and the spec says the max range is 200uF. and the function for the diode(2mA) LED(1mA) and continuity check works well.

[Shannon]

We can find the main IC is COB package on the PCB, when we open the shell.

But you can find that this is a good mechanical structure design, with silicone anti-skid material around it to increase comfort, and the arc-shaped shell is also convenient to hold.

Some details are not perfect. We found a manual divider, two stacked resistors connected in series with another resistor, and other traces of capacitor modification,

The tweezer's arm at the end is directly welded on the PCB, with some traces of flux. I'm very curious about the material of this tweezer's arm. It should be stainless steel with a coating. It has high strength and can be directly welded on PCB. It is obvious that the tweezers do not use the four-wire Kelvin measurement method.


Guess that the integrated chip in the middle is a multimeter measurement chip, which integrates the measurement mode of resistance, capacitance, diode, etc. as for the function of LED, it seems that it is very rare on the multimeter, so we need to make a constant current source with 20V compliance voltage to output 1mA current to measure the voltage on the LED.

The input terminal can see mz4 series PTC, the resistance value is about 1.5k at room temperature, such a circuit is very common in the multimeter, usually used to protect the input terminal, when the voltage is too high, the PTC will heat up, the resistance value will increase, and the chip will be protected.

Under the automatic resistance measurement mode, the current flowing through the resistance to be measured is 0.177ma when it is less than 1kohm. When it is more than 1kohm, the current flowing through the resistance changes with the change of resistance value. When the DUT is 10K, the current is 20uA, and when the DUT is 100k, the current is 2uA. Therefore, at this time, the voltage at both ends of the resistance is 200mV. It looks like it is the same as the multimeter. It takes about 6S to measure the 10ohm resistance by switching the series-connected divider resistance and DC excitation. The value keeps increasing from 0 to 10ohm until it is stable.

Under the automatic capacitance measurement mode, I used an oscilloscope to measure the voltage waveforms at both ends of the capacitor. The waveforms in the figure are the waveforms when the DUT is 10nf.


It seems that the RC charging circuit is used to calculate the charging and discharging time, and I found one thing. This instrument supports the measurement of 100uF, but the test time is longer. It takes about 25s, my God, There is no movement on the screen until the test results are displayed. The capacitance test circuit is shown in the figure below. An RC OP-AMP oscillation circuit can calculate the capacitance value of capacitance C by measuring the frequency of the output square wave and the known resistance value
the calculation function is
f = 1/(2RC*ln(1 + 2R1/R2))
assume R1=R2, then f = 1 / (2RC * ln(3)) and ln(3) = 1.099
it's not hard to get the value of C


For the measurement of LED, I found that it can support the series connection of several LEDs, and the minimum excitation voltage can be up to 20V. But I didn't find any inductance. It seems that the charge pump is used to provide a higher voltage, and there are many large capacitors on the PCB.

[tautech]

100uF = No response ?  Really ?
Of uF values it can resolve, will it display ESR ?

All the good SMD tweezers can which negates the need to get a dedicated ESR tester.

[Electro Fan]

Nice post.

Did you make the calibration board or is that a commercial product?

[indman]

The First step is buying the cheapest tweezers and do some test.
HP-990B is a nice option. More exactly, this one is not LCR tweezers, the official name is small multimeter, that is specially used to measuring SMD(Surface Mounting Device), there are resistors, capacitors, Diode, Zener and LED. When I went through the manual, I think it's a good tool based on the price, many tested cases have been considered, such as relative measurement.

This is not a very good tweezers model! More successful in terms of functionality was the HP-4070C  model with an installed DTM0660L processor.
This processor allows you to measure capacitance values ​​up to 100mF (100000microF)

[Shannon]

100uF = No response ?  Really ?
Of uF values it can resolve, will it display ESR ?

All the good SMD tweezers can which negates the need to get a dedicated ESR tester.

In fact, 100uF can be measured, but the time is a little longer, which makes me think that there is no response. The actual measurement time of 100uF is 25 seconds

the capacitor measurement method is the same as a handy DMM, and there is no gear switching

yep, you are right, good SMD tweezers can measure ESR and many other parameters, this one is not so good, the price of this one on amazon is about $26

[Shannon]

Nice post.

Did you make the calibration board or is that a commercial product?

yep, I design the calibration board, and one Chinese friend helped with manufacturing,
0.1% 25ppm resistors, from 10ohm to 1Mohm
and the capacitor is not so accurate, I planned to use 1% capacitors, but then we found the capacitors are too expensive for few pieces, so we used 5% accuracy in this version

[Johnny10]

 

[Shannon]

The First step is buying the cheapest tweezers and do some test.
HP-990B is a nice option. More exactly, this one is not LCR tweezers, the official name is small multimeter, that is specially used to measuring SMD(Surface Mounting Device), there are resistors, capacitors, Diode, Zener and LED. When I went through the manual, I think it's a good tool based on the price, many tested cases have been considered, such as relative measurement.

This is not a very good tweezers model! More successful in terms of functionality was the HP-4070C  model with an installed DTM0660L processor.
This processor allows you to measure capacitance values ​​up to 100mF (100000microF)

You are right, HP-4070C is better than HP-990B, even though they all used DMM IC to measure resistor and capacitors.

By the way, how much did you speed for HP-4070C, It doesn't seem to have that much stock

[indman]

By the way, how much did you speed for HP-4070C, It doesn't seem to have that much stock

I do not understand what speed do you want to know? Please clarify.

[Electro Fan]

By the way, how much did you speed for HP-4070C, It doesn't seem to have that much stock

I do not understand what speed do you want to know? Please clarify.

That might have been meant to be “spend”.

[Shannon]

By the way, how much did you speed for HP-4070C, It doesn't seem to have that much stock

I do not understand what speed do you want to know? Please clarify.

FT... it's typing error
By the way, how much did you spend on HP-4070C, It doesn't seem to have much stock on amazon.

[Shannon]

This is a good LCR tweezer, if you only spend 26 dollars, it can measure the general resistance and capacitance requirements, and it seems to be more friendly to SMT devices.

Well, it's time to say the disadvantages

1. The structural designer does spend a lot of effort on the appearance and structure, but it's hard to say that this is a beautiful tweezer, and the quality is not very good. There is a dedicated box to hold it, but the box is not so good-looking

2. This is a SMT tweezer, but it can only measure the devices that have been soldered on the PCB. Because it is very easy to clip the chip components, even 0805, which gives people a very unstable feeling to operate it carefully, and the tweezer's arm is directly soldered on the PCB. This is a convenient way to fix it, but it is not a high-strength way

3. The tip of the tweezers is not so sharp. Every time you measure the device on the PCB, you always worry about not touching it or slipping it off

4. In the natural state, the opening distance of the tip of the tweezers is too short, which is estimated to be 7mm, so it is not convenient to measure larger devices

5. The measurement speed is too slow. It takes 7 seconds to measure 10ohm resistance and 25 seconds to measure 100uF capacitance. OMG, life is short

6. PCB quality is not very good, there are many traces of manual change

7. Without the Kelvin test method, holding the tweezers arm in hand will affect the measurement results. The display value is about several picofarads, which is not so important for the accuracy of the tweezers

8. The quality of the probe is not good, the spec is 500V 10a, I dare not use it to measure high voltage, I have been shocked, I know the taste

[Electro Fan]

Lots of HP-4070Cs on eBay (but don’t know about the sellers).

Another alternative on Amazon (but I haven’t compared the specs or performance vs the 4070C):
https://www.amazon.com/Mastech-MS8910-Resistance-Capacitance-tester/dp/B00F3RQCFW

[indman]

By the way, how much did you speed for HP-4070C, It doesn't seem to have that much stock

I do not understand what speed do you want to know? Please clarify.

FT... it's typing error
By the way, how much did you spend on HP-4070C, It doesn't seem to have much stock on amazon.

I spent about $14 on my HP4070C, but that was a few years ago. I will disappoint you a little - if you buy this model now, then 99% of you will receive exactly the same filling as the HP-990 model. HoldPeak has also started using chip caseless. But I know for sure that DM1106(upgrade DTM0660) is now installing UT-116C UNI-T into its model

[Shannon]

By the way, how much did you speed for HP-4070C, It doesn't seem to have that much stock

I do not understand what speed do you want to know? Please clarify.

FT... it's typing error
By the way, how much did you spend on HP-4070C, It doesn't seem to have much stock on amazon.

I spent about $14 on my HP4070C, but that was a few years ago. I will disappoint you a little - if you buy this model now, then 99% of you will receive exactly the same filling as the HP-990 model. HoldPeak has also started using chip caseless. But I know for sure that DM1106(upgrade DTM0660) is now installing UT-116C UNI-T into its model

DTM0660 or DM1106 is a widely used Handy DMM IC, so these tweezers belong to the same classes based on the DC measurement method.

Actually, when I got HP-990, I'm confused that if LCR tweezers (SMD tester) can't be like tweezers, and can't be like LCR meter as well, why don't we use DMM, I can play DMM two test probe as chopsticks
And I think UT-116C has a similar problem, the structure can't clamp an SMD component, and can't support inductor measurement as well, in my opinion.

So generally speaking, this type LCR tweezer(SMD tester) may be just a toy most of the time
Fortunately, it's not an expensive man's toy

It's just my idea, do you have similar confusion? 

[Shannon]

A real LCR instrument usually uses AC excitation to measure, because inductance and capacitance are more impedance characteristics than resistance characteristics. In other words, capacitance and inductance not only affect the amplitude of (AC) electrical signal but also affect the phase of the "AC" electrical signal.

Therefore, the impedance characteristics of the device to be tested can be determined by measuring the influence of DUT on (AC) amplitude and phase. The capacitance of the capacitor and the inductance of the inductor can be obtained through the impedance theoretical calculation formula of the capacitor and inductor.

In theory,
The impedance formula of capacitance is Z = 1 / JWC, w = 2pi * F
The impedance formula of inductance is Z = JWL, w = 2pi * F

R = V / I
Z(f) = V(f) / I(f)
If the (AC / DC) voltage on the DUT and the (AC / DC) current flowing through the DUT can be measured, the (impedance/resistance) of the DUT can be calculated

If you don't have an LCR meter on hand, but you have a signal source and an oscilloscope, you can use the resonance method to measure capacitance or inductance. The accuracy is not so high.

Auto balancing bridge is an LCR measurement method that is often used, as shown in attached image.

This structure of the measurement method can support to 100kHz, but in order to ensure accuracy, the signal source and voltmeter here will choose high-precision DAC and ADC, and with complex digital processing chip, they are expensive and inconvenient to use.

[Shannon]

A month ago, I knew the cheapest "AC measurement" LCR tweezers. The model is dt71, which costs only $70, so I bought one to measure the index and analyze the working principle.

But I'm a little disappointed that the dt71 measurement method is not so pure "AC". Let's see.

First look at the dt71 spec, and then compare with the test results. After self-calibration, the measurement of 0.1% resistance shows that the indicators are all within 1%, not including offset.


Compared with hp-990b, it has no function of relative measurement
DT71 is 3 digits LCR measure tweezers, the dynamic count is 999, and display count is not fixed, for example, the display value is 0.0ohm, when short the two probe.

the noise and stability issue should be taken into account, a part of that noise and drift MAY be caused by the quality of capacitors, but the main issues come from test equipment.
By comparison, resistance spec is within the claimed in the manual, for the capacitor specs is a disaster in my mind, noise count is large than offset spec, a little pitty.
Then it is found that a strange thing is that the data manual usually uses "AC" to measure the impedance will claim the frequency of the excitation signal used in the measurement results.

When using an oscilloscope to measure the signal of resistance gear, it is found that... The figure below shows the voltage waveform at both ends of the probe when measuring resistance. Then adjust the measurement mode to the capacitance gear and observe the waveform of the excitation signal. The excitation waveform looks like an AC signal, but it doesn't seem to be a pure AC signal. The figure below shows the excitation waveform of the capacitance.

The test waveform is very close to the capacitance measurement waveform of hp-990b, but the frequency of DT71 test waveform is much higher, and there are two different test frequencies. In essence, the measurement method of dt71's capacitance and inductance is still the same as that of hp-990b. The difference is that dt71 uses two different amplitude excitation signals, which can cover a wider measurement range of capacitance and inductance.

It can be seen from the waveform that the frequency of the excitation signal reaches 1MHz at the fastest time. An LCR meter that can support a 1MHz excitation signal is very expensive.

[Shannon]

It's time to summarize dt71

Advantages:

1. The price is cheap. You can buy commercial LCR tweezers for $70, which can support L, C, R, D, frequency, and voltage

2. Rich functions, in addition to the LCR function, can also realize signal generator, multimeter, and other functions

3. The structure is relatively novel, a bit like a compass. There are many smart designs in it, such as using a permanent magnet to ensure that the tweezers arm is open, using a bite structure at the front end to make the two tweezers arms align at the end, and so on

4. Using the popular touch button, the menu is simple, one-button can complete all operations

5. With calibration function, it is convenient to calibrate the accuracy of short circuit and open circuit

6. Support left and right-hand operation can automatically switch



Disadvantages:

1. The measurement of LCR is still based on a principle similar to DMM. Without a pure sine wave, it will lead to a difference in accuracy and stability. The most important thing is that there is no equivalent series impedance of capacitor and equivalent DC impedance of the inductor

2. There are many test functions, but there are still many problems to be solved. The THD of the output sine wave is very poor, and the frequency of the output sine wave cannot be adjusted

3. The structure is not so stable, the tip of the tweezers is difficult to align, and it is not very convenient to measure the SMD device, even if the SMD device has been welded to the circuit board

4. The key misoperation is serious, the screen display misoperation is serious, and the display is often reversed

5. You can't use it when charging. It's a stupid way. You have to separate your head from your body...

6. Kelvin measurement method is not used, and the accuracy in general, so it can only be used for the most basic use

7. The handle is poor, mainly because it is too light, so it is easy to touch the arms of the tweezers together

The figure below is the default alignment state of the tip of the tweezers. It's hard to imagine that it can be measured by clamping an SMD device

[fabriciofx]

Hi!

Where did you buy this callibration board?

At.te.

[Shannon]

Hi fabriciofx,

I designed this Calibration board by myself and manufacturing it by my Chinese Friends.
Those resistors are 0.1% 25ppm, caps are 5% (1% caps are expensive for few parts).

it's quite convenient.
If possible, I will manufacture some of them for those who are interested. 

[Shannon]

Smart tweezers st5s is a well-known commercial precision measuring tweezers. I believe it is a real pure AC inspired LCR meter, rather than the two products mentioned above. It's time to take a look at st5s, which is generally recognized as the best tweezers.

As I said, ST5S is a very high precision, and it's very convenient to use, so the price is beyond my plan. The price on Amazon is $311, and the shipping cost is $46. OMG. I thought it was very expensive many years ago, but it's still very expensive for me now

Finally, through the second-hand platform, I bought a second-hand st5s for $120. I thought it was quite worthwhile. Although it was not very good, the button cap was missing, and the tweezers arm was bent. It seemed that its owner didn't care about it. He tried to bend the tweezer's arm to make the tweezer's point aligned.

When I got this tweezer, I really liked its performance. It was very powerful and accurate. When I found that it supported ESR and |Z| measurement, I could confirm that this LCR meter really used pure AC signal for measurement, because if the test methods in hp-990 and dt71 were used, it was impossible to measure the ESR characteristics of capacitance, which was great, makes me know the quality of the capacitors.

The test frequency supports four frequency points: 100Hz, 120Hz, 1kHz, 10kHz, and supports the automatic identification of components. Dt71 also supports the identification of component types, but st5s can automatically adjust the frequency of the excitation signal to measure the device under test.

The main parameter display is standard 4 bits, so the display range is from 0000 to 9999, and the highest resolution is close to 0.01%. The subparameter index is also 4 bits, but there is no clear description of the performance of the subparameter. If the main parameter is very accurate, the accuracy of the subparameter is usually very high.

The chapter "how it works", clearly states that st5s uses Kelvin 4-wire measurement method to measure, and finally sees the necessary conditions to realize precision measurement.

It is worth emphasizing that st5s supports two kinds of excitation amplitude, which is very good because when measuring SMD components on PCBA, these components are usually connected with the pins of semiconductor devices. If the excitation amplitude is too large, the measured impedance is the result of connecting the diodes inside the semiconductor devices in parallel, so the excitation signal with low voltage amplitude is very useful, which does not exceed that of semiconductor devices At the same time, the parameters of the device can be measured. St5s supports excitation signals of 0.5vrms and 1Vrms, and the accuracy of excitation voltage is 2%.

[Shannon]

The resistance index is very good. There is a note below the index, which indicates that the range of 1 ~ 100ohm is minus the offset impedance. The best index is 0.2% + 3counts, and the accuracy under different frequencies is clearly marked, except for 120Hz, because the index of 120Hz is very close to 100Hz.



Compared with the test results and indexes, the resistance indexes are all within 0.2% + 3 of the best indexes, which is unbelievable. When testing the capacitance, in addition to the capacitance value, the equivalent RP or equivalent RS parameters of the capacitance can be obtained through the sub-parameters, and the quality of the capacitance can be detected. In addition, through st5s, it is found that the ceramic capacitance will change in the measurement process, which should be due to the piezoelectric effect of the ceramic capacitance, resulting in a small change in the capacitance value. St5s can detect this slow change phenomenon.


The figure below is the waveform of the excitation signal. The device to be tested is the probe of the oscilloscope. The input capacitance is about 10PF. The default test frequency is 10kHz. The waveform is a very beautiful AC 10kHz 0.5Vrms sine signal.
The internal circuit looks very complicated, salute to the designers.

[Shannon]

ST5S summary:
This is the best commercial LCR tweezers I have ever used

Advantages:
1. Very, very accurate
2. The function is very powerful, which can be compared with the standard LCR meter
3. Support Kelvin measurement
4. Compact design

Disadvantages:
1. A little expensive
2. The structure design is the best LCR tweezers I have ever used, but there is still a gap between real tweezers
3. Although it is very accurate, it does not support self-calibration, which may be a disadvantage

[tautech]

For 15 years I have used the earlier ST3 and even back then the cost was a disincentive to purchase although with a quality tool the initial cost hurt is now long forgotten. They are my goto component tester as with that cost also provides you an ESR tester ....another piece of equipment you don't need to buy. 

Some think tweezers are only suitable for SMD and to that I say BS as the tweezer legs can be spread to cover most through hole devices.

Would I buy another ? In an instant !