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

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Shannon:

--- Quote from: indman on February 02, 2021, 04:42:17 pm ---
--- Quote from: Shannon on February 02, 2021, 03:33:04 pm ---
--- Quote from: indman on February 02, 2021, 03:15:28 pm ---
--- Quote from: Shannon on February 02, 2021, 02:47:11 pm ---By the way, how much did you speed for HP-4070C, It doesn't seem to have that much stock

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

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

--- End quote ---
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

--- End quote ---
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 :palm:
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?  >:D

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 8)

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.

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