Products > Test Equipment

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

(1/13) > >>

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 ?  :wtf: 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:

--- Quote from: Shannon on January 31, 2021, 05:21:41 pm ---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.

--- End quote ---
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) ;)

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod