That is approximately what it measure in circuit. I can pull it an measure it accurately tonight.
Does anyone know what IC U7 is? It is not the SDB628 mentioned above; the pin wiring does not match.
U7 is very similar to SY7208CABC in the location of the contacts, but this cannot be said with 100% certainty, because the HYDSI or HYDWE marking does not appear anywhere yet.
P.S Here is an even more likely interpretation of U7 - this is AN_SY7200A (marking HY6VE). If you believe the documentation for this chip, then the output current is calculated by the formula I=0.2/R1. If R1 = 40 ohms, then the test current I=0.2/40=0.005A, that is, 5mA.
Now it remains to solve the riddle from Chinese friends -why does this circuit not withstand a given R1 current and allows it to conduct a much higher current?
I recently purchased one of these so-called "improved" LCR-TC1 transistor tester units. U1 (MCU) is mislabeled as a Mega328. The display shows "FNIRSI" when performing the self-calibration. This tester contains the U7 circuit (as discussed in this thread) to generate 30V DC for testing zeners.
For me the zener test feature worked properly a couple of times without excessive zener current, but then it failed in the mode where the zener test current isn't limited.
After it failed I measured up to 500mA into a short circuit placed on the K and A terminals. I did this test very briefly. Fortunately conducting this short-circuit current test one time did not cause any additional failures. The U7 circuit continues to generate 30V (but without any current limiting). All other functions of the tester still work OK.
I believe the failure occurred when I connected an 11V zener across the K and A test terminals
while the tester was already powered up.
Here is what I suspect causes U7 to fail:
1) When powered up with an open circuit on the K and A terminals, U7 continuously generates the full 30V DC output. The filter capacitor on the 30V rail, although small, still holds a substantial amount of energy when charged up to 30V.
2) Connecting a component across K and A can immediately dump the entire 30V directly into the current sensing pin of U7, causing this pin to permanently short circuit.
3) According to the U7 datasheet the current sensing pin is only capable of handling 4V
To prevent the failure of U7 I intend to experiment with placing a resistor of perhaps 4K7 or 10K between the high end of the 40 ohm sensing resistor and the current sensing terminal of U7. Hopefully this will prevent failures of U7. The worst case test will be to have the tester powered up and then apply a short circuit between K and A.
U7 is intended for use as a current-limited LED driver. In this application the string of LEDs is
permanently connected. Therefore the current limit will activate gradually as the output voltage rises soon after power up. In this case the feedback loop is always closed, preventing excessive voltage from reaching the current sensing pin of U7.
But in the transistor tester application, the output voltage rises to the full 30V when K and A are open circuit. The output voltage rises to 30V because the current-limiting feedback loop is open. Then, when a short-circuit or low resistance is applied across K and A, the entire 30V is instantly dumped into the current sensing pin of U7. This is what damages U7.
I will post my results after testing. First I must obtain a supply of replacement U7 chips. I will try to get several types that have the same pinout, including the SY7200A.
-EB
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