Electronics > Repair
Help identify glass encapsulated diodes
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晓看风云:

--- Quote from: Kim Christensen on August 26, 2024, 04:57:46 pm ---
--- Quote from: 晓看风云 on August 26, 2024, 06:41:23 am ---After using your testing method, I further validated this result. When the voltage is 1V, my power supply displays 0.000A, and the current remains at 0.001A at any stage between 2-15V. I think this should indicate that it is a 1mA current limiting diode and is working completely normally!
--- End quote ---

Though the current meter on your power supply doesn't have enough resolution, it appears to be working as expected. If both diodes check out the same, you can put them back in circuit if you haven't done so already.

Looking at your circuit again, it looks like the transistor circuit is a current buffer for the LT1007 Op-Amp. I assume the output load is connected at the junction of R19 & R22. If the circuit is operating properly, the voltage at R19 & R22 should follow the voltage at the output of LT1007 (pin 6), within a slight error, while connected to an appropriate load:



--- End quote ---
I have tested both and the results are the same. They work normally. So I have already put them back into the circuit board. As you said, the point between R19 and R22 is the output point, which ultimately reaches the relay contact and then the output terminal. Is the function of this current buffer to increase the current load?
晓看风云:

--- Quote from: fzabkar on August 26, 2024, 05:06:34 pm ---I expect that the junction of R19 and R22 would be connected to pin #2 of the op-amp, possibly via a potential divider. The output would then follow the input at pin #3, scaled down by the divider.

--- End quote ---
You are right, I roughly remember that the input was a signal from the MCU! Now I have encountered a new problem. The Q23 component with XL silk screen printing does not measure properly, and the voltage drop between pin 1 and pin 2 of the diode is 0.060V. I found an NPN transistor with the model number 2SD2704K on my search, but I'm not sure if it's correct. I noticed that there are several XL screen printed components on the circuit board, and they measure the 1st and 2nd pins online to be 0.060-0.070V. Even a few have pins 1 and 2 directly connected together on the circuit board. I am very confused, this is completely different from the NPN transistor I know.
Kim Christensen:

--- Quote from: 晓看风云 on August 26, 2024, 05:45:28 pm ---The Q23 component with XL silk screen printing does not measure properly, and the voltage drop between pin 1 and pin 2 of the diode is 0.060V.
--- End quote ---

R19 (22 \$\Omega\$) is across Q23's pins 1 & 2 (b-e junction) and therefore will make Q23 appear to be faulty when doing a diode check in circuit.

R22 will do that same for a diode check on Q24's b-e junction.

You have to interpret in-circuit measurements of components with caution. Other components connected to them can give false results, making you chase ghosts.



--- Quote from: 晓看风云 on August 26, 2024, 05:34:50 pm ---As you said, the point between R19 and R22 is the output point, which ultimately reaches the relay contact and then the output terminal. Is the function of this current buffer to increase the current load?
--- End quote ---

If it's driving a relay, I suspect it's one of those single coil, set/reset, relays that require a bipolar pulse to turn off and on. Yes, the transistors increase the current capability of the LT1007 because it can't drive the relay load directly.
晓看风云:

--- Quote from: Kim Christensen on August 26, 2024, 05:58:31 pm ---
--- Quote from: 晓看风云 on August 26, 2024, 05:45:28 pm ---The Q23 component with XL silk screen printing does not measure properly, and the voltage drop between pin 1 and pin 2 of the diode is 0.060V.
--- End quote ---

R19 (22 \$\Omega\$) is across Q23's pins 1 & 2 (b-e junction) and therefore will make Q23 appear to be faulty when doing a diode check in circuit.

R22 will do that same for a diode check on Q24's b-e junction.

You have to interpret in-circuit measurements of components with caution. Other components connected to them can give false results, making you chase ghosts.



--- Quote from: 晓看风云 on August 26, 2024, 05:34:50 pm ---As you said, the point between R19 and R22 is the output point, which ultimately reaches the relay contact and then the output terminal. Is the function of this current buffer to increase the current load?
--- End quote ---

If it's driving a relay, I suspect it's one of those single coil, set/reset, relays that require a bipolar pulse to turn off and on. Yes, the transistors increase the current capability of the LT1007 because it can't drive the relay directly.

--- End quote ---
The Q23 here is not on the circuit board I provided, I just wanted to facilitate understanding and let you know that it is a switch tube starting with Q. In fact, it is a silk screen printed XL component, and the measurement is also very outrageous when removed, which is completely different from the diode characteristics of ordinary NPN transistors such as BE or BC. So I don't know if the 2SD2704K I found is correct.
Kim Christensen:
That's the problem with surface mount components. XL can mean many things.
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