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SN74HC14: The indicator light cannot go off completely

Weijie Gao
Prodigy 10 points
Part Number: SN74HC14

Hello, the following is the material test situation, please analyze the reason, thank you

Test conditions: add 77-110v dc voltage signals to 110VG (x1-dbz26) of 110VCS (q-1) using the regulated power supply. Indicators 1a-4b on the panel are on. When the voltage is lower than 60V, indicators 1a-4b is off

Abnormal situation: the 4A indicator light on the panel cannot be completely extinguished, and the voltage between the measuring chip and the ground is about 1V

1. Replace NXP brand materials and the fault disappears

2. Replace the samples provided by the original TI factory, and the fault disappears

3. Replace batch 2115 chips and the fault disappears

In addition: There is no problem that the two front screen prints are vertical lines. There is a small round pit in the front of the batch 2117+. Please see what is the difference

  • 0
    TI__Guru* 96591 points

    Hello,

    Weijie Gao said:

    Test conditions: add 77-110v dc voltage signals to 110VG (x1-dbz26) of 110VCS (q-1) using the regulated power supply. Indicators 1a-4b on the panel are on. When the voltage is lower than 60V, indicators 1a-4b is off

    Abnormal situation: the 4A indicator light on the panel cannot be completely extinguished, and the voltage between the measuring chip and the ground is about 1V

    The voltages and errors you listed seem unrelated to the HC14. It does not take in 110V and does not include any LEDs. I assume you are referencing your system, which I do not have any details on. I need you to explain the issue with the SN74HC14, not your system.

    -

    The device was recently updated to a new process, so there are some very minor differences between the new material and the old.

    In order to help troubleshoot the issue, I need to learn more about the application of the device. How is it used?

    Please give me details of the operation and issue surrounding the SN74HC14.

    Exactly what issue(s) is(are) observed _at the SN74HC14_? Are the input and output signals the same as a 'known good' device? If not, what issue is observed?

    Show a schematic of the device with all input and output connections (including any device, cable, connector, or PCB trace the SN74HC14 connects to)

    Show a scope shot of the "good" input and output signals.

    Show a scope shot of the "bad" input and output signals.

  • 0 in reply to Emrys Maier
    Prodigy 10 points

    Description of fault phenomenon:

    When sbbak1 channel sends 110VDC, LD7 indicator light is on. After the 110VDC is removed, LD7 should be off, but actually LD7 is still on (brightness is halved), and the two pins of d8 are about 2.3V. The voltage of pin 1 is about 1V to the ground. Abba test is also abnormal for this channel.

    Troubleshooting:

    1. Replace NXP brand materials and the fault disappears

    2. Replace the sample provided by Ti, and the fault disappears

    3. Replace 2115 batches of chips and the fault disappears

    4. From the fault phenomenon, the chips in question are 2117 batches, and the silk screen logo is the one with a circle  

    order number

    T01397751

    If the problem still cannot be solved, can you send it back to the original factory for testing? It is suspected that there is a quality problem in this batch.

  • 0 in reply to Weijie Gao
    TI__Guru* 96591 points
    Weijie Gao said:
    If the problem still cannot be solved, can you send it back to the original factory for testing? It is suspected that there is a quality problem in this batch.

    You can start an FA with your supplier -- I cannot help with returns here, only troubleshooting the technical issue.

    Weijie Gao said:

    Troubleshooting:

    1. Replace NXP brand materials and the fault disappears

    2. Replace the sample provided by Ti, and the fault disappears

    3. Replace 2115 batches of chips and the fault disappears

    4. From the fault phenomenon, the chips in question are 2117 batches, and the silk screen logo is the one with a circle  

    order number

    From the above description, it sounds like you haven't done any checking of the actual issue at the component level. Your troubleshooting steps indicate a system-level issue and solution, but do not describe a device-level problem. For me to troubleshoot, I need to know exactly what's going wrong with the device -- ie how is SN74HC14 failing?  

    Please test operation of both a good and failing device -- we prefer to see scope shots of operation so we can analyze what is going wrong.

    -

    I can tell you where I would start -- from the schematic, it looks like the inputs have a much longer discharge (2x) than their charge rate, and the change from the old material to the new may cause a change in the delay here.

    -

    This is the equivalent circuit for the input shown:

    Here you can see that when the BJT is on, VX ~= VCC, the input voltage will eventually charge to VCC, so VIN = VCC after 5*R*C = 5*4.7k*4.7u ~= 2.2 ms

    When the BJT is off, VX will discharge to 0V after 5*R*C, but R here is 4.7k + 4.7k = 9.4k, which means discharge takes about 4.4 ms.

    When changing from one material to another, the input thresholds will vary, which can cause a variation in delay at the output, and may be the source of your failure.

    I cannot be sure - but this is where I would start looking. I'd like to see scope shots of the known good input / output, and the failing input / output.