High negative voltage on scope

[ogedik]

Hi,

I have a question about high negative voltage on osilloscope. I am trying to start and stop control DC motor with TIP122 transistor by signaling D3 pin on ESP-C3-13 kit. There's no problem when I set HIGH D3 pin and scope displays ~12V without any noise. Things getting weird when I set LOW D3 pin. When I set D3 to LOW, scope show -67 V with unstable graph.

I checked FFT graph and found out many noise inside the signal, but it is an easy circuit and shouldn't have kind of noise. If I face with noise I should see same problem at FFT graph on +12V too. Am I correct?

So, can anybody make a comment why I am facing -67 V with unstable graph when I set D3 pin to LOW?

As an additional information, multimeter measure 0 voltage level whereas I see that high negative voltage level on scope when D3 is LOW.

Please note that scope probes attached to motor pins. Even if I drop motor from circuit, I face with same problem.

Please find scheme and scope outputs.

-67V with large scale:

-67V with large scale

IMG_1308.jpg (2.56 MiB) Viewed 2310 times

-67V with FFT (FFT graph is red):

-67V with FFT (FFT graph is red)

IMG_1312.jpg (3.1 MiB) Viewed 2310 times

12v stable:

12v stable

IMG_1310.jpg (2.46 MiB) Viewed 2310 times

Thank you for your helps,

Orkun Gedik

[ogedik]

Schema is here:

schema3.jpg (751.61 KiB) Viewed 2307 times

Hi,

I have a question about high negative voltage on osilloscope. I am trying to start and stop control DC motor with TIP122 transistor by signaling D3 pin on ESP-C3-13 kit. There's no problem when I set HIGH D3 pin and scope displays ~12V without any noise. Things getting weird when I set LOW D3 pin. When I set D3 to LOW, scope show -67 V with unstable graph.

I checked FFT graph and found out many noise inside the signal, but it is an easy circuit and shouldn't have kind of noise. If I face with noise I should see same problem at FFT graph on +12V too. Am I correct?

So, can anybody make a comment why I am facing -67 V with unstable graph when I set D3 pin to LOW?

As an additional information, multimeter measure 0 voltage level whereas I see that high negative voltage level on scope when D3 is LOW.

Please find scheme and scope outputs.

-67V with large scale:
IMG_1308.jpg

-67V with FFT (FFT graph is red):
IMG_1312.jpg

12v stable:
IMG_1310.jpg

Thank you for your helps,

Orkun Gedik

[Craige Hales]

check the Q1 part number. Double check the pinout.
The scope ground must be connected to the circuit ground (not a motor pin.)
https://electronics.stackexchange.com/q ... transistor hints the 10K resistor might be too big.

Scope probe to the base on Q1:
It's been a long time, but I think it has to be high (5V) to turn it on and low (gnd) to turn it off.

Scope probe to the collector:
if Q1 is on, the collector should be near ground, not 12V.
if Q1 is off and the motor provides a path to 12V, the collector should be near 12V.
if there is no motor installed, I think you might expect to see noise when Q1 is off.

[ogedik]

check the Q1 part number. Double check the pinout.
The scope ground must be connected to the circuit ground (not a motor pin.)
https://electronics.stackexchange.com/q ... transistor hints the 10K resistor might be too big.

Scope probe to the base on Q1:
It's been a long time, but I think it has to be high (5V) to turn it on and low (gnd) to turn it off.

Scope probe to the collector:
if Q1 is on, the collector should be near ground, not 12V.
if Q1 is off and the motor provides a path to 12V, the collector should be near 12V.
if there is no motor installed, I think you might expect to see noise when Q1 is off.

Hi Craige,

Thank you for your precious information. Very informative for my learning path.

Best regards,

Orkun Gedik