High negative voltage level on scope

Using Arduino General Electronics
1

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.

schema3
schema31920×1350 48.3 KB

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

IMG_1312
IMG_13121920×2560 465 KB

-67V with large scale:

IMG_1308
IMG_13081920×2560 357 KB

12V:

IMG_1310
IMG_13101920×2560 358 KB

Thank you for your helps,

Orkun Gedik

2

  • With only 12v and 5v seen in the schematic, 67v implies a grounding issue.

  • Confirm when output goes LOW the scope shows a solid LOW.

  • Show us good images of your actual wiring.

  • If you make the output go HIGH and stay HIGH, what is Vce with a volt meter ?

  • What does 3v3 measure ?

1 Like
3

It appears that the scope probe is not grounded to Arduino GND.

1 Like
4

I forgot to say. Probes attached to motor pins which drives with 12V+

5

Hi LarryD,

Thank you for your response. Please find the answers below;

  1. Good point. I use regulator circuit and its output is fine.
  2. Yes I can confirm.
  3. Ehm... Ok I will send a post for this.
  4. Vce ~12v on motor pins and 3v3 on D3
  5. Can you clarify little bit more :slight_smile:

Thank you.

6

Is that 1N4001 diode soldered directly across the motor terminals?

1 Like
7

You have learned something about the dramatic electrical noise generated by brushed DC motors.

See this tutorial on reducing it: Pololu - 9. Dealing with Motor Noise

1 Like
8

If D1 is bad (or missing or not connected) you WILL get a high-voltage negative spike when the motor switches off (flyback diode).

The flyback diode should be rated for at-least the maximum motor current (i.e. the "stall current").

1 Like
9

No. Have little bit distance. Around 10cm.

10

I will to re-solder diode. Then reply you back. Bu it is weird that even if I remove motor from device and attach probes on terminals, I see same result.

11

What do you mean by device?

12

Circuit I mean. Sorry :roll_eyes:

13

OK but what is this?

9a7f1
9a7f11920×1350 66 KB

14

ams1117-regulator-modulu-5v-komponent-samm-1232-41-B
ams1117-regulator-modulu-5v-komponent-samm-1232-41-B700×700 53.9 KB

This one:

https://tr.aliexpress.com/item/1005004871263507.html?spm=a2g0o.productlist.main.3.28255d2a1dbGZz&algo_pvid=6e33f82e-48c4-4f28-ae11-79304ad472a1&algo_exp_id=6e33f82e-48c4-4f28-ae11-79304ad472a1-1&pdp_npi=4%40dis!TRY!6.64!5.78!!!0.23!!%402101e9d216928902182254181e39d9!12000030830897174!sea!TR!4274059567!&curPageLogUid=zU7YFlp8FgkF

15

Again OK.
I noticed from he scope that the intererence seems to be at 50Hz.
You seem to be picking up intererence from the AC mains. If you connect your scope ground to the circuit ground and measure to either terminal on the motor, do you still see the -67 spikes?

16

If you take diode off you will have both negative and positive voltages, that voltage is produced by DC motor, put a capacitor 100 nF in parallel to motor terminals.
https://www.youtube.com/watch?app=desktop&v=O6GTZS10-us

Which terminals, show that on schematic.
Do you have spikes when motor is not running?

1 Like
17

Yes, I am getting huge negative spikes. This make me confused actually

18

Hmmm. I conclude that as you and jremington mentioned that this is a grounding issue, if I am correct. This is because when I set D3 pin low, no ground connected to negative motor/probe pin. And thus I see spikes on scope. On the contrary, when I set HIGH on D3, ground connect to motor/probe GND flows through TIP122 and can see stable +12V.

At this stage the question arise into my mind that this huge negative voltage/spikes make problem? Or do you have further suggestion? And why mutlimeter do not show negative spikes with same terminals?

Thank you so much for your helps :slightly_smiling_face: Learning phase is painful but I enjoy with it too much

Orkun

19

I will try this. Thank you tom321

20

No it's not a problem you are just using your scope wrong.

And why mutlimeter do not show negative spikes with same terminals?

Most multometers can only read DC and 50-60Hz AC signals