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I think sharing the video will be the easiest way of describing the situation.

The transistors are electrically isolated from the heatsinks, I have checked with multimeter.

Few days ago, I have asked this question, it was related to the same circuit. There were answers and comments helped me, however, I think there might be a more descriptive answer, and wanted to focus this question on the reason for this behaviour.

I observed that, if I touch the heatsinks, as long as I keep contact, the noise disappears, also grounding the heatsink solves the problem. But, why an already electrically isolated metal piece creates noise? Some kind of radiation?

For my previous question, there were answers which states after isolating the heatsink, it is not a must to ground it. Which, comes not totally true, as I understand from my experiments.

Thanks in advance.

muyustan
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The transistor collector metal exposed at the back, the metal heatsink, and the insulation between them means you built a capacitor. As the heatsink is isolated it is very high impedance node so even a small amount of capacitance between collector and heatsink will couple the collector waveform to heatsink. An oscilloscope is also quite high impedance measurement tool, especially when used with a 10x probe, so it will only slightly add capacitive and resistive load to the heatsink. When you touch the heatsink you also add capacitive and perhaps resistive load if you touch ground so the waveform amplitude is less when touching the heatsink.

Justme
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  • But the problem is not being less in amplitude, by touching, I somehow omit the noises, distortions etc. – muyustan Dec 25 '19 at 09:39
  • OK, so in this case the circuit has issues due to stray inductance from long wires and stray capacitance from the breadboard, and so grounding/touching the heatsink actually adds loading capacitance to transistor collector and it removes the issue that causes distortion. – Justme Dec 25 '19 at 10:28
  • hmm, so, implementing the circuit by soldering properly should solve these – muyustan Dec 25 '19 at 11:17
  • It might, if component placement, layout design, etc are properly done. – Justme Dec 25 '19 at 11:25