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I am a service engineer for a company that manufactures lab equipment. I want to ask for electrical engineering help with a problem.

We make a product that consists of two parts, one of which is a heatable water bath.

I have a customer who since he has had the unit, has had a problem where the instrument emits a loud noise only when the heating elements are turned on.

I took a microphone to the instrument hooked up to my scope on FFT, and measured the noise to be at 3.1 kHz. The heating control PCB and power supply PCB are both located in the heating bath part of the unit. The bath contains 2x 750 Watt resistive elements in parallel.

In order to resolve this, we replaced the heating bath with a new one, assuming the noise was coming from one of the PCBs. The customer plugged the new one heating bath in and it is still making the noise when the heating elements are on.

I took the old heating bath home, and when I plugged it in I do not hear the noise. The heating bath has resistive elements which are controlled by a triac off of 110 V.

The only variable that makes sense is that the noise is caused by something with the mains power supply in his lab, If anyone has any insight it would be much appreciated. Could the mains 110 V in a lab be coming from a buck/boost converter which is exhibiting audio noise at high current demand? The origin of the noise seems to be from the water bath and not the outlet.

What could cause a loud 3.1 kHz tone to be produced when a heating element is powered on in one environment, but not in another? The rest of the instrument does not contain any electrical components related to the heating circuit.

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    \$\begingroup\$ Is the device powered by real mains voltage, or via some equipment like UPS or inverter? \$\endgroup\$
    – Justme
    Apr 28 at 15:39
  • \$\begingroup\$ Thanks for the comment and suggestion... I am not sure on the wiring before it reaches the outlet, but from the outlet is directly wired to the heatingbath, I was thinking of going back to the lab with my scope to look at the waveform coming from the outlet, but I can also ask the professor to reach out to the facilities dept and inquire how their outlets are wired and see if they are using any sort of inverter. I'll update when I get more information. Thanks! \$\endgroup\$
    – user284206
    Apr 28 at 15:51
  • \$\begingroup\$ Hi Justme, you are correct, the client confirmed he was using a UPS, when he used the regular outlet the noise was not present. \$\endgroup\$
    – user284206
    Apr 28 at 17:59
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It's possible the noise originates in the heating elements. Triacs, depending on the way they are driven, can turn on sooner or later in the cycle, if they turn on later (relative to the zero-crossing) there is a sharp increase in current at double mains frequency, and an attendant force impulse from the magnetic fields that can stimulate mechanical resonances.

If your heater construction is subject to that kind of resonance, then you could have audible noise. Incandescent bulbs controlled by triac dimmers can emit noise for this reason. There is a fundamental frequency at 120Hz or 100Hz, but also a relatively high frequency pinging that results from the mechanical resonance.

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  • \$\begingroup\$ I would think there would be an additional piece of the puzzle to explain why the audio noise is not produced when I power it on at home, versus when it is powered in a lab? \$\endgroup\$
    – user284206
    Apr 28 at 16:08
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    \$\begingroup\$ Yes there would be. Do you have any ideas what is different? \$\endgroup\$ Apr 28 at 16:09
  • \$\begingroup\$ The customer just informed me that the outlet he was using was wired to a UPS, when he used a normal outlet the sound went away. Thanks for your help! \$\endgroup\$
    – user284206
    Apr 28 at 17:59
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Thanks for all the helpful feedback. The client just indicated that he was using a UPS with the instrument and when he used a normal mains outlet the audible noise was not present. The audible noise was therefore caused during high current draw of the heaters (2 x 750 Watt in parallel) when powered by the UPS.

Reading everyone's answers, the actual cause of the noise is likely due to timing issues with the triacs from overloading the UPS resulting in mechanical resonance, or a switching frequency from the UPS when it is under high load.

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  • \$\begingroup\$ The output of some UPSs is more like a square wave than a sine wave. Perhaps you could find out what type the customer is using and add some advice to the instruction manual. \$\endgroup\$ Apr 28 at 18:23
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Obviously, the power supply causes or produces the 3.1kHz noise. Maybe the power supply is overloaded when the cold heater draws a high current surge until it gets hot.

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  • \$\begingroup\$ The heating cycle works through hysteresis so the elements are only powered when the temperature is below the set point, then turn off during the overshoot then back on again when the T drops below the set point. I agree that the extra current demand of the heating elements being on, 2x 750 W in parallel, is likely loading the power supply and causing the noise, but he is located in an industrial lab and I am in a residential home. I don't know enough about potential mains wiring differences in the two different environments to know why they would be different. \$\endgroup\$
    – user284206
    Apr 28 at 15:27

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