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schematic

simulate this circuit – Schematic created using CircuitLab

An old bicycle headlamp + battery pack. The batteries were 3.7V li-ion but broke down. I don't know how were they connected in the pack thus assume serial connection (output voltage 3.7V nominal).

I connected the light to a wall brick (old, there's a bulky transformer) of specification

Output:
3.7V 355mA

The torch has several modes of shining. When is shines, there's noise coming from the light (not the wall brick) with volume proportional to the brightness. There's a blinking mode which makes is obvious the light emitting circuit generates the noise.

Assuming power ripples, I inserted a filter in the power loop (2.2mF electrolytic, a tiny ceramic in parallel and two inductors of 680uH +-10%). Noise being present even at the lowest brightness setting should have tipped me off that it's not this.

No change. Any ideas how to power the torch in a home environment? It shines bright and I would hate to toss it.

Vorac
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    Easiest way is to get an AC-DC 4V wall wart. 3.7V lithium batteries are 4.2V at peak charge. Not sure if you can find a 4V one though. You *might* be able to get away with 5V but I guarantee nothing. – DKNguyen Mar 14 '21 at 06:12
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    It's possible that the noise you are hearing from the lamp will be present even when powered by a battery. What is the circuit diagram of the low-pass filter that you placed at the output of your wall-brick? Add a drawing. – HypeInst Mar 14 '21 at 06:15
  • @HypeInst thanks, I added something (but don't know how to edit it to indicate V1). The light did not buzz when running with it's original battery pack. – Vorac Mar 14 '21 at 06:26
  • "The light did not buzz when running with it's original battery pack" - that is really surprising... – Solar Mike Mar 14 '21 at 07:18
  • @Vorac your low-pass filter's roll-off is into the audio range. Does your torch make the noise at full power? – HypeInst Mar 14 '21 at 20:59

1 Answers1

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Step down regulators going to step-up regulators are notorious for acoustic and electrical noise.

I had one that was chaos or white noise and the ferromagnetic noise was so loud from the little SMD inductor in a noisy lab, it sounds like a water faucet.

The cure was a low ESR e-cap like the low ESR (<50 mOhm) of a Li Ion battery. But beware not to make it too big as the surge current and overshoot of the buck can be really bad.

Tony Stewart EE75
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  • Hmm, so you suspect `355mA` is insufficient? But the torch misbehaves even at the lowest load! Is there a way to test your hypothesis? I have access to the filter and an oscilloscope. – Vorac Mar 15 '21 at 03:36
  • @Vorac when the torch is at its lowest brightness, its power-modulation circuitry is most active. When the torch is at its maximum brightness, the power-modulation is essentially OFF. So in order to determine if the torch's power-modulation circuitry is generating the audible noise, simply turn the torch to its highest continuous brightness setting. If the noise disappears, then the torch's circuitry is the culprit. – HypeInst Mar 15 '21 at 17:29
  • @HypeInst "there's noise coming from the light (not the wall brick) with volume proportional to the brightness" – Vorac Mar 15 '21 at 19:08