EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: bobcat2000 on September 09, 2020, 06:06:43 pm
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Good morning,
I am shopping for a mini amp. Amazon has a bunch of these. One of this Fosi says it won't spark when you plug the dc jack to the back. I see a video in youtube that the so call spark is more like spark-flying-all-over-the-place-that-can-burn-your-finger kinda spark.
I am wondering what kind of "Protection Circuit" Fosi put in the amp to prevent it to spark or what make it spark in the first place.
Would you guys teach me please? Thanks!
This is the link to that amp.
https://www.amazon.com/Bluetooth-Audio-Amplifier-Receiver-Integrated/dp/B076XSBCCL?ref_=ast_sto_dp (https://www.amazon.com/Bluetooth-Audio-Amplifier-Receiver-Integrated/dp/B076XSBCCL?ref_=ast_sto_dp)
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Assuming the claim of no sparking is true, it’s simple: they didn’t put in much, or any, capacitors on the power rail. (Well, I guess it’s possible they did, but with an inline resistor to limit inrush current.) The amp IC itself doesn’t draw much current when idle. But the beefy power supply capacitors that one should have will instantaneously draw a ton of current to charge up initially. (A class D amp like this can easily have 3-5A current pulses while operating at an average current of under an amp, so you either need big caps or a rather oversized power supply.)
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...But the beefy power supply capacitors that one should have will instantaneously draw a ton of current to charge up initially.
Thanks!
I see. So, this is the capacitors inside the amp that are drawing too much electricity from the dc adapter.
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Well I wouldn’t call it “too much”, insofar it’s their job to charge and discharge quickly. All it means is that one should avoid plugging in the DC plug with the AC adapter powered up.
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The power stage's bulk capacitance can only be reduced to a certain level before it causes adverse effects on performance. It could just have a soft-start circuit on the input, which could be as simple as a transistor with a resistor and capacitor driving the gate so it takes a bit of time to turn on when voltage is first applied. Or it could be a transistor that's used to turn on the output stage (including its bulk capacitors) from the MCU/bluetooth SoC at some point after power is first applied.
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EDIT: woops sorry, I was thinking jacks on the front, not the power jack on the back. You can ignore most of this post. Inrush current is the cause (just like other people say), a series resistor (might not even have to be an NTC type) would be enough to prevent the problem; or they could micro-manage the voltage delivery with a microcontroller if they really wanted to spend more money. Regardless the sparking should be harmless as long as you don't intentionally try to make a bad connection; I wouldn't worry about wanting this feature.
I see a video in youtube that the so call spark is more like spark-flying-all-over-the-place-that-can-burn-your-finger kinda spark.
You will not have massive sparks and burn your finger unless your building wiring or power supplies are already dangerously faulty.
You may see sparks when inserting/removing TRS jacks because of their "short everything on insertion" design, but all devices are designed to handle this. It's unavoidable with this connector style.
You can and will see tiny sparks when connecting devices powered by isolated switchmode power supplies (I first noticed this on DVD players). They leak a little bit of high-frequency AC through class-Y capacitors. This is designed to be a harmless amount of electricity.
I am wondering what kind of "Protection Circuit" Fosi put in the amp to prevent it to spark or what make it spark in the first place.
Resistors on the inputs make the most sense. Cheap, easy, effective.