Monday, July 26, 2021

When off is not off, or "why is my amplifier pulling 30mA when it's off?"

 I have a little shed out the back with a 100W solar panel on it and a 13.8v battery. Right now it's just running a Raspberry Pi 2 powering an ADS-B decoder which works pretty well.

So then I added a little bluetooth audio amplifier from Fosi - . It's small, it's relatively efficient, and it's dumb as bricks to use.

But then I discovered something amusing.

It draws like 30mA even when turned off.

Here is the current meter when plugged into the battery side, but with it turned off.

.. and then I disconnect the power to it entirely, and  ...

High levels of what the heck.

So, I did what I always do when I find silly electronics questions - I disassembled it!

Here's the active power islands when it's turned off.

The MOSFET at the bottom is an IRF part that's .. always on. Then every other thing circled? That's also always on. That includes what look like opamp bits for the bass/treble filtering, and something op-amp-y feeding into the bluetooth audio offload PCB (that blue module.)

So what's the switch switching? Well, here's what's on the reverse of the PCB, showing the two power switch pins:

Note that one of those power pins is grounded. Yes, it's just some logic line feeding a silicon switch somewhere. And it's definitely not switching the IRF MOSFET at the beginning of the circuit - that'd be too easy.

Anyway. This amplifier will just pull ~ 350mW when turned off. I'm going to wire up a separate little switch and LED to feed power into the amplifier so I'm not discharging the battery for no reason.

But in any case, that's kinda hilarious.

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