why use a step-up converter like this ?? (confused)

[giuhalftrack]

Hi, I'm still learning electronics, I have this metal detector (Seben Extreme, MD5002) very bad clone of an old commercial TR-GEB-DISC detector (don't know what was the original brand, someone saids whites).

My question is about using this step-up converter:

Battery pack = 12V
linear regulator (7809) = 9V
step up converter rise the voltage to ca.12V

I do not understand why lower voltage from 12V (battery) with the (7809) and the rise this voltage to 12V (again) with a converter...

I've tried to power the machine with 9V (after 7809) and it works as aspected... so why don't put 12V of the battery directly into the machine (after step-up converter) directly ???
I know this converter are noisy... really cannot understand..

I'm a geologist so i see things in another way, and my impression for this design, is that layers of different era and composition stack up in this badly copied circuit

[Qiaozhi]

I suspect the requirement here is to provide a regulated +/-5V supply. If you run the circuit directly from a 12V battery, then you would have an input voltage that fluctuates over quite a wide range, and consequently the 5V supplies would do the same. In this case the circuit needs to regulate the incoming supply first (~12V down to 9V), then boost it to the required level (say 12V) and use a rail splitter to derive the +/-5V supplies. This makes maximum use of the battery capacity, allowing the input voltage to drop to ~10V before the 9V regulator drops out.

[giuhalftrack]

I suspect the requirement here is to provide a regulated +/-5V supply. If you run the circuit directly from a 12V battery, then you would have an input voltage that fluctuates over quite a wide range, and consequently the 5V supplies would do the same. In this case the circuit needs to regulate the incoming supply first (~12V down to 9V), then boost it to the required level (say 12V) and use a rail splitter to derive the +/-5V supplies. This makes maximum use of the battery capacity, allowing the input voltage to drop to ~10V before the 9V regulator drops out.

ah ok, your explanation is clear, and about the noise ? do you think is safe use this converter ?

[Qiaozhi]

ah ok, your explanation is clear, and about the noise ? do you think is safe use this converter ?

It looks like a simple boost converter, so it should be perfectly safe.
I assume the converter is driven by pulses from the TX oscillator (via R46) so any noise will be synchronized to the transmitter, and can easily be filtered out.

[giuhalftrack]

Yes is drived by the TX oscillator, thank you.