For the Basic Relay Bypass, I have some questions about the circuit's power supply. Looking at the schematic, if I understand correctly, the intent is to power the board directly from the DC jack (i.e., in typical PedalPCB projects, with this relay bypass, you'll run two wires from each pole of the DC jack, one to the actual effect circuit board, and one for the relay board).
That means pin3/VI of the 78L09 (i.e. voltage input) should generally see about 9VDC (likely a bit more for a fresh battery or typical modern pedal power supplies). My first observation, there is no polarity protection. All the PPCB effect boards I've seen have a 1n5817 in series with the DC input, I always assumed that was for reverse polarity protection (at the expense of a small voltage drop).
Second question: I believe that 78L09 is a 9V LDO-type regulator. If I'm not mistaken, it looks like it generally expects around 12V or more in order to output 9V. So is it actually able to output a constant 9V here, if the input is generally say <= 9.5VDC?
Then the two 10k resistors make a voltage divider, producing VREF, which should be 4.5V, if the 78L09 is putting out a constant 9V. It looks like VREF is used for the relay's coil power. But there will be some voltage drop across R3/100k and the 1n5817 diode, right? Isn't this cutting things a little close? And what if the source supply is a bit under 9V (e.g. low battery, or we put a reverse-polarity protection diode in front of pin3), won't VREF fall below 4.5V? If VREF is too low, it won't activate the relay coil.
I'm probably missing something here, but that's why I'm asking!
Thanks!
That means pin3/VI of the 78L09 (i.e. voltage input) should generally see about 9VDC (likely a bit more for a fresh battery or typical modern pedal power supplies). My first observation, there is no polarity protection. All the PPCB effect boards I've seen have a 1n5817 in series with the DC input, I always assumed that was for reverse polarity protection (at the expense of a small voltage drop).
Second question: I believe that 78L09 is a 9V LDO-type regulator. If I'm not mistaken, it looks like it generally expects around 12V or more in order to output 9V. So is it actually able to output a constant 9V here, if the input is generally say <= 9.5VDC?
Then the two 10k resistors make a voltage divider, producing VREF, which should be 4.5V, if the 78L09 is putting out a constant 9V. It looks like VREF is used for the relay's coil power. But there will be some voltage drop across R3/100k and the 1n5817 diode, right? Isn't this cutting things a little close? And what if the source supply is a bit under 9V (e.g. low battery, or we put a reverse-polarity protection diode in front of pin3), won't VREF fall below 4.5V? If VREF is too low, it won't activate the relay coil.
I'm probably missing something here, but that's why I'm asking!
Thanks!