Post by thread:[EE]: Powering a PIC from the Phone Line

PIC Listers, Sometimes you need to power a microcontroller from the phone line for example in a Caller ID unit or a line powered modem. This is fine when the phone is offhook and you have several milliamps to enjoy. However, when you are on-hook and you can only draw a maximum of 50uA from the 50V phone line (2.5mW), you need to use some smart-thinking to keep your microcontroller alive. If you are running a RTC from a 32kHz crystal, such as is possible on any of the small PICs (but more conveniently the F628 and F877 which have a second oscillator) you can get by on about 15 or 20uA. This is easily done using a 1megohm dropper resistor from the front end bridge rectifier to provide a leakage supply which charges a capacitor. This is how most modern phones are powered. If you need more power then a simple buck converter can be made from a 4060 CMOS counter, an couple of high Vce transistors and a dozen or so passive components. The whole lot will fit onto 3 square inches of pcb space - using conventional components and a DIL 4060. I have one running on a bit of stripboard and it produces 300uA at 5V making it about 60% efficient overall. Total cost - about $3. Access to the schematics is via the Rat_Ring group site, and you can subscribe to that from here: http://www.geocities.com/kenboak/rat_ring.html Rat_Ring is a discussion forum for those interested in telephony and telecom applications of PICS Regards, Ken Boak -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads