Post by thread:battery/wall xfmr switching

At 01:51 10/02/99 -0600, you wrote: >Hello, >I'm finishing up my first (?real?) PIC project. A chess clock. I'd like >to power it with either a 9V battery or a little wall transformer. > >I'm guessing there is an IC out there to switch between supplies. Or >maybe there is some other trivial way to do this. Assuming there's a regulator on the PIC circuit, use a 9V battery and a 12V supply, with a diode in series with each. Then if the 12V drops below 9V, the battery will take over supply. Quick and dirty, and usually works just fine ;-) Nigel

erik wrote: > > Hello, > I'm finishing up my first (?real?) PIC project. A chess clock. I'd like > to power it with either a 9V battery or a little wall transformer. > > I'm guessing there is an IC out there to switch between supplies. Or > maybe there is some other trivial way to do this. > > Any help would be much appreciated. > > Thanks, > Erik One way would be to use diode "Or"-ing (use a power supply that has over 9V output - which would include most 9V wall warts!) - Common cathodes, an anode to +9V and +WallWartIn, and the system picks the higher voltage automagically. Cheap, too. (Do lose a diode drop's worth of power - an FET or BJT circuit might be better if switching between 2 battery sources.) One project used this with a bridge rectifier they had scads of, as a reverse voltage protection/spike protector (connect the common anodes to ground, though Battery & Wall Wart then need to share grounds.) "Think Devious" <G> Mark

suggestions: - include a small relay (powered from the wall wart) which switches from battery to mains - use a switch: one side to the battery, the other side to the mains: doubles as on/off when you pull out the mains. regards, Wouter. ---------- {Quote hidden}> From: erik <spam_OUTerikTakeThisOuTNETWURX.NET> > To: .....PICLISTKILLspam@spam@MITVMA.MIT.EDU > Subject: battery/wall xfmr switching > Date: Wednesday, February 10, 1999 08:51 > > Hello, > I'm finishing up my first (?real?) PIC project. A chess clock. I'd like > to power it with either a 9V battery or a little wall transformer. > > I'm guessing there is an IC out there to switch between supplies. Or > maybe there is some other trivial way to do this. > > Any help would be much appreciated. > > Thanks, > Erik

Or you could even use a power socket with an NC switch built into one of the contacts, as most fo them have. Much easier, it's what the vast majority of mains/battery appliances use. (Although their long term reliability is not terrific) Mike Rigby-Jones mrjonesKILLspamnortelnetworks.com {Quote hidden}> erik wrote: > > > > Hello, > > I'm finishing up my first (?real?) PIC project. A chess clock. I'd like > > to power it with either a 9V battery or a little wall transformer. > > > > I'm guessing there is an IC out there to switch between supplies. Or > > maybe there is some other trivial way to do this. > > > > Any help would be much appreciated. > > > > Thanks, > > Erik > > One way would be to use diode "Or"-ing (use a power supply that has > over 9V output - which would include most 9V wall warts!) - Common > cathodes, an anode to +9V and +WallWartIn, and the system picks the > higher voltage automagically. Cheap, too. (Do lose a diode drop's > worth of power - an FET or BJT circuit might be better if switching > between 2 battery sources.) One project used this with a bridge > rectifier they had scads of, as a reverse voltage protection/spike > protector (connect the common anodes to ground, though Battery & Wall > Wart then need to share grounds.) "Think Devious" <G> > > Mark >

w. v. ooijen / f. hanneman wrote: > - include a small relay (powered from the wall wart) which switches > from battery to mains > - use a switch: one side to the battery, the other side to the mains: > doubles as on/off when you pull out the mains. Michael Rigby-Jones wrote: > Or you could even use a power socket with an NC switch built into one > of the contacts, as most fo them have. Much easier, it's what the > vast majority of mains/battery appliances use. (Although their long > term reliability is not terrific) Both these (or all 3) have a problem. I'm just wondering whether he really wanted a "no-break" switching to retain its RAM contents? My feeling is that you use a (Schottky) diode from the battery and a bridge (or half-bridge) rectifier from the power transformer which presumably will produce a somewhat higher voltage than the battery. It doesn't have to be much higher though, as having the reservoir capacitor *after* the diodes guarantees that any supply from the mains keeps the battery diode biassed off. This presumes either that the PIC can tolerate the higher supply voltage or you use a low-leakage regulator and that in either case, low leakage reservoir capacitor after the diodes. This is where things actually become complicated. You don't generally need a switch in the mains supply; it almost certainly uses so little power (or shuts itself down) that you can afford to leave it running. If it has no shut-down function, you will however want a battery on/ off switch, but this can be a simple single pole. OTOH, you could place the switch between all the diodes and the reservoir capacitor. -- Cheers, Paul B.