The regulator needs about 2.5 to 3 volts head room. The circuit is what is often called a boost circuit. If my calculations are right, it should produce about 15.5 to 16V on C10. This gives the regulator enough over voltage to work as a regulator. If the supply you have is not regulated it won't be able to be used directly.
TR2 and L1 transformer form an oscillator. When TR2 conducts, it causes a field to build up in L1. When TR2 turns off, the field in L1 tries to collapse. The voltage build up until the diode conducts charging C10. This is often called fly-back. Coils like to keep conducting at a constant rate. Since TR2 turns off, the coils voltage will continue to rise until it finds a path to send the current ( the diode ). When the voltage gets high enough across the resistor divider, R16/R15, TR3 turns off. This removes the bias needed to turn on TR2, shutting down the oscillation. If the voltage on C10 drops to the point that TR3 conducts, the oscillation will start again, boosting the voltage on C10 again. Dwight ________________________________ From: cctalk <cctalk-boun...@classiccmp.org> on behalf of Adrian Graham via cctalk <cctalk@classiccmp.org> Sent: Wednesday, August 23, 2017 12:33:44 AM To: General Discussion: On-Topic and Off-Topic Posts Subject: Re: A little power circuit explanation please > On 23 Aug 2017, at 00:49, Rob Doyle via cctalk <cctalk@classiccmp.org> wrote: > > It's a little switching power supply. It steps up the 9V input voltage to > something a few volts greater than 12V to feed the 12V regulator. > > If your 12V is correct, it is probably working. > This is from 2 months ago but I was puzzling over the 12V circuit in this particular machine and also maybe suffering from a red herring since the PSU I was using is from a ZXSpectrum+2 which is an unregulated 12-14V lump so the coil was already being fed 12V. A > Rob. > > On 8/22/2017 4:30 PM, Adrian Graham via cctalk wrote: >> Hi folks, >> Failing Atari PSU aside I remembered a question I had ages ago but never >> posted about the power circuit of another 80s home micro, the Enterprise 64. >> This machine is powered by a 2A 9V unregulated PSU and internally there’s a >> pair of 7805s and a 78L12 to smooth things out. >> There’s also a small transformer coil in there too (L1 on the following >> schematic) and I’m not entirely sure what it’s for. Here’s the schematic of >> the original circuit, any enlightenment gratefully received since I have an >> Enterprise 64 with a dead coil :) >> http://www.binarydinosaurs.co.uk/ep64PowerRegulators.png [http://www.binarydinosaurs.co.uk/ep64PowerRegulators.png] <http://www.binarydinosaurs.co.uk/ep64PowerRegulators.png [http://www.binarydinosaurs.co.uk/ep64PowerRegulators.png] > >> Cheers! >> — >> Adrian/Witchy >> Binary Dinosaurs - Celebrating Computing History from 1972 onwards >
