Making a floating 9VAC 1A 60Hz supply - Page 1
Forum section: electronics / projects

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 27, 2016, 11:42:29 pm
content: The Commodore 64 requires a floating (transformer isolated) 9VAC supply as well as a 5V DC rail. Usually the 9V is done with big iron 60Hz transformers.Can this be done using a Class-D amplifier, fed a suitable signal, driving a smaller high frequency transformer and a filtering network after the transformer?Behold this NASA-level sketch:The linear supply could be the 12VDC part of a PC power supply, or maybe even off-line.The class-D could be anything from a one-chip solution to maybe discrete parts.Now the question is, could using a smaller high frequency transformer work to provide the isolation? What kind of filter will I need? The red feedback part will be optically isolated and whatever is driving the 60Hz (or 50Hz) signal will be driven by the feedback loop.What do you think?

author: Seekonk
author_rank: Super Contributor
timestamp: April 28, 2016, 05:05:23 am
content: I have a number of IR half bridge drivers, a HV 555 with power fet. this could be a use feeding a cap and a transformer. Want to do a shout out for these if you haven't noticed them.http://www.ebay.com/itm/DC-AC-Pure-Sine-Wave-Inverter-SPWM-Board-EGS002-EG8010-IR2110-Driver-Module-HK-/262348487870?hash=item3d15303cbe:g:qHkAAOSwZ8ZW8fo9Add a minimum of two FET and all problems solved for $6.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 28, 2016, 05:35:19 am
content: This smacks of "using a sledgehammer to squash a flea".I used to have a book with a full schematic of a C64,but I think I threw it out as well when I sent the cursed thing to the landfill.------Do Inotsound like a fan?I seem to recall that the 9v AC went to a conventional switchmode supply on the main board.These usually have a bridge rectifier at the input,so will operate happily on a suitableDCsupply.

author: johnkenyon
author_rank: Regular Contributor
timestamp: April 28, 2016, 05:01:50 pm
content: Quote from: vk6zgo on April 28, 2016, 05:35:19 amThis might helphttp://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/The only caution I would give is that on the two diagrams I looked at the 9V AC feeds the user port, so while you might be able to get away with a suitable DC feed for the C64, you also need to look at what peripherals are being used.The first diagram I saw had some parts using "unregulated 9v" coming from a BR+Cap, and other parts fed from a regulator - one diagram had a basic "zener+transistor+2 resistor" regulator, the other had a 7805 fed normally from a bridge rectifier, and a 7812 fed from what looks like a diode pump circuit.Personally I'd just use a transformer with two separate windings - one to feed the 9VAC, the other feeding a suitable 5v regulator module. If the regulator module was isolated, I'd use a single winding transformer and feed both the module and the 9VAC.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 29, 2016, 12:10:46 am
content: Sigh. There's no switcher in the 64, and if I could get away with DC, why would Commodore have put 9VAC there in the first place?The AC not only feeds peripherals on the user port, but the 50/60Hz sine wave drives the TOD clock with interrupts. And you need a floating supply for the SID chip. Don't ask me why, I didn't design the stupid thing. So worst case, common DC on the 9VAC lines risks frying the 64; or best case software that relies on the TOD incrementing will fail.I want to get away from the 60Hz magnetics because they're big and getting harder/expensive to find. It's 30$ for a Hammond part that's only 500mA. I think I could throw together a hack for less than that.It's not just the 64 that uses this, but the SX-64 and the 128.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 29, 2016, 02:33:52 am
content: Quote from: Alex Eisenhut on April 29, 2016, 12:10:46 amSo they could put the DC power supply in the main case----not uncommon in the 1980s.QuoteOK,I was wrong,--long time ago,& I never was that enthusiastic!QuoteWhy buy a Hammond part?I've never had anything to do with them,but they have a reputation for being costly.In Oz,you can buy a transformer from Jaycar for $A11.95 which would probably do the job ---It's almost certainly Chinese,but what the heck? So is everything.Surely there is something similar in your part of the world?