I'm doodling with a future circuit. The application (Commodore 64) expects 5VDC (which I will provide with a pass-through) and 9VAC 50Hz. I do not have any working circuit or a schematic, because the circuit does not yet exist other than as an idea, but I have this very crude block diagram:

enter image description here original

The circuit is fed by an external power supply, with 5V@10A and 12V@10A. I can use whichever I want. Even both. Since I want 9VAC in the final step (~13V û), I want some headroom for the H-bridge. So I pick 18V. That's what I want the DC/DC to achieve for me.

I think I need isolated DC/DC, but I'm not sure. What I'm sure about is that the 9VAC needs to swing around 0V, so no DC-components or offsets on it. With isolated DC/DC, I'm hoping (yes, because this area is completely out of my league) that the 9VAC will look like any 9VAC wallwart would produce. At least, that's my goal.

The 9VAC is supposed to be able to deliver 750mA continuously, so I will design for 1A. Bonus is if I can manage to double it.

There's a lot of new things here for me, so I need to break things down to make them graspable. At this point, I will put the PIC/Bridge/LC aside for a while and focus on the DC/DC.


  1. I know MAX1523 will work here, but it's way too expensive. Will LM2735 (datasheet) be of any use for my design?

  2. What part(s) in my block diagram are responsible for the maximum amps at the outputs? The DC/DC, the H-bridge, or both?

  3. Flyback topologies seem to use a transformer for isolation. Datasheet for MAX1523 suggests using a dual inductor with ferrite core (to save space). If this sort of isolation is used, will the 9VAC output be of the type I want (no DC-offets, etc)?

  • \$\begingroup\$ What is the application here? Why can't you simply feed the 12VDC directly to whatever device is expecting the 9VAC wall-wart? It should just work, unless it's something extremely specialized. \$\endgroup\$
    – Dave Tweed
    Feb 7, 2019 at 12:31
  • \$\begingroup\$ The application (Commodore 64) expects 5VDC (which I will provide with a pass-through) and 9VAC 50Hz. If I feed 12VDC there, things will most likely break beyond repair. \$\endgroup\$
    – bos
    Feb 7, 2019 at 12:39

1 Answer 1


OK, from the service manual, here is the circuit you're driving:

C64 power input circuit

Since the 9VAC is used to feed a voltage doubler in order to produce the +12V bus, you do indeed need AC. However, the waveform is not critical — a square wave (with some filtering to eliminate RFI) would work just as well. Also, the frequency is not critical either, unless something connected to the "User Port" cares about it.

So, I'll modify my previous comment. You could just feed pins 6 and 7 with complementary square waves that swing between 0V and 12V using your H-bridge, and the circuit will be happy. No need to boost the voltage, and no need for isolation. Nothing will be damaged.

The "+9 unreg" rail will be close to 12V, but that's the nature of "unreg". It probably was that high anyway, assuming the 9VAC input is an RMS rating.

  • \$\begingroup\$ Thank you. This answer leads to new things I need to read up about. I've had this fix idea that the 9VAC had to swing around 0V to be acceptable, while you claim swinging between 0 / 12V would yield the same result. This led me to think; does it mean that the offset (in this application, or in general) is not important at all? In other words, would feeding a square wave of 100 / 112V work just as well? \$\endgroup\$
    – bos
    Feb 7, 2019 at 13:39
  • \$\begingroup\$ No, of course not. That would be way too much voltage. \$\endgroup\$
    – Dave Tweed
    Feb 7, 2019 at 13:41
  • \$\begingroup\$ Regarding the user port and 50 Hz, I will have to assume something attached to it relies on the 50 Hz AC-frequency. Because if I don't, I might end up in unexpected behaviour. In this case, would 50 Hz square wave match 50 Hz sine wave? If not, what should I read up about to learn how I can compensate for the different waveforms? \$\endgroup\$
    – bos
    Feb 7, 2019 at 13:52
  • \$\begingroup\$ Why do you "have to assume" anything? Don't you already know what you'll be plugging in there? Anyway, I can't imagine that anything that was designed for that port would care about the waveform. \$\endgroup\$
    – Dave Tweed
    Feb 7, 2019 at 14:04
  • \$\begingroup\$ Ofcourse I know what I will be plug in there - cartridges - but what I don't know is how these cartrigdes works "under the hood". There are literally thousands of different cartridges from different manufacturers. There is no way I will be able to open them all just to see if they use the 9VAC-lines, and then trying to figure out if the frequency is important to that particular cartridge. That's why I will have to assume. \$\endgroup\$
    – bos
    Feb 7, 2019 at 14:10

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