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Old 10 August 2015, 11:59   #1
Mounty
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Developing a new A500/1200/600 power supply. -12V needed?

Hi,

I am in the early stages of developing a new power supply for the A1200/A500/A600.

Instead of going for the multi-box/adapter approach, I am developing a single-box solution.

I am going for a colour-co-ordinated enclosure with a status LED. This would look better than an ATX psu I think, and would not need an adapter.

The thing is, it would be significantly cheaper if I left out the minus 12 volt line. This is only used for RS232 comms (null modem connections etc).

What are your feelings on this? Can we live without RS232?
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Old 10 August 2015, 13:06   #2
Tmw2n
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Could you not stick in an LM7915CT and associated gubbins and get -12V that way? should add less than a pound to parts cost of the psu, and -12V shouldn't need more than an amp I'd expect.
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Old 10 August 2015, 13:10   #3
Mounty
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About 100mA.

I wouldn't use a LM7915CT the losses would be too high. Instead I would use a MC34063ABN-based circuit.

It's the gubbins that makes all the difference, you'd be suprised what a few mosfets, inductors, electrolytics etc adds up to.

But why bother if no-one needs it?
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Old 10 August 2015, 13:21   #4
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I think it also goes to the expansion connector on the A1200 at least, not sure if any expansion uses it though.

How about designing it into the PCB, but not populating the components unless someone really needs it?
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Old 10 August 2015, 13:33   #5
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Doesn't the audio circuitry need -12v?
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Old 10 August 2015, 14:15   #6
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Yes I believe so. I once forgot to connect -12 when converting my ATX power supply and there was no audio.
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Old 10 August 2015, 14:16   #7
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No idea about the -12v line, but I need exactly 5volts delivered to the psu plug of the Amiga. Jens was going to put a sensor there to control it.
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Old 10 August 2015, 14:35   #8
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Yes, -12V is also needed for audio. It'll still work without it but will be totally distorted. I can't imagine any expansions used it, but the fact that it was on the expansion connector allowed the Mediator board and similar to power the computer (including -12V line) without connecting anything else to the motherboard.
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Old 10 August 2015, 14:42   #9
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You need -12 V for both serial and sound AFAIK.
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Old 10 August 2015, 16:30   #10
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Yup, as said before, -12V is essential to audio.
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Old 10 August 2015, 16:35   #11
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yup, -12 will be needed to drive opamps, so audio :-)
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Old 10 August 2015, 17:43   #12
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-12V is needed... just in case it was still unclear.
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Old 10 August 2015, 17:45   #13
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Be nice if you could make it both Amiga/C64 compatible
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Old 10 August 2015, 20:12   #14
Mounty
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Fair enough if the audio needs it, it has to be there, no question.

I'll look into C64 compatability. Shouldn't think that will be a biggy. Unless it needs minus 5v..
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Old 10 August 2015, 20:22   #15
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Quote:
Originally Posted by Mounty View Post
I'll look into C64 compatability. Shouldn't think that will be a biggy. Unless it needs minus 5v..
You also need 12 or 9V AC for a C64 (12V for old ones, 9V for new ones). I wouldn't bother trying.
I think your main problem here would be to get the square DIN connector. Other than that, it's a pretty standard power supply.
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Old 10 August 2015, 20:43   #16
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Quote:
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You also need 12 or 9V AC for a C64 (12V for old ones, 9V for new ones). I wouldn't bother trying.
I think your main problem here would be to get the square DIN connector. Other than that, it's a pretty standard power supply.
Square? I thought the C64 used a round connector.

You're right though, it doesn't look practical to build in C64 compatability.
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Old 10 August 2015, 21:25   #17
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Square? I thought the C64 used a round connector.
The AMIGA has a square DIN connector and this will be your problem.
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Old 10 August 2015, 21:31   #18
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Quote:
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The AMIGA has a square DIN connector and this will be your problem.
I may have a supplier for this connector, but if that does not happen, then the buyer would need to transfer the cable from his broken psu.

I think everyone expects that anyway. It's widely known these connectors are extremely difficult to source.
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Old 10 August 2015, 21:39   #19
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Yes, two seconds to find them http://www.vesalia.de/r50e_amiga_har...pare_parts.htm

It would be nice if you could make a Mechy Apower style PSU.

To make it unbeatable it would have to add the 5v sensor in the power connector to supply exactly that.
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Old 10 August 2015, 21:59   #20
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If no C64 support, build one separate. A few members have looked into making one, even Jens, but no one so far has done it and released one

As for the square plug, it was made by Mouser Electronics, Stock No. 171-4405
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