Power supply options A1200

[jarp]

Of course it had to be more difficult than that. My Amiga 600 does not seem to draw pretty much any current from -+12V lines (judging from schematics if no peripherals are connected then only audio op-amps will draw any power from those lines) and that's out of RT-50B specs (min draw for +12V is 0.2A and for -12V it is 0.1A).

As end result +12V source is out-of-specs too. I get +13.3V. +5V and -12V are well within specs though. I added 120ohm wire wound resistor to +12V line which draws 0.1A - still under 0.2A but enough for +12V line to drop to 12.6V. 56ohm resistor gives me 12.2V which is perfect.

Problem solved except 56ohm resistor will generate 3W worth of heat requiring heat sink and space being premium it will be difficult to mount it neatly. I was thinking that perhaps I should just accept +12V being out-of-specs... I checked specifications of all chips using +12V line and they all are rated at least for -+15V. But then again I am always against leaving something half-complete...

[demolition]

+13.3V is perfectly fine. The only thing that will happen is that the audio opamps produce a little extra heat but they are still within safe limits. The important one is the 5V line.

[jarp]

Originally Posted by demolition

+13.3V is perfectly fine. The only thing that will happen is that the audio opamps produce a little extra heat but they are still within safe limits. The important one is the 5V line.

That's the conclusion I came to also. At least when looking at schematics there should be nothing rated below 15V. Though I made slight mistake, I actually get 13.6V which is still below 15V but closer absolute maximum ratings. I'll leave it as is (I would be able to fit resistor and heatsink into enclosure but that's still some extra heat there).

There is of course possibility that PSU is oscillating and unstable, but I get so steady voltages that probably not? Would need that oscilloscope I've been wanting for so long to be sure.

Ps. Thanks for your reply!

[demolition]

Alternatively, instead of using resistors, you could also use two silicon diodes. Then you would get a voltage drop of around 1.3V which would not change much as the current changes. You can get such diodes in big packages which are able to handle some power, for example in TO-220 packages which can even be attached to a heatsink, although I don't think that should be necessary here.

[bebek]

There is a way to make enough space for Pico PSU in A1200. Remove power connector, filter, modulator, U12, all caps around and filters for composite video. All of it is not required as nobody uses composite nowadays .

[Akira]

No need to remove much or anything at all.
Although I remove RF modulators systematically, you can fit a unit on top of those components, bolting the picoPSU adapter to the lower plastic case. I put a bit of insulation below to not let it short anything (like the lower metal shield).

Stedy's adapters are available again, by the way, and I recommend this once more as the best solution.

[spannernick]

Isn't that this in a way..?
http://www.ebay.co.uk/itm/Commodore-...8AAOSwPcVV0RGV

[demolition]

Yes, that is just an external version.

[Daedalus]

Originally Posted by demolition

Alternatively, instead of using resistors, you could also use two silicon diodes. Then you would get a voltage drop of around 1.3V which would not change much as the current changes.

This will work, provided the +5V regulation isn't governed by the +12V regulation as it is in modern ATX PSUs. Personally I'd prefer to go with the dummy load resistors to be sure the PSU is working as it was designed. 5W resistors are easy to get hold of and should be enough to stabilise things.

[idrougge]

Are there any adverse effects in using a 7805 on the 12V line to add a load and gain some additional effect on the 5V?

[demolition]

Originally Posted by Daedalus

This will work, provided the +5V regulation isn't governed by the +12V regulation as it is in modern ATX PSUs. Personally I'd prefer to go with the dummy load resistors to be sure the PSU is working as it was designed. 5W resistors are easy to get hold of and should be enough to stabilise things.

These are two different things. I was talking about how to reduce the voltage from 13.3V to 12V and using two diodes would be more effective than using resistors since the voltage drop across diodes is relatively constant with changing currents where for resistors it changes proportionally.
It could also be that the reason for seeing 13.3V is because of an insufficient load and in that case a dummy load in shape of a power resistor could be helpful, but then it is to be added across the supply rails and not in series with the device.

Originally Posted by idrougge

Are there any adverse effects in using a 7805 on the 12V line to add a load and gain some additional effect on the 5V?

You should not connect the output from the 7805 to the PSU 5V line. It could well cause trouble with the regulation in the PSU. As the voltage is not exactly the same, which ever of the two has the higher voltage will end up driving everything, or at least try to.
You can however use a 7805 separately to drive some peripheral, like a HDD, and in that way ease the load on the 5V line. Just be aware that with a 7V voltage drop it won't take much current to heat it up quite a bit.

[Daedalus]

Yep, that's true, though I wasn't talking about dropping the voltage across resistors. What I was thinking was that if the regulation isn't great (and 13.3V for a 12V line is a fair indication that's the case), the voltage is likely to fluctuate with the load. If that's the case, having a fixed voltage drop might see the supply dip below 12V as the load increases, say when a CD drive spins up or similar. On a standard A1200 this mightn't be an issue assuming the internal floppy doesn't need 12V, but stabilising the output would be a more robust approach IMHO.

[jarp]

Originally Posted by Daedalus

Yep, that's true, though I wasn't talking about dropping the voltage across resistors. What I was thinking was that if the regulation isn't great (and 13.3V for a 12V line is a fair indication that's the case), the voltage is likely to fluctuate with the load. If that's the case, having a fixed voltage drop might see the supply dip below 12V as the load increases, say when a CD drive spins up or similar. On a standard A1200 this mightn't be an issue assuming the internal floppy doesn't need 12V, but stabilising the output would be a more robust approach IMHO.

Are we still talking about my Meanwell RT-50B? Yeah I would not drop voltage by using resistors or diodes connected in series because like you said, if there's enough load from Amigas end PSU will regulate itself better and as end result +12V would then drop below 11V.

I experimented with power resistors and like said, 68 ohm resistor connected between +12V and ground will make voltages to stabilize properly. Only problem with dummy load is heat. 68 ohm resistor translates to 2W which may not sound much but need proper heat sink anyways. I just ordered 10W power resistors with integrated heat sinks, perhaps they fit better in enclosure than my hacks.

I would not say that voltage regulation is poor on RT-50B. There is clear min spec for current and it is 0.2A. Amiga draws pretty much nothing. Thus to be fair you really can't expect output voltages to be within specs either.

[Daedalus]

Yeah, I didn't mean the PSU was poor quality or anything, I consider poor regulation as drifting voltages regardless of cause - use outside specification (under minimum load), or bad design or quality can all end up in the same situation.

[jarp]

Again I would need a new power supply, this time I do not have any spare Amiga PSU enclousures, but square DIN connectors are now obtainable (just got shipment of 10)!

So, does anybody know any desktop power supply which could work? So far MeanWell GP50A is only one I've found; however it too has too high min load specs for +-12V rails. I've not found anything else (which can be actually bought or is not insanely expensive) and I've searched long time.

[Drag0nFly]

Sorry for not answering your question @jarp, but reading this thread I am starting to get second thoughts about using my recently-ordered MW RT-50B PSUs to power my A1200+A500 systems -

https://www.sunpower-uk.com/product/...0B/default.htm

Obviously I read several threads beforehand stating that it basically was a "drop-in" replacement (Nightfallcrew+others), then I discovered this info and also people claiming "flickering" when used with (ceramic) recapped A1200/A600 machines.

I must say though that the GP50A actually looks like the better option (minimum load specs for for +/- 12V rail aside) than the RT-50B – if only I had spare DIN connectors, that is.

Now it looks like attempting to repair the 4.5A original Commodore PSU is the better option. It died for no reason at all recently, and does not appear to be damaged in any way...

[jarp]

I have suspected long time that RT-50B puts out ripple and/or noise at least when operating out-of-specs (load below minimum). I have no scope so I cannot prove it, but if others are reporting flickering then perhaps. I've recommended RT-50B as well so my apologies.

The thing is; i've not found any single PSU, external, open frame, whatnot, which would have suitable min load specs (0A for +-12V). Perhaps most of the PSUs will work anyways, but would need scope to verify that PSU won't put too much crap out.

Dunno what would be ideal PSU replacement then? Should we just add dummy load to +-12V rails? PicoPSU?

Ps. Square DIN connectors can now be bought as new. Search lemon64 forums for it.

[mpattonm]

I have made few replacement units, check:
https://www.facebook.com/centurionte...45949142386392

[Drag0nFly]

€42 for a 2.5A version? Sorry, *way* too expensive. Half the price of what I paid for my A1200.

[indigolemon]

Tbh, that seems more than fair for a brand new power supply built to meet the Amiga's (by modern standards) odd power requirements! I think if you paid €84 for your A1200 that's more of a strange price than the Power Supply at €42 - it seems in line with what a reasonable ATX supply would cost for a PC.