+2a/b power question

zedexspectrum

Will a power supply from a +2a/b be sufficiant to run a +3 if no other devices are connected to it? i know its not ideal but its all i can put my hands on.

guesser

It will certainly work fine if you disconnect the internal floppy drive power connector in the +3... No good if you wanted to use the 3" drive though.
If you don't it may overload the PSU because the +2A supply is only rated to supply 12v at 200mA while the +3 PSU is rated at 700mA.

I suspect the place it would fail is the bridge rectifier supplying the plus and minus 12v regulators using diodes rated for 1A, as opposed to having a separate rectifier for +12v on the +3 PSU with diodes rated for 1.5A.

Much like the 128k thread the other day though I've not measured what current the +3 actually draws when running the drive motors, or what current a +2A PSU will actually give before it fails.

Twiggy

Would one be able to use a +3 power supply with a +2a?

1024MAK

Twiggy wrote: »

Would one be able to use a +3 power supply with a +2a?

Yes, you can use a +3 PSU with a black +2A :-)

Mark

balford

Question :) Is the -12v line used anywhere on the +2a/+3?

If not, a generic 12v supply capable of 2A or so could feed both the +12v line (directly) and the 5v line (via a suitable switch mode 5v regulator) to a DIN plug.

Something I'm thinking about anyway...

B

guesser

balford wrote: »

Question :) Is the -12v line used anywhere on the +2a/+3?

Yes, but it will work fine without it.

It is only used by the serial port driver.

DEATH

As far as I know, the -12V line is only used for the RS 232 port. I used a PSU with no -12V line for years on my old +3 without any issues. Only found out the -12V didn't work when I tried to use the RS 232 port and couldn't figure out why I wasn't getting anywhere.

Using a +2A supply on a +3 - well it WILL work, but you will stress the PSU and certainly not to be reccomended. Stressed +2A supplies don't live long ;)

bugpat

The -12V line is only used by the RS232 port. So if you don't need the RS232, you can forget the -12V (as I did for my +3, using a small PSU coming with those cheap 3.5' hard drive's enclosures, and having only +12V / +5V).


[EDIT : Ooops, didn't refresh and didn't saw guesser and Death's answers !!! :) ]

balford

Sweet, I'm sure a 7912 reg hanging off the +12 supply would work too, as the current draw is only 50mA or so, a heatsink wouldn't be required.

B

1024MAK

balford wrote: »

Sweet, I'm sure a 7912 reg hanging off the +12 supply would work too, as the current draw is only 50mA or so, a heatsink wouldn't be required.

No, no heatsink required, but you may want to check what you wrote :p, as this would not work :o. Not sure how much magic smoke would be produced, as I haven?t tried it :D.

If you need to generate the -12V rail for the RS232 from the +12V rail, use a suitable DC-DC converter ;).

Mark