Rectifiers

[motorollin]

I have looked at datasheets for several rectifiers and none of them specify whether they output positive or negative DC voltage. Can the same component be used to rectify AC to either positive or negative DC voltage, or do some rectifiers output positive DC voltage and others negative?

[motorollin]

Oliver wrote:
Are you looking to rectify an AC wave from a transformer?

Yes. I want to replace my A1200 PSU with one which is built in to the plug. I can't find one in this design with 12v, -12v and 5v so I'm thinking of getting one which outputs 12vac and then using rectifiers to output 12vdc and -12vdc, and then a transformer to step the 12vdc down to 5vdc. Hopefully I can build this in to a circuit small enough to either fit inside the PSU housing or be attached to the bottom of it.

Oliver wrote:
To understand the rectification process, think of the diodes as controlling current direction.

OK, so it only allows the current to go one way and not to flow back the other, this making it DC. Is that right? Does that mean you can just swap the output of the rectifier around to get -12vdc?

Oliver wrote:
The output of the transformer is floating, i.e. it is not referenced to 0V earth potential, unless you ground one of its electrodes.

I'll take your word for that ;-)

Oliver wrote:
Are you designing a power supply?

No, adapting one :-)

[motorollin]

Oliver wrote:
Can't you buy a miniature switch mode supply for this?

I don't know. Could you give an example of the type of supply you mean?

Oliver wrote:
Do you know how much current is required for each line?

Yep. I need +5vdc@3.0A, +12vdc@500mA and -12vdc@100mA. That's according to the back of the A1200 PSU.

Oliver wrote:
Your approach will not work as planned. A transformer will only transform AC to AC. Running DC through a transformer will only overheat the primary coil. If you want to get these AC volages, I suggest you use a multitap transformer, then rectify the AC to get unipolar waves.

Do I really need a multitap transformer for this? Why can't I just use a 12vac power supply and tap off three spurs from its single output? One would be rectified to +12vdc, the other to -12vdc, and the last rectified and stepped down to +5vdc. Wouldn't that work?

Oliver wrote:
Have you got some regulators in mind?

No, since I have absolutely no idea what I'm doing :lol:

Oliver wrote:
The rectified waveform will need to be filtered and regulated for the computer. Compuer supplies are usually done with switch mode regulators. You may need some more detailed help to get this designed well.

Is that an offer? ;-)

[motorollin]

@JLF65
Yes I'm hoping to replace the A1200 PSU. Thanks for the links, but the power supply you linked to isn't built in to the plug, which defeats the object of building a replacement since my aim is just to have one cable.

[motorollin]

Thanks for all the info guys. I think this is too complicated for my basic knowledge of electronics. I had hoped it would be as simple as knowing what voltage was going in and what voltage was coming out of the transformers/rectifiers, but it seems there are more considerations that that. Also, I don't think I can get the PSU I want anyway, so I'll just stick with the original A1200 supply.

Thanks anyway!

[motorollin]

Oliver wrote:

pyrre wrote:
Why not build a pico psu inside an a500 psu box...

I think the point was to eliminate the need for a sizeable external PSU box, by placing all components in a plugpack.

Indeed. The only other way would be to build the PicoPSU in to the Amiga itself and then have a 12v supply in plugpack form factor which powers the PicoPSU. The problem is though that I have had a PicoPSU before and couldn't find a 12v plugpack PSU with enough current to power the PicoPSU.

My knowledge of electronics is not good enough to be sure anything I design would be safe. I thought this would be more simple to design, but given the possibility of it failing I'll have to just leave it I think.

[motorollin]

I'm prepared to do some reading but at the moment I don't have time. My degree is taking most of my spare time so this will have to be relegated to a summer holiday project :-(

[motorollin]

Oliver wrote:

motorollin wrote:
My degree is taking most of my spare time...

Yeah, that's pretty much what I had expected. I was actually a little surprised that you would be taking on a design project at this time.

Well, I didn't think it would take so much designing when I first had the idea ;-)

Oliver wrote:
All the best for your studies.

Thanks!