Jump to content

2-component LC power supply (amateur hour bodge)


Recommended Posts

Multiple people have asked me recently about my bodge-up LC II PSU that I use in the rackmount LC II, so I'm going to post about it here.  It comes hedged about with a number of warnings, though: this is amateur hour hardware, and may be a bad idea in ways I don't know about.  Obviously this comes with no guarantees at all beyond my statement that I've built two of them and they're both basically stable (one is in use in my rackmount LC, the other I use to test boards with).  I don't use them with machines with spinning discs in, though.

 

For most uses, the macATX or similar is a far better option that this, if only because someone's actually tested it properly.  But availability of those varies over time, since they're enthusiast-type projects.  That's just a fact of life for small-run hardware, honestly.  But I would probably have used one of those if they had been invented at the time I was building the rackmount LC :-).

 

That said, part of the point of posting this is to point out that the LC boards at least are really not difficult to power; and while obviously using/repairing "real" PSUs is desirable to keep machines as near stock as possible, the loss of a power supply is by no means the end of the machine.  And if you have spare LC boards you want to test, this is a reasonable way to do it.  I just built this up on a veroboard for my own use, and it was quick and easy, and I am not good at this.

 

The approach I took is: take a standard 5V 2A wall adapter, because most of the load in my LC is on the 5V line.  Then use two isolating DC/DC converters to generate +12 and -5v.  +12v is easy; and actually it being an isolating DC/DC converter is overkill,  but this is the part I ended up with after getting overwhelmed by choice, panicking, and buying the first thing that looked vaguely like it would do the job.  To generate the -5V, I used an isolating DC/DC converter that takes 5V in and pushes 5V out, but the output is isolated from the input.  If you then tie the positive side of the output to 0V, you end up with -5V.

 

Note that the -5V supply is only used, as far as I can see, for differential signalling on the serial ports.  So if you don't want to use LocalTalk or RS422 devices, you can leave it off; RS232 devices will (probably) work fine, though I haven't tested it.  Since the rackmount LCII is an AppleTalk router, the -5V is non-negotiable for my use case.

 

Here's a badly-drawn schematic:

 

1457701186_ScreenShot2020-11-18at12_10_10.png.810791b602550839564939b9e4684f7a.png

 

The +5V and 0V (which I've just noticed the + on, argh) lines come from a barrel jack on the left in mine.  The top converter, the 0512, is, as its name suggests, the 5V -> 12V converter; the 0505 at the bottom is, as its name suggests, the 5V -> 5V converter.  These probably ought to have some RFI suppression capacitors around them; the datasheet for the REC3A-0505 provides suitable values.  Since mine is running full time in what amounts to a Faraday cage, I didn't bother.  Also, I was lazy.

 

Also, don't be put off by all the parameters/choices of the DC/DC converter.  I aimed for something with roughly the current capacity of the PSU already in the machine, then narrowed down my selection by the highly scientific method of "picking the cheapest one that looked easy to solder and was also in stock on that day".

 

Hope this is useful to someone, anyway :-).

Link to post
Share on other sites

This is my prototype/board testing one.  It doesn't have the barrel jack on it because I use it with my bench PSU, and the multiple wires are arbitrary and paranoid.  The one in the rackmount is a bit neater, but I can't get that machine out trivially without unplugging lots of stuff :).

 

62740316323__21854930-267D-42A9-9DB9-A099444DF35C.JPG

Link to post
Share on other sites
18 hours ago, cheesestraws said:

Note that the -5V supply is only used, as far as I can see, for differential signalling on the serial ports.  So if you don't want to use LocalTalk or RS422 devices, you can leave it off; RS232 devices will (probably) work fine, though I haven't tested it.  Since the rackmount LCII is an AppleTalk router, the -5V is non-negotiable for my use case.

 

I’m not familiar with the LC series, but I know that early analog boards (128k/512k/Plus) have -12V (and not -5V) and I thought that was likewise for RS-422/LocalTalk. Do the specific voltages matter, or do they just have to be far enough apart? Wikipedia suggests to me that ±5V is correct for RS-422, so maybe the Plus is doing something else that I don’t understand.

Link to post
Share on other sites
4 hours ago, PotShotScott said:

I'm curious - what function does the rack mounted LC perform?

 

It's my AppleTalk router between LocalTalk and Ethernet.  It's often cheaper and easier to find an LC or LC II than it is to find a hardware LocalTalk bridge, and the software is better, too.  And once recapped those things are really rather reliable, so it makes a lot of sense.  Of course, I promptly demolished much of the cost advantage by putting solid state storage in it and building a rackmount case for it, but hey, it was fun, and it is out of the way physically now :-)

 

2 hours ago, ymk said:

A charge pump might be another way to get the -5V rail if current requirements are low.

 

I think a charge pump is probably the more sensible way of doing it, honestly; the approach here is optimised for "I just want it to work and don't want to spend time debugging it" so it's rather overengineered in the -5V department :).

 

1 hour ago, sfiera said:

Do the specific voltages matter, or do they just have to be far enough apart?

 

I think you're allowed up to a 6V difference, but it's allowed to be noticeably lower than that.  So I'd assume that the Plus has another use for its -12V as well, but I have no idea what that is...

Link to post
Share on other sites
On 11/18/2020 at 9:41 AM, cheesestraws said:

This is my prototype/board testing one.  It doesn't have the barrel jack on it because I use it with my bench PSU, and the multiple wires are arbitrary and paranoid.  The one in the rackmount is a bit neater, but I can't get that machine out trivially without unplugging lots of stuff :).

 

62740316323__21854930-267D-42A9-9DB9-A099444DF35C.JPG

How wide is that breadboard? Will that fit in an LC case?

Link to post
Share on other sites

That specific board I think too wide to fit inside the case for an LC PSU, but you might well be able to find a corner for it in the case.  I think @erichelgeson was looking for / might have found a prototyping board the right size for an LC PSU case.  Again, that board was one I had in the cupboard, rather than one that was bought especially for this project, if that makes sense.  You could fit the thing on a noticeably smaller protoboard than that.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...