Non C64 Power Board Design

[m00dawg]

I spent all this time building a beautiful C64 PSU board only to kill my only good C64 PSU doing something stupid while testing it. So, after pondering over few ideas, I am now opting to make a non-C64 optimized PSU. I figure if I can get another C64 PSU great, but if not, I have something else to keep me busy :)

My idea is to get a +12-16VAC wall-adapter and convert it to 12, 9 and 5VDC. Basically, I am taking the resulting voltage after the rectifier and smoothing capacitors, and then using a series of regulators to produce the other supplies (with a ceramic 100 uF cap after each step). Since you cannot drive a 7812 with a 12VAC supply, I'm planning on adding jumpers to bypass one or more of the voltage regulators so that one could use a smaller AC adapter.

The idea is to offer up options that could be used both as a core MB-SID power supply, but also for other projects potentially. I also wanted to allow for mixing and matching 6581 and 6582/8580 SIDs.

Anyways, I whipped up a quick schematic and board layout (attached) and was hoping to get more scrutiny from the community :) Specifically, I am curious as to if the ceramic caps after each voltage regular will help reduce noise that may come from the other regulators (since this was a concern of some). I am also wondering if I should use fuses and/or diodes in places to avoid killing my wall adapter and/or other components, much like I did with my C64 PSU.

Thoughts would be appreciated :)

powerboard.zip

[Goblinz]

Are you sure your C64 PSU is fried? I thought mine was so took a look inside and found a burnt out fuse... May be worth a look!

[m00dawg]

It probably is a fuse or something simple, but I haven't figured out how to open it without breaking things in horrible ways :) Basically, the +5VDC works and 9VAC appears to work, but cannot handle any load. If you place load on it, the voltage drops down to 1-2V. I've tried numerous tests and checks to see if components were in backwards, etc. All checks out, and since I already had a working C64 PSU board (that is now not working), seems to indicate it's the PSU.

I'll continue to figure out a solution for the C64 PSU. I already have a printed board that seems like it will work great once I have a working C64 PSU. But, I think this might be a fun side-project to pursue since I won't get another PSU for another couple of weeks at minimum, and if I break another one, it'd be nice to have a backup plan :)

[asafnetzer]

I would love to see this project materialize, it would save me the trouble of finding C64 PSU and would save a lot of space on my desk  ;D

Asaf

[m00dawg]

Me too! I know it can be done, but the question is if using voltage regulators is the way to go over, say, using transformers directly and generating the separate voltages using them. I don't know enough about this stuff to know what the impact will be. I do plan on building a prototype board hopefully this weekend and, if that works well, I'll spring for getting printed boards made.

I've already made some modifications to the design I sent previously - I'll post an update once I'm happier with the configuration (I'm thinking of changing a few things to make it more configurable).

[asafnetzer]

I really have no knowledge regarding power supplies so I can't really help here.

I've been using Ray Wilson's design for some of my modules before.

http://www.musicfromouterspace.com/analogsynth/WALLWARTSUPPLY/WALLWARTSUPPLY.php

Just a question, maybe a dumb one, When providing ~15V from the wallwart, wouldn't the 5V regulator need to burn ~10V through heat ?

Asaf

[m00dawg]

That's why I step down the voltage using multiple regulators. So it works like this:

15VAC -> ~14VDC -> 7812 -> 7809 -> 7805

I have a pin header after each voltage regulator such that you can pick from the ~14VDC, 12V, 9V, and 5V, depending on what you need (and if the devices you are powering have voltage regulators themselves).

I'm thinking of also using jumpers so that you can select which voltage regulator to send the rectified voltage from. That way, if you only need 5V for your module, you can use a 9VAC wallwart and, thus, bypass the 7812 and 7809 voltage regulators (since they won't work with lower voltages anyway).

That said, I'll have to check out the power board you are using as it looks like it does similar things and offers -12V. My design only works around +V so it won't work for projects needing bi-polar supplies (I haven't gotten quite that far yet in my own designs :). I suppose my design could be extended to offer both + and - voltages, but I think the first step is to get what I have working and to see if it still produces negligible degree of noise.

[nILS]

(yes I didn't read all the posts)

Every additional vreg adds noise. How about some additional bigger filter caps? All the multi-vreg designs I've seen so far had an additional 10-100uF electrolytic cap per vreg.

[doc]

Another issue in this design:

The 7812 will get very hot. The current of all stages in sum run through the 7812. I think you are not able to get more than 500mA out of the 7805 before the 7812 regulates down because of thermal reasons.

Another possibility without thermal and noise issues:

Do each rail with its own transformer. Or use a Dual-transformer with two secondary windings.

For the 9V version use a 2x12V transformer and for the 12V version a 2x15v (Maybe a 2x12V also works because transformers often deliver a 2V - 5V higher voltage than labeled).

Good regulators can handle up to 30V of input voltage (in fact they are getting very hot then).

greets

Doc

[m00dawg]

Every additional vreg adds noise. How about some additional bigger filter caps? All the multi-vreg designs I've seen so far had an additional 10-100uF electrolytic cap per vreg.

I was thinking about that. I wasn't sure what the common practice was in regards to that. That said, I thought electrolytics tended to allow a fair amount of wobble in the voltage due to their nature? Wouldn't ceramic caps be better there?

The 7812 will get very hot. The current of all stages in sum run through the 7812. I think you are not able to get more than 500mA out of the 7805 before the 7812 regulates down because of thermal reasons.

Yeah I figured that it would. My hope was to minimize that by using 14-15VAC wall plug (just enough for the 2V overhead for the voltage regulator to work). I was also going to put a big heatsink on all three regulators. I have some extra ones lying around but haven't really done much work on looking at the more standard heatsinks.

Do each rail with its own transformer. Or use a Dual-transformer with two secondary windings.

For the 9V version use a 2x12V transformer and for the 12V version a 2x15v (Maybe a 2x12V also works because transformers often deliver a 2V - 5V higher voltage than labeled).

Well, I thumbed through my Allied catalog looking at transformers. I'll admit that's new territory for me and I was concerned that, were I to put a transformer inside my case, it would create interference? That said, I do think that is a great route to go. Actually, you can get transformers that output different voltages as well correct (such as +15, +10, etc.)?

Do they make multi winding transformers in wall-wort or brick format? I wasn't able to find much in my searching other than the standard single output wall-warts.

Thanks for all the suggestions everyone! It's great to see a community so helpful and dedicated to the cause! Hopefully as I learn more of this stuff I can start contributing back more as well :)

[m00dawg]

Actually, on the note of the voltage regulators getting hot, one alternative would be to use multiple voltage regulators for each "rail". So, for the +5V, I would have 3 dedicated just to the +5V (a 7812, 7809, and 7805). The +9V would then have it's own 7812 and 7809. A bit wasteful, and somewhat more expensive space and money-wise, but that might avoid pumping a huge amount of current through a shared 7812.

*shrug*

[m00dawg]

So here's an update. I dropped the jumper idea in this one since it seemed more trouble than it was worth. So this design offers 12, 9 and 5 and, thus, requires the use of a 14VAC input. I also did away with sharing the voltage regulators, so each output has it's own regulators in series. I get the feeling I might need more filter caps, but am still trying to read up on those sorts of things. Anyways, enjoy :)

powerboard-alt.zip

[m00dawg]

Here's another update. Now with fuses! I'm not quite sure if I might be overdoing it here, at least for the rectified DC output since I have both a 1A and 250mA fuse. I did that because all of the other DC outputs jump off that one but I am also supplying it as a raw output (in case another component already has a voltage regulator for instance.

powerboard-alt.zip