PSU Question in relation to Midibox SID

Design Concepts
1

Hi there,

After leaving my unfinished project on the shelf because of a PSU problem I couldn’t fix, I’m giving it another shot to get it working.

This questions have probably been answered before, so if there’s an existing thread that could be interesting feel free to post a link (I did use the search, but that just came up with too many topics to check :wink: )

The story: My C64 PSU just couldn’t cope with more than one set of Core and SID drawing power, so I decided to build a PSU from scratch. Luckily, my dad just retired from work so he’s got lost of time now, and he’s reasonably experienced with electronics, so he’s offered to help me a bit. We found a nice transformer in the local used parts shop, which delivered 2 x 9Vac (8,5Vac to be precise), at about 4A. We removed several windings from the coil, frequently measuring the delivered volts, and right now it’s delivering 5Vac and 9Vac (5,8Vac and 8,5Vac). We also got 2 nice overrated bridge rectifiers to convert the AC to DC.

So far so good, now the questions:

In the schematics, on the 5V part a 2200uF/16V cap is used, while on the 9V part a 2200uF/25V cap is used. Why the difference, and can I use 2200uF/25V caps for both parts?

I am planning to use only 8580 SIDs, so there’s no need for 12V, right? So, is there any reason to implement the connection between +5V and -9V right after the 7809 to cranck up the voltage to 14V, only to reduce it to 9V on the individual board later on? (Is the connection called a “feedback loop”?)

Because I’m not using the C64 PSU, there’s no 5Vdc incoming, rather 5Vac. I already implemented a bridge rectifier in the schematic, but I think I also need a 7805, since that is also part of the C64 PSU, right?

If for any reason I do need the connection between +5V and -9V, do I connect the +5V before or after the 7805?

The general idea is to put all the power-conversion components on one board, to supply clean current directly to the other boards without the need to alter the current on every individual board. Can I put the 10mF and 100nF caps on the PSU board as well, or do they need to be close to other components?

I hope this makes sense, because I’m not really into electronics at all  :-[

Thanks in advance!

Alex.

2

Hi Alex,

In the schematics, on the 5V part a 2200uF/16V cap is used, while on the 9V part a 2200uF/25V cap is used. Why the difference, and can I use 2200uF/25V caps for both parts?

the maximum voltage of the cap should be at least 2 times higher than the AC voltage.

I am planning to use only 8580 SIDs, so there’s no need for 12V, right? So, is there any reason to implement the connection between +5V and -9V right after the 7809 to cranck up the voltage to 14V, only to reduce it to 9V on the individual board later on? (Is the connection called a “feedback loop”?)

In this case there is no need for 12V. But since your transformer only delivers 9V AC, and the 7809 requires a little bit more (appr. 11V) to regulate the DC voltage, you propably need to use the same method like with the C64 PSU.

If for any reason I do need the connection between +5V and -9V, do I connect the +5V before or after the 7805?

My strong suggestion: draw a schematic of the circuit you are planning to build, and post it here. 1000 words are no guarantee for correctness, only with the schematic somebody can say if it will work or not

Best Regards, Thorsten.

3

Hi TK, thanks for the quick reply!

We also tested the output with the rectifier and a 4700uF capacitor in place , the output was about 10,5 Vdc would that be enough to power the 7809?

I have drawn 2 schematics, with different places of the feedback. I’ll try to get them hosted somewhere soon.

Alex.

4

Here are the schematics:

http://home.quicknet.nl/qn/prive/alex.span/midibox/psu1.png

and

http://home.quicknet.nl/qn/prive/alex.span/midibox/psu2.png

Thanks in advance!

Alex.

5

Hi Alex,

psu2.png is the better one, but note that the GND output of the 7809 branch is not ground, it’s on the same level like the 5V output of the lower branch.

On the other hand: if you’ve measured 10.5VDC before the 7809, than this is excellent! So, in this case, you could connect the AC output of the transformer directly to J1 of the SID module

Which voltage to you measure before the 7805? Because I fear that it’s not high enough

Best Regards, Thorsten.

6

Hi TK,

the voltage of the 5Vac is about 5.8Vac, without rectifier or capacitor, so this can be easily brought to 7V or more in front of the 7805 with a good capacitor I guess.

If I understand correctly, the ideal voltages before 7809 and 7805 are 11V and 7V? And if that is so, I can do without the connection between +5V and the 9V part?

Do I read correctly that you advise me to connect AC current to the SID boards (I know there’s another rectifier there) if it’s 10,5V or (slightly) above? My plans were to create a nice clean 9Vdc on the PSU board and supply this directly to the AID boards, omitting the rectifiers there. Are there serious objections against doing so, apart from risk of swapping polarity?

Cheers,  Alex.

7

If I understand correctly, the ideal voltages before 7809 and 7805 are 11V and 7V? And if that is so, I can do without the connection between +5V and the 9V part?

yes!

This connection is only required if somebody wants to use the C64 PSU

Do I read correctly that you advise me to connect AC current to the SID boards (I know there’s another rectifier there) if it’s 10,5V or (slightly) above? My plans were to create a nice clean 9Vdc on the PSU board and supply this directly to the AID boards, omitting the rectifiers there. Are there serious objections against doing so, apart from risk of swapping polarity?

No, there are no objections so long you know what you are doing - take care for the polarity, once swapped, it will destroy your chips! You could add a shottky diode in parallel to the 5V/ground rail of the core module, and the 9V/ground rail of the SID module in order to protect the modules against polarity issues. The anode has to be connected to ground in this case

Best Regards, Thorsten.

8

Hi,

I adjusted the schematics like this:

Any comments?

I didn’t add the diodes yet, are schottky diodes the same symbol as normal diodes?

I plan to put 4 connectors on each branch, to wire all the boards in a star topology.

Something I didn’t think about yet: the SID boards use +9v and +5v, but only use the ground of the 5v, right? I have a feeling I cannot connect the grounds of both 5v and 9v, is that correct? Is it a bad idea to wire the SID boards with +9v, +5v, and the GND from the 9v branch?

Thanks, Alex.

9

Hi Alex,

the link to the schematic is missing…

No, schottky diodes have a slightly different symbol. But before you are doing a seperate order: you can also use a common diode like 1N4001, it will do the same so long the PSU is not too strong

The ground of the 5V and 9V pin is the same, the SID chip doesn’t provide a clear seperation between analog and digital ground. When thinking about wiring: star topology is the best, this means, that a ground connection between SID and CORE is not required (this would result into an unwanted ground loop)

Best Regards, Thorsten.

10

Hi,

oops  :-[ never noticed the schematics where missing…

well here they are anyway: http://home.quicknet.nl/qn/prive/alex.span/midibox/psu.png

OK, I’m going to connect the SID modules to the GND of the 9V branch, and the Cores to the GND of the 5V branch. I’ll take your advice on the diodes into account. I’ll post my definitive schamatic as soon as I’m done with testing, I suspect I might need slightly bigger caps right behind the trafo, like 4700uF.

Thanks!

11

Hi,

some small questions:

If I’m going to replace the 2200uF caps by bigger ones, should I also change the 330nF caps for bigger ones?

If I’m going to use the schottky diodes, where in the chain is the best place to put them? Right at the end, just before the power goes to the boards?

Thanks in advance,

Alex.

12

Hi all,

I’ve completed my PSU!  ;D

Voltages delivered are 5,06 Vdc and 8,93 Vdc. I’m sure the 5V line is OK, is 8,93 V enough for the SID line (8580’s ofcourse)?

Some pics:

http://home.quicknet.nl/qn/prive/alex.span/midibox/psuboardtop.jpg (looks pretty 8) )

http://home.quicknet.nl/qn/prive/alex.span/midibox/psuboardbottom.jpg (looks awful :stuck_out_tongue: )

Final schematic:

http://home.quicknet.nl/qn/prive/alex.span/midibox/psufinal.png

Hope to etch a PCB for this one.

By the way, there re a few (6 or so) of these trafo’s left at the store I bought mine, they are pretty easily modify-able to supply 8,6Vac (standard) and about 5,8Vac (modified), and they cost about 3 euro’s. If anyone’s interested drop me a message, and I’ll see wht I can do for you.

Cheers,  Alex.