[XCSSA] Power Supplies Continued

[X-otic Computer Systems of San Antonio]

Ah ok. I was never really sure how the ground prong fit. I mean I know  
it's an earth ground to prevent grounding to, say, the operator, but  
beyond that never was quite sure what to do with it.

As for the regulators, I actually use 7809 and 7805's right now. The  
difference is that they are in series. So, basically:

9-12VAC -> Rectified -> Smoothed (2200uF, 330nF, 100nF), 7809 -> 7805 - 
 > More Smoothing Magic (330nF, 100nF, 10nF) -> 5DC
							   \  -> More Smoothing Magic -> 9DC

This seems somewhat inefficient because the 7809 has to power both 9DC  
and give power to the 7805 so that it can produce 5DC. So what I was  
thinking, instead, was something like this:

9-12VAC -> Rectified -> Smoothed -> 7809 -> More Smoothing Magic -> 9DC
5-7VAC ->  Rectified -> Smoothed -> 7805 -> More Smoothing Magic -> 5DC

The trouble here is I either need two transformers or a multi-tap that  
will give me something close to those voltages. I have yet to find one  
of those :/

The reason I need +9 and +5 are because the SIDs use +9 and everything  
else uses +5. That's part of the problem. Some estimates put the  
current draw at around 1000mA off the +5. The SIDs only chew up around  
300mA on +9, however. So, if I use 9-12VAC for everything, I end up  
having to throw off quite a bit of excess heat off the 7809 and  
7805's. A gigantic heatsink might solve that problem, but it still  
seems a bit gacky :)

Now the board I'm likely going to buy already has things to use a C64  
PSU with it. So I could either use that, or perhaps modify my design a  
bit and provide 9AVC and 5DC and use the same C64 style plug...hmm...

Thanks, as always, Fredrick! How's the AppleDore coming along, by the  
way?
	



On Dec 27, 2008, at 4:39 PM, X-otic Computer Systems of San Antonio  
wrote:

> Howdy...
>
> Personally I would fuse the hot line of the 120 VAC input.
> I would not ground the circuit to the third prong, however if you  
> have a
> metal case, it would be a good idea to ground it.
> The third prong is a safety ground, not a signal ground so to say...
>
> I'm not familiar with building a power supply but I can have a look at
> it if you want to.
> I would find a regulator (7805, LM1086, LM2940 or similar) that fits  
> my
> need and follow the manufacturers recommendations...
>
> Start by specifying your needs and then search for a regulator that  
> fits.
>
> /Fredrik
>
> X-otic Computer Systems of San Antonio wrote:
>> Alright I've got another couple of questions about power supplies :)
>> While I am likely going to make things easier on myself and buy a  
>> pre-
>> made mainboard and parts kit for my MidiBox SID, I still need to
>> figure out how to power the thing. Fredrick was kind enough to let me
>> have an extra C64 PSU, but I'm actually thinking it might be fun to
>> make my own power brick.
>>
>> The idea is to have 1 or 2 transformers (I'm having trouble finding
>> multi-taps that output 9 and 5VAC) connected to mains and then  
>> rectify
>> and regulate those. My previous method called for using voltage
>> regulators in series with a big heatsink. That works, but it seems
>> kinda lame and intentionally less efficient.
>>
>> So, my questions are then - for fusing the 120VAC input, do I just
>> need to fuse the positive line (and not the neutral)? And, assuming I
>> use a 3-pronged cord, do I ground everything to the 3rd prong
>> (basically connect it to the ground plane of the board, assuming I  
>> get
>> a printed board)? Also does anyone know what those laptop style cable
>> and connectors are called? You know, the ones that has a standard 3-
>> prong on one and hexagonal female on the other? It looks sort of like
>> a nuclear warning sign. Dell uses it alot on their connectors. I
>> figured that would be easier to use than trying to strip a normal AC
>> table and all that stuff.
>>
>> If anyone wants to see my (very much alpha) design, I'd be happy to
>> send it your way!
>>
>> Thanks! Oh and Happy New Year!
>>
>> Tim
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>>
>
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