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Midibox PSU


NorthernLightX

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  • 2 weeks later...

The latest news is that the guy who was going to etch the board did them all mirror-image...  >:(

Also the drilling is sometimes a bit small, too small for the components that have to go through the particular holes.

After some testing this does not seem to be too much of a problem, the holes can be widened with a small drill and a steady hand, and the mirror-imaging only affects the the regs; they are not placed optimally for heatsinking anymore.

So Wisefire, I have a mirror-image board lying around with your name on it.

No tests on the power delivery yet, have been MUCH too busy...

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mirror image.. hmm.. thats less then optimal.. oh btw.. the order from reichelt.. specially the parts for my MBFM.. im missing some parts.. :S

but about that mirror image thing.. is it still gonna cost me 7 euros?

Please send me a mail about this, better to duscuss that in private.

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  • 4 weeks later...

hey NLX,

i was wondering.. ive got 2x 12VAC wirepairs..

where should i attach the second pair??

Hi Wisefire,

youve got 2x 12v + and 2x 12v -

one of the + wires goes to the ground pin, and one of the - wires too. The other + and - wires go to their respective pins. This you could have guessed from the schematic. (I am happy to answer questions, this was just a hint where to look for answers if I'm taking a long time ;) )

Cheers,  Alex.

[edit]

I just checked the schematic, and I must have had an earlier schematic in mind, because the trafo is not pictured in this one. If you still have questions don't hestitate to contact me.

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the green red (1 wire pair) and the brown blue (another wire pair) both feed out AC.. this is normal and im not confused by that.. however ive got 2 pairs of 12VAC coming out of the ringkern.. and i only see one connection on your board for it (and a ground pin which i assume goes straight to earth)

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OK, I think I have been a bit unclear. I'll try to explain again:

The trafo gets 230VAC from the mains socket. Those are the yellow wires.

The secondary wires are paired, since I know exactly which trafo you have I can tell you which wires do what, but you could have read it from the trafo too: Green and Red are a pair, and Brown and Blue are a pair.

If you solder the Green wire to the first pin, the Blue wire to the third pin, and both Red and Brown to the middle pin, it should work.

The real + and - are only created after the rectifying diodes, With + and - I ment the branches on the PBC to create positive and negative tension. Hope this clears things up a bit.

Cheers,  Alex.

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I did some work on this myself and here is a simplified +/-12 +5 V one.  I added some protection diodes to the 12V lines so when they are used to drive op amps (which is the case for me) they will prevent the power up cap short.  I also added spaces for more big caps.

psu_art.jpg

psu_scm.jpg

And the Trace Art:

The regulators are shown flat but should be mounted vertically for use with heat sinks, i could not find a pin compatible to-220 package in the library so i just used those

The Transformer I used was a Hammond 166L24 24ct VAC / 2 Amps  (mouser p/n:546-166L24)

and the power resistor was an 8 ohm 10 watt Xicon type 280-CR10-8.2-RC (mouser p/n:280-CR10-8.2-RC)

I will be testing it on my SID x4 and get results up promptly.

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  • 2 weeks later...

I'm hopeless at electronics, so please bear with me. i'm having trouble understanding the following things with this psu:

1. what is the reason for all the capacitors on the psu, i understand you need one or two for smoothing out the a.c. but why so many?

2. the 220 on the led circuit, is that 220ohms or 200kohms or what? i assume 220ohms, but thought i'd ask.

3. i can see a 7805 outputting 5V and a 7812 outputting 12V, but on TK's optimised psu, there's a 7809 (which i'm assuming puts out 9V because that's what you use on a sid module for the 8580 SID) outputting 14V??? i just don't get it. i can understand that something like a resistor knocks voltage down to a value that is relative to the input voltage, but i thought voltage regulators put out what they're designed for - a set voltage - what ever their input voltage is (up to a certain limit of course).

any explanations are welcome.

thanks.

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The caps are for smoothing.  There is a formula for calculating what you need for smoothing based on load, but I generally go overboard with over designing stuff, hence the 4 big caps (I may only use one for the -12).  The idea is that you want to remove any oscillations in the power.  The higher the load, the more pronounced they will be so the bigger the cap, the less the effect.  The small caps are there since they charge quicker than the big caps.  I'm driving both the 7812 and 7805 from the positive out of the rectifier so the draw there will be relatively large and the big caps should be necessary.  The big honking resistor is to lower the voltage before hitting the 7805, otherwise putting 12V directly on the 5V regulator would make it fairly hot.  I added diodes on the +/- 12 rails (per the 7912 sheet) to prevent a power up short since caps apparently short for an instant before charging and this would be bad for a the op amp i am driving.

In TK's psu, notice that the 7809 pin 2 is connected not to ground but to the +5VDC line, therefore 9+5 = 14V

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The caps are for smoothing.  There is a formula for calculating what you need for smoothing based on load, but I generally go overboard with over designing stuff, hence the 4 big caps (I may only use one for the -12).  The idea is that you want to remove any oscillations in the power.  The higher the load, the more pronounced they will be so the bigger the cap, the less the effect.  The small caps are there since they charge quicker than the big caps.  I'm driving both the 7812 and 7805 from the positive out of the rectifier so the draw there will be relatively large and the big caps should be necessary.  The big honking resistor is to lower the voltage before hitting the 7805, otherwise putting 12V directly on the 5V regulator would make it fairly hot.  I added diodes on the +/- 12 rails (per the 7912 sheet) to prevent a power up short since caps apparently short for an instant before charging and this would be bad for a the op amp i am driving.

thanks for taking the time to explain that. very informative.  ;D

In TK's psu, notice that the 7809 pin 2 is connected not to ground but to the +5VDC line, therefore 9+5 = 14V

ahh yes, simple when you know, how embarrassing!  :-[

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That is to be expected if you do not have anything connected behind your reg. The regs need a load to regulate properly.

is it possible to put a load on the psu before attaching it to the midibox core etc to find out the voltages, or would you need to know the current draw of your setup?

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I got some testing done today and so far so good.  Reading 5/12/-12 and tested it today on the modifed SID board with success.  Subjectively, the SNR was at least as good as the optimized c64 PSU.  I'll hook it up to the spectrum analyzer next week and compare ;D  I only had time to run one SID/Core pair with step B control surface so I have yet to run all 4, but thats next.

The modification to the sid board was exactly like on the core, pull the regulator and rectifier, bridge the regulators in/out with a jumper, and supply +12 via available pin.  I used the + leg of the removed recitfier which puts the power in on the corner of the board.  Works perfect

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right i've just built this circuit too, with some minor changes, trying to incorporate some of the 'optimised psu'. basically sending 15VDC to the SID module (which still has it's 7812 in place) with a 7815, and then using the 8R2 resistor to try and drop the voltage for the 7805. however, i've found that the resistor idea doesn't seem to knock the voltage down too much, only about 1V, so the 7805 is recieving 16.5V! (the output of the rectifier being 17.5V) and as a result it gets pretty hot. i read on here somewhere that chaining VRs is not a good idea from a noise point of view, but i was thinking of running the 16.5V through a 7812 or 7809 before the 7805 to try and spread the load a bit. it's this a good idea, or not?

ps, i have a 12VAC input, but i'm thinking 9V would also work.

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