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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
MFOS Power supply stripboard
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thundarr



Joined: Jun 07, 2009
Posts: 124
Location: Newcastle upon Tyne

PostPosted: Tue Jun 30, 2009 12:02 pm    Post subject: MFOS Power supply stripboard
Subject description: First attempt and a few questions
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Hello there, folks.

Having recently built my WSG (My first foray into electronics which, to my surprise, worked first time), I have been trying out various oscillators with 9V batteries, and realised I will need a bi-polar supply for testing many of the designs I have found, and eventually building further projects.

So, I decided to try Ray Wilson's Wallwart PSU (here) out, and figured I would practice doing a stripboard layout at the same time.

If it's not too much bother, would anyone mind checking this stripboard layout to see if I have made any obvious glaring errors? I am particularly cautious with working anywhere near mains power, so I thought it best to triple-check and get any opinions I can. The two transistors are meant to be voltage rectifiers. I have removed a couple of the 3300u capacitors as a halfway-house between the 6 in the schematic and the 2 for the bare minimum.

I have a couple of other related questions too... Please excuse my ignorance here Wink

1. Is stripboard suitable for this task, as far as being able to handle the appropriate current?
2. I am unsure what exactly the ground connects to here. My first thought was the enclosure, as with the single pole 9V designs I have tested, but that seems like it might be a dangerous plan in this instance. Smile Where should it connect to?
3. Do the voltage rectifiers have to be screwed down parallel to the board, or can I leave them perpendicular?

EDIT: Removed the old dodgy layout. See halfway down the page for a working one.

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Last edited by thundarr on Wed Oct 07, 2009 5:55 am; edited 1 time in total
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magman



Joined: Feb 04, 2009
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PostPosted: Tue Jun 30, 2009 2:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

From a first pass, I can see a few things wrong with this layout.

I think D1 and D2 are reversed. The Cathode is normally the white stripe and for D1 this should be connected to the input of U1 for one rail, with the anode of D2 being connected to the input of U2.

I would check the polarity for all of the capacitors. The stripe on an electrolytic capacitor is normally the negative, not the positive.

The 7912 regulator, U2, has a different pin-out than a 7812 (Rays circuit diagram has the correct pin numbers), so you will need to redo all of the connections around this IC.

I think D5 and D6 are also reversed.

So basically, I would say that U1 and D3 are correct and the rest need to be looked at.

Regarding your other questions, it looks like you are using 7xL12 regulators, which have a maximum rating of 100mA, so you won't have to worry about the current carrying capacity of the tracks or the orientation of the IC's (they'll be quite happy just sitting proud of the board). If you plan to use the normal 7x12 regulators (without the L in the middle), these can supply up to 1A, but then would need heatsinks, for low power use though, they can just stand vertically in free air without heatsinks, if you find them getting hot in use (as you load up the PSU with more modules) you may need to look at adding heatsinks then. Again, there should be no issue with the current carrying capacity of the stripboard.

Last but no means least, if you need a bipolar supply for a circuit, it will have connections for both supply voltages as well as the ground. As you are using a wallwart PSU, there should be no other issues with grounding.

Hope that helps.

Regards

Magman
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thundarr



Joined: Jun 07, 2009
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PostPosted: Wed Jul 01, 2009 3:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Cheers for the prompt reply Smile I feel so foolish now, hehe.

I had some sort of brainfart and thought that the stripe on both the capacitors and diodes was the positive, for some reason.

I had also assumed that the 7912 pins were just the reverse of the 7812, thanks for pointing that out Smile

I'll have another go this afternoon... Many thanks for the extremely informative reply Smile
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magman



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PostPosted: Wed Jul 01, 2009 4:08 am    Post subject: Reply with quote  Mark this post and the followings unread

No worries,

If you don't ask your unlikely to learn.
There's no point in re-inventing the wheel.
If in doubt, Ask!

Very Happy

I could probably come up with a few more sayings and platitudes, but what I'm trying to say is that your kind of question is what this community is here for. Feeling foolish remotely is much better than the smell of burning electronics if you get it wrong in the real world (but that's part of DIY electronics sometime as well Wink ).

Ask away

Magman
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PostPosted: Wed Jul 01, 2009 5:08 am    Post subject: Reply with quote  Mark this post and the followings unread

Yeah man. There are no stupid questions, only stupid people! Or, um, just kidding there... ask away, it don't hurt!

Les

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thundarr



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PostPosted: Wed Jul 01, 2009 5:24 am    Post subject:
Subject description: Second attempt :)
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Ok, here is the new attempt. Let's see if this is a little closer to the mark Smile

EDIT: Removed the old dodgy layout.

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Meandering ambient drone doom
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Last edited by thundarr on Wed Oct 07, 2009 5:54 am; edited 1 time in total
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magman



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PostPosted: Wed Jul 01, 2009 6:21 am    Post subject: Reply with quote  Mark this post and the followings unread

That looks to be a lot better.

The only thing missing is the Ground connection out off the layout, I would suggest picking this up from the bottom right corner of the layout.

Next step is build it and test it.

Regards

Magman
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thundarr



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PostPosted: Wed Jul 01, 2009 6:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Cheers... Going to order some parts now.

Having trouble finding 3300u 35V capacitors, but I have found some 4700u 35V and 3300u 25V ones. Ray mentions 4700u capacitors when specifying example product codes, so I guess these would be the best to go for.

Thanks again Smile
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magman



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PostPosted: Wed Jul 01, 2009 7:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Your choice of the 4700u 35V is the best bet. For capacitors in parallel you sum the values, so 2 x 4700u per regulator is just about equivalent to the 3 x 3300u's that Ray suggested in his original design. It is a bit of overkill for a small PSU, but it won't do any harm.

It is not a good idea to go for a lower voltage though (as the output from the walwart could get close to 25V at peak and you should never specify a component at close to its limits).

It's also worth getting all of the components in hand before starting the build, as things like lead spacing can differ quite a bit between different specs of components.

Regards

Magman
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Uncle Krunkus
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PostPosted: Wed Jul 01, 2009 7:47 am    Post subject: Reply with quote  Mark this post and the followings unread

I can't quite tell which strips the caps are across.
I'd go for the 4700uF 35V caps. The only worry with using bigger caps is that the inrush current when you power up might stress your wall wart and/or fuses. Other than that, bigger is better as they dampen the ripple more.
You are wise in sorting out your PSU requirements first. Wink

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thundarr



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PostPosted: Wed Jul 01, 2009 9:09 am    Post subject: Reply with quote  Mark this post and the followings unread

Everything ordered... Just the long wait now!

I'll report back when I find out if it works / explodes / implodes / destroys my flat.
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thundarr



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PostPosted: Sat Jul 04, 2009 6:10 am    Post subject: Reply with quote  Mark this post and the followings unread

All built and tested with 1k loads, giving me +11.97V between + and ground and -11.94V between - and ground. That's close enough for me!

Re-arranged some components to allow more room for the 7812 and 7912 to lie flat and tidy some things up. The finished schematic is attached if anyone would find it useful.

Many thanks again for the help Smile


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magman



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PostPosted: Sat Jul 04, 2009 6:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Glad to hear it worked out OK.

Did you go for the 78L12 or the full fat 7812 regulators in the end (you're suggestion on laying the regulators flat suggest 7812's)?

Regards

Magman
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thundarr



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PostPosted: Sat Jul 04, 2009 9:40 am    Post subject: Reply with quote  Mark this post and the followings unread

I went for the 7812s... Might have a look for some heat sinks if they get troublesome. Smile
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Danno Gee Ray



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PostPosted: Sat Jul 04, 2009 12:21 pm    Post subject: problem Reply with quote  Mark this post and the followings unread

I hate to be a downer, especially now that you say it's working, but I see a very big problem with your layout as finally posted. If I'm reading it right, you have one of the input AC legs going straight through to the DC output ground. This is very dangerous. If you happen to connect the AC Hot signal to this instead of the one tied to nuetral, you stand a chance of getting a very bad shock from your kit.

The input AC should have a ground which is carried all the way through the circuit as the output ground as a safety feature. If you cannot use that, then at least run a gound that is not tied to any leg of the input AC signal.

Make sure you build Power Supplies carefully. The wrong mistake can be your last.

Dan

PS...I just rechecked Ray's layout and see that this is in fact what Ray has done. I may be too cautious in this, but do not like running one AC in rail as a DC ground, unless you KNOW it is the AC leg tied to Nuetral.
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Uncle Krunkus
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PostPosted: Sat Jul 04, 2009 4:32 pm    Post subject: Re: problem Reply with quote  Mark this post and the followings unread

Danno Gee Ray wrote:
If you happen to connect the AC Hot signal to this instead of the one tied to nuetral, you stand a chance of getting a very bad shock from your kit.


Remember that this is all on the secondary side of the transformer isn't it? Confused
That would only be about 18VAC wouldn't it? Active or Neutral.

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Danno Gee Ray



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PostPosted: Sat Jul 04, 2009 5:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes,

That's why I put in the PS. after the original posting. It is on the low voltage side of the transformer. Just seems to go against my training and gut feelings to float it like that...

If I was too alarmist, My apploogies.

Probably should have said nothing, the fact that all the engineers out there said nothing shoulda been a clue...Disregard my last, and stripboard away!
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Uncle Krunkus
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PostPosted: Sat Jul 04, 2009 5:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

Oh, don't get me wrong, I agree there should be a break in that strip.
I just meant that it would be unlikely that a dangerous voltage would come down there. (though not impossible, transformers do short out in unpredictable ways Wink )

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thundarr



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PostPosted: Sat Jul 04, 2009 8:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

Interesting... what would you change to the circuit to allow for this? I realise it may not be necessary, just trying to work out exactly what was suggested Smile
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Uncle Krunkus
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PostPosted: Sat Jul 04, 2009 9:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

It would actually take more than just cutting a track (because of the way the rectifying diodes are arranged) so I'd just leave it as it is for now.
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PostPosted: Sat Jul 04, 2009 9:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm can't think of anything you could do with this design to eliminate the suggested risk - and I wouldn't be the least bit concerned. Even if you went to a full wave rectifier and center tapped transformer the suggested risk of a primary to secondary short exists...
I'm not saying it can't happen, but the odds of a short in the wall-wart that would route mains voltage to the secondary is pretty slim.

Using a 'wall-wart' for a supply like this is about as safe as you are going to get.

That's this guys opinion....

bruce

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Danno Gee Ray



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PostPosted: Sun Jul 05, 2009 12:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well, I have to agree with Uncle K. and Ref, I went back and looked at some stuff and all i can say is...What freaking planet was I on?

The wall wart / transformer isolates the supply from line AC...
low voltage on the secondary reduces potential danger...

I guess I just saw it in situ without the transformer and threw up crosshairs, aimed and fired without thinking it through.

In the end all I can really say is...My Bad...
and well corrected by the crew.
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Uncle Krunkus
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PostPosted: Sun Jul 05, 2009 4:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

Fair enough Danno, we all get "carried away" at times, life's like that when your passionate about things. Very Happy

I still wonder why Ray did it this way though. And your concern is what got me interested. Wink

Using an AC wall wart means there is no grounding to the mains involved.
So why not isolate the ground at this point? This may be someone's first PSU on their modular, and as such, it should also be their first "star point" ground connection. (which I would not want connected to one side of an AC wall wart!)
The difference is only an extra two diodes and a slight change in the layout.
Am I missing something?

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Danno Gee Ray



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PostPosted: Sun Jul 05, 2009 5:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

I would tend to agree with you. I'm so used to seeing full wave rectifiers in bipolar supplies, with the centralized ground as you described.

Not sure why Ray designed this as he did, and I'm not on thick enough ice to speculate.
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PostPosted: Sun Jul 05, 2009 5:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well, we want the modular to be grounded to earth ground, which is the third (round) plug on a three-prong power plug. Using a wall-wart as Andy mentions means that there is no such ground on the power supply. This is not good because there will be noise induced into the circuit due to the floating ground. If you ever hook up an output to a computer's line-in jack then the computer will ground the system.

I would suggest running a ground wire to the system from a separate plug. That sounds kind of weird, having two plugs, one just for the ground, but it's one safe way of doing it. Hmm, I wonder, with all the floating synths there must be out there, i guess it's not that big of a deal. I floated my scope for years and had no troubles, so I guess it's ok but for safety's sake at least, ground it.

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