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 Forum index » DIY Hardware and Software » Developers' Corner
Power Supply Question
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kxspxr



Joined: Nov 20, 2008
Posts: 35
Location: Denmark

PostPosted: Wed Mar 18, 2009 3:56 am    Post subject: Power Supply Question
Subject description: PSU for my homebuild modular
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Hello all - I have a few questions about power supplies I was hoping someone could answer. I'm almost finished with my modular system and I loathe the idea of using 8 9V batteries to power the whole thing. I never worked with wallwart power supplies in my projects so I don't know how to go about that.

Every module in my modular has a 7805 +5V regulator and if I am using a power supply to power the whole thing, I would only need one of those, right? This leads to another question: If so, can I use an ordinary 9V 50-100mA wallwart power supply, and how would I go about distributing the power to all the modules? They need to have the same common ground too, right?

Right now every module is actually stand-alone units and do not share ground unless two modules are connected using patch cables. Basically my questions are: (to sum up) How do I make a power bus and distribute the power to the modules? (I have the power supply) AND do they have to have a common ground (my guess is: Of course they do) But again, I don't have ANY experience with wallwart supplies. I'm kinda eager to get this right since I am almost done bulding the modular...

-kasper

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scriptstyle



Joined: Jan 22, 2008
Posts: 250
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PostPosted: Wed Mar 18, 2009 5:53 am    Post subject: Reply with quote  Mark this post and the followings unread

make sure to add the current of each module together before you put a 100ma wallwart on a system that requires something like 750ma. Also I'm quite sure your regulators are good up to a amp. It may have been better to use one regulator closer to the wallwart with a mini decopling board there. But I'm not exact on what your application is.
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kxspxr



Joined: Nov 20, 2008
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Location: Denmark

PostPosted: Wed Mar 18, 2009 9:46 am    Post subject: Reply with quote  Mark this post and the followings unread

scriptstyle wrote:
make sure to add the current of each module together before you put a 100ma wallwart on a system that requires something like 750ma.


How can I do that?

scriptstyle wrote:

Also I'm quite sure your regulators are good up to a amp. It may have been better to use one regulator closer to the wallwart with a mini decopling board there. But I'm not exact on what your application is.


It's basically a modular lunetta I'm building. The VCA's, EG's and Mixer isn't kept within the Lunetta style but the rest is. I thought about doing just that, using only one regulator. Can you supply a link or a schematic for a decoupling board? Or is it just a 47uf cap from +V to GND before the 7805? Is it possible to power 8 or more units to this decoupling board just like that, with wires from +5V and GND to all the units? I don't know anything about powering something if it's not from batteries, so I'm kinda precautious Smile

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scriptstyle



Joined: Jan 22, 2008
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PostPosted: Wed Mar 18, 2009 12:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

Nothing wrong with being precautious! Search the lunetta forum for the psu thread. Decoupling board/regulator board on the lunetta thread it's called a psu. Call it what you want? Do you have a dmm?
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kxspxr



Joined: Nov 20, 2008
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PostPosted: Fri Mar 20, 2009 6:56 am    Post subject: Reply with quote  Mark this post and the followings unread

scriptstyle wrote:
Nothing wrong with being precautious! Search the lunetta forum for the psu thread. Decoupling board/regulator board on the lunetta thread it's called a psu. Call it what you want? Do you have a dmm?


Thanks! Found it - seems pretty easy. Have a bunch of 7805 - I'll just one of those. What's a "dmm"?

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Blue Hell
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PostPosted: Fri Mar 20, 2009 12:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

dmm = digital multi meter, usually ...
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kxspxr



Joined: Nov 20, 2008
Posts: 35
Location: Denmark

PostPosted: Fri Mar 20, 2009 5:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
dmm = digital multi meter, usually ...


Ah of course! Yeah I have a dmm.

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scriptstyle



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PostPosted: Fri Mar 20, 2009 9:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

Once you have your power setup. I would hookup each modual indivdually and check DC current with your multi meter at the conection to the psu over the 5v.
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kxspxr



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PostPosted: Sat Mar 21, 2009 10:53 am    Post subject: Reply with quote  Mark this post and the followings unread

scriptstyle wrote:
Once you have your power setup. I would hookup each modual indivdually and check DC current with your multi meter at the conection to the psu over the 5v.


Thanks - I'll try that!

-kasper

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Tony Deff



Joined: May 25, 2008
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PostPosted: Sun Mar 22, 2009 2:16 am    Post subject: Power Supply question
Subject description: Multiple regulators
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Quote:
Every module in my modular has a 7805 +5V regulator and if I am using a power supply to power the whole thing, I would only need one of those, right?


Why so anxious to throw them out?
"Local regulation" has the advantage of isolating each unit from each other, and acting as virtual decouplers because of their low output impedance, so you would then need little or no decoupling on the output side of each regulator, which is far from the case when the power arrives by wire.

Take all returns to a single Star-point, which in the good old days used to be where the chassis was earthed. Just take care to work out which units are carrying significant currents or high frequencies. Imagine passing the returns of these units through a small resistor (= , thin wiring or bad soldering, following indirect routes). It is then easy to imagine what can happen to sensitive modules by grounding them "on top" of this resistor, so it all becomes very intuitive.
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kxspxr



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PostPosted: Wed Mar 25, 2009 3:17 am    Post subject: Re: Power Supply question
Subject description: Multiple regulators
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Tone-Deaf wrote:

Why so anxious to throw them out?
"Local regulation" has the advantage of isolating each unit from each other, and acting as virtual decouplers because of their low output impedance, so you would then need little or no decoupling on the output side of each regulator, which is far from the case when the power arrives by wire.


Smile As I said, I have absolutely no experience with PSU's. Have been avoiding them every design I ever made and build. I chose the safe route of 9V batteries. Since I'm self taught in electronics and none of my friends knows anything about electronics (or anyone else I know for that matter) I never dared trial/error with PSU's. I designed the units to run off 9V batteries but figured now would be the right moment to leap out of my shell and reach for some new knowledge! Thanks! I'll keep the 7805's right where they are.

Quote:

Take all returns to a single Star-point, which in the good old days used to be where the chassis was earthed. Just take care to work out which units are carrying significant currents or high frequencies. Imagine passing the returns of these units through a small resistor (= , thin wiring or bad soldering, following indirect routes). It is then easy to imagine what can happen to sensitive modules by grounding them "on top" of this resistor, so it all becomes very intuitive.


I decoupled all my CMOS modules with a 0.1uF close to the IC, the opamp circuits (I use LM5532) are decoupled 10uF. What do you mean with "Take all returns to a single Star-point" -what do you mean by returns? Ground? -So basically I can connect wires from +9V and GND on the PSU to all the units, provided the units doesn't draw too much current compared to what the PSU can handle? Excuse me for asking about this again, but I need to be sure I understand this before I go around setting fire to my apartment in the process of trying to power my modular Smile

-Kasper

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Tony Deff



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PostPosted: Thu Mar 26, 2009 5:49 am    Post subject: Returns
Subject description: Fire Insurance
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Quote:
I need to be sure I understand this before I go around setting fire to my apartment in the process of trying to power my modular


It is taken for granted that you have Fire Insurance before switching-on the soldering-iron. The Return is the receipt from your Insurance Company that indicates you are now covered to set fire to your apartment, but watch-out for clauses in the contract that prohibits arson or criminal neglect. It is not enough for you to say to your Insurance Company that you were only trying to follow advice from me. (Nor do I have enough money for you to try and claim from me).

Another form of return is, indeed, the power supply lead that you have allocated as "ground" or "earth" (on Mars, this is called "Mars"). So you have Supply (which can vary a few Volts and cause no effect because the regulators are "high-impedance" and so you can wire them like a rat's-nest) and Return, to which everything in your circuits becomes referenced, including the regulators, and every op-amp).

Supposing you had a power amplifier somewhere in your system, gulping-in 2 Amp peak currents from the supply wire, through the speaker, and "returning" it down a long, thin wire back to the Power Supply Unit (PSU). Suppose the resistance of this wire (including solder tags and joints, terminal blocks and whatever) was 0.1 Ohm.

There will be a potential difference along the length of this "return" of 200milliVolts (in rough peaks). Depending on where along this wire you "ground" the other modules, they could have 200mV unintentionally injected into them, interfering with your required signals.

At the best, you will have "hum" (100Hz); at the worst, oscillation or some other weird and mysterious symptom.

Another, far better, "return" is the money that comes pouring through your letterbox from people buying your equipment. This can only happen if the equipment works well, and that means the power supply "returns" are carefully arranged. A "star-point" is the single point where all the Returns meet, and at this point it is often connected to the "earth" of the house (water pipes etc) via the mains plug. On a circuit-diagram, it is drawn to look like a "star", with all the wires coming-in at different angles.

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kxspxr



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PostPosted: Fri Mar 27, 2009 3:07 am    Post subject: Re: Returns
Subject description: Fire Insurance
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Thanks for your thorough explanantion! It's making sense which is good.

-Kasper

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