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crashlander42
Joined: Oct 21, 2006
Posts: 292
Location: Orlando, FL (US)
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 Posted: Sat Mar 03, 2007 2:31 pm Post subject:
 Some questions about DIY power supplies |
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I just put together my first diy power supply (and I'm still alive!) using this layout and have a couple questions.
My voltages coming out of it are not exactly 12v. I have +11.75 and -11.83. Is this type of variation normal?
I seem to have my positive voltage coming off of my 7912 and my negative is coming off my 7812. Is that even possible? Positive voltage off of a negative regulator?
Any one built this before that might have some insight into whats wrong, or maybe it's just my multimeter.
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blue hell
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| Posted: Sat Mar 03, 2007 2:42 pm Post subject:
Re: Some questions about DIY power supplies |
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| crashlander42 wrote: | I seem to have my positive voltage coming off of my 7912 and my negative is coming off my 7812. Is that even possible? Positive voltage off of a negative regulator?
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That's definitely an unwanted situation ...
I'd check the voltage after D1 and D2, after D1 there should be a positive voltage and after D2 a negative one. If not put the diodes the other way around - I hope that he regulators did survive this ... but they do have every right to give up under such a condition I'm affraid. Also C1 and C2 will not be too happy about this, so don't leave it on too long.
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also .. could someone please turn down the thermostat a bit.
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fonik
Joined: Jun 07, 2006
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Location: Germany
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| Posted: Sat Mar 03, 2007 3:43 pm Post subject:
Re: Some questions about DIY power supplies |
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| crashlander42 wrote: | | Any one built this before that might have some insight into whats wrong, or maybe it's just my multimeter. |
just to be shure: how did you use the multimeter? you don't mixed the connections? com to ground?
sometimes we do these strange things... (i recently built a MFOS dual VCA, hooked it up and it didn't do anything - just nothing. everything seemed to be okay - and eventually i recognized that i've forgotten to put the IC into the sockets!!!!)
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blue hell
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| Posted: Sat Mar 03, 2007 4:31 pm Post subject:
Re: Some questions about DIY power supplies |
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| fonik wrote: | | sometimes we do these strange things... |
Oh yes we do, good question about the polarity !
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Jan
also .. could someone please turn down the thermostat a bit.
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crashlander42
Joined: Oct 21, 2006
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fonik
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| Posted: Sun Mar 04, 2007 4:04 am Post subject:
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hey, that's great. simple solutions are what we want sometimes (i was very happy when i recognized i only forgot the ICs!) 
BTW the variation is normal, i think. you may want to take a look at the datasheet.
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Uncle Krunkus
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| Posted: Mon Mar 05, 2007 5:22 am Post subject:
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Yeah,
The spec's only promise somewhere between 11.5 and 12.5 for the 7812. I imagine the 7912 wouldn't be any more accurate than that.
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stefano
Joined: Nov 23, 2006
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| Posted: Fri Mar 09, 2007 7:30 am Post subject:
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| Uncle Krunkus wrote: | Yeah,
The spec's only promise somewhere between 11.5 and 12.5 for the 7812. I imagine the 7912 wouldn't be any more accurate than that. |
I built the same project and I get a -12V from negative voltage and a +10.5V voltage from the positive one. Should I change the 78xx regulator ? |
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Uncle Krunkus
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| Posted: Fri Mar 09, 2007 2:12 pm Post subject:
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I would check everything else first. (diode & cap polarity?) The 78XX has a different pinout to the 79XX, is that okay? It's pretty rare that a regulator would be so far out without just dying. Does it run hot?
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crashlander42
Joined: Oct 21, 2006
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Location: Orlando, FL (US)
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| Posted: Fri Mar 09, 2007 6:55 pm Post subject:
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Once I corrected my multimeter problem (  ) I found I was getting +11.98 & -11.94. Which seems reasonable considering the specs for the regulators. Make sure you've got good heat sinks. Those things get hot.
While we're on the topic. This seems like a good quick cheap (cost me about $3) and easy power supply project, but I'm sure it wouldn't run a modular. Is there a DIY power supply project out there for greater power needs?
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zipzap
Joined: Nov 22, 2005
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| Posted: Sat Mar 10, 2007 3:04 am Post subject:
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I think it´s quite ok. I would use a full wav rectifierer (4 diodes in a ring). Better because it produces less ripple and the regulators have an easier job.
The rest of the design is common and has been used many times.
The cabs can be increased (i use 10000uf), and you can use regulators with different power ratings. I use the 1A types, enough for quite a number of modules. you can also use more of those regulators in parralell if one is not enough. just give each regulator a 10uf and a 0.1uf cab. But don´t connect the outputs, they are to be used seperately for different mudules.
Only grund is connected (right by the big cabs)
Only thing to look after is that the transformer and the diodes can handle the power. The 0.1 cabs must be right by the regulator, i soldered them directly to the pin.
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zipzap
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crashlander42
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Uncle Krunkus
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| Posted: Sun Mar 11, 2007 3:43 am Post subject:
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| zipzap wrote: | | I think it´s quite ok. I would use a full wav rectifierer (4 diodes in a ring). Better because it produces less ripple and the regulators have an easier job. |
Keep in mind though that you can't do this if using a transformer like the one in the first schem. To have full wave rectification on a bipolar supply you'll need a centre tapped transformer, like the one in that last schem.
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Uncle Krunkus
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| Posted: Sun Mar 11, 2007 3:59 am Post subject:
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| crashlander42 wrote: | | While we're on the topic. This seems like a good quick cheap (cost me about $3) and easy power supply project, but I'm sure it wouldn't run a modular. Is there a DIY power supply project out there for greater power needs? |
Because it uses a single tap transformer, the positive going cycle goes to the +ve regulator, and the negative one to the -ve. This means that the current you draw from either side can only be half of what the transformer is rated at.
The transformer is always going to be the most expensive part of a PSU like this, and will be the deciding factor in it's current handling capabilities. The diodes, regulators and caps don't cost very much and will be basically the same, right up to one which supplies 1Amp per rail. (which would be fine for most medium sized modular setups)
The type of transformer I'd recommend is a toroidal one like this
http://www.jaycar.com.au/productView.asp?ID=MT2113&CATID=19&keywords=&SPECIAL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=539
About AU$50
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crashlander42
Joined: Oct 21, 2006
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Location: Orlando, FL (US)
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| Posted: Sun Mar 11, 2007 8:53 am Post subject:
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AH! I'd been wondering what those big copper doughnut lookin things were.
I've had a lot of luck with finding cheap parts. I have this Skycraft place down the street from me thats been selling surplus for NASA and defense contractors for the last 30 years. They even had LM709's. I'll have to check them for copper doughnuts.
Thanks!
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