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Simple Power Supply Question
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Simple Power Supply Question

by msymms on Wed Sep 22, 2010 3:59 pm

Hi All,
While I await parts for my other project (and thanks to all who have supplied their wisdom) I thought I would pose a question that seems simple to answer, but, since I am no EE, the solution eludes me.
Given a +V DC supply (pick a voltage 12, 5, 3 etc.) how does one get separate outputs of +V and -V (same voltage is not necessary)?

-mark
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Re: Simple Power Supply Question

by Hfuy on Wed Sep 22, 2010 4:10 pm

Using some sort of switching converter. They can be had off the shelf:

http://www.tracopower.com/

...or many others. You can build them, but they are notoriously twitchy devices to make and unless you need them in large numbers it probably isn't worth it.
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Re: Simple Power Supply Question

by oPossum on Wed Sep 22, 2010 4:21 pm

For low current, a capacitive inverter like the ICL7660.
For moderate current, a switching regulator like the MC34063A.
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Re: Simple Power Supply Question

by msymms on Wed Sep 22, 2010 10:52 pm

Thanks, the 34063 seems like it would work, but the Max current is 100ma. What I have is a 12 volt bench PS I have for my radios. I have a project with a need for +/- 12V with 1.5A on each side, or as close to that a I can get.

Is there a simple solution?

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Re: Simple Power Supply Question

by Hfuy on Thu Sep 23, 2010 7:35 am

Is there a simple solution


Sure - something from this range:

http://www.tracopower.com/General-Purpose.215.0.html

The THL 20-2422WI would do it from a 12V input.

You need to be quite careful with these (or any switching power supply) because they do tend to produce rather noisy power, which will certainly screw you up if you're doing radio. We used one of those on an audio mixer and had to site the converter physically distant from the mixer itself, and apply quite a bit of LC filtering and anti-parasitic techniques to produce an adequately quiet power supply (and it still isn't as good as the iron-transformer mains supply).

HF
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Re: Simple Power Supply Question

by msymms on Thu Sep 23, 2010 11:19 am

Hfuy,
You are obviously more well versed at this than I. Clean power is always preferable. To answer your question, this is not for my radios. The initial source output is ~ +13.8V 15-20A. The first block of the new project has to convert the power to +/- Voltages. From there I got it, it is just this gap I am trying to bridge.
If I use the above described method, and then run the the resulting outputs through a voltage regulator circuit (which is part of my project), would that "quiet down" the power? Or is a solution involving transformers a better way to go?

Thanks for the advice.
-mark
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Re: Simple Power Supply Question

by Hfuy on Thu Sep 23, 2010 12:24 pm

It is difficult - probably very nearly impossible - to create an SMPS that is as quiet as a decent iron transformer based power supply, especially if you are sensitive to high frequency noise, as you are in anything with analog circuitry (audio, RF, data logging, etc). If you can deal with it being mains powered, the way to go is to use a mains transformer with plus and minus windings a few volts above your desired output, then rectify, smooth and regulate as required. To get really good results will require mains filtering on the input side, and filtering both sides of the regulation. It will be big and heavy and inefficient and it will require mains power, but to some extent those are the breaks.

This may help less in an environment where there are lots of SMPS or triac based lighting dimmers running off the mains anyway, as both types of device tend to suck big chunks out of the mains waveform and can give you high frequency transients that your iron power supply won't necessarily filter. This also applies to creating a mains supply and trying to run it from a battery-powered mains inverter, even types that advertise "true sine wave" output.

The only way to do any better than that is to use batteries, which may actually end up being cheaper, smaller, easier, and more convenient.

P
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Re: Simple Power Supply Question

by msymms on Thu Sep 23, 2010 12:44 pm

Wow. I think I need to go with the traco unit. I will need to step up to the 40w unit so I can get 1.5A on each rail. I am hoping that after channeling each through a regulator circuit any noise will be lessened. I can also put some filtering caps downline of the traco unit.
Are there other companies making this type of component? I haven't found any.

Thanks for your help
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Re: Simple Power Supply Question

by Hfuy on Thu Sep 23, 2010 1:55 pm

Vicor, Recom.

HF
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Re: Simple Power Supply Question

by oPossum on Thu Sep 23, 2010 2:04 pm

CUI
Delta
Emerson
Murata
Panasonic
Power One
Powerex
TDK-Lamda
Texas Instruments
Astec
Volgen
Cincon
Fujitsu
Mean Well

Many others

Search DigiKey or Mouser for what you need. Price will probably $50 to $100. Much cheaper to just use another 12 volt supply for the negative rail.
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Re: Simple Power Supply Question

by zener on Thu Sep 23, 2010 2:23 pm

oPossum wrote:Much cheaper to just use another 12 volt supply for the negative rail.

Yes. I would consider just getting the split supply you need. Might be very small and cheap. Look at http://www.mpja.com

This one does +- 12 at 1A for 15.00 http://www.mpja.com/prodinfo.asp?number=18204+PS

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Re: Simple Power Supply Question

by Entropy on Thu Sep 23, 2010 2:52 pm

oPossum wrote:CUI
Delta
Emerson
Murata
Panasonic
Power One
Powerex
TDK-Lamda
Texas Instruments
Astec
Volgen
Cincon
Fujitsu
Mean Well

Many others

Search DigiKey or Mouser for what you need. Price will probably $50 to $100. Much cheaper to just use another 12 volt supply for the negative rail.

Key here - If you can find a fully isolated power supply, then you can connect the +12v output of one unit to the ground of another, and the ground of the first unit becomes -12v.

However, the power supply MUST be fully isolated, otherwise you'll just wind up with a nasty short.
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Re: Simple Power Supply Question

by zener on Thu Sep 23, 2010 3:01 pm

I am thinking unisolated supplies are pretty uncommon. No?

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Re: Simple Power Supply Question

by oPossum on Thu Sep 23, 2010 3:09 pm

Yes, *very* uncommon.

A switching supply that has the ground of the 120 VAC input connected to the negative of the output is uncommon. The ground is usually just for the chassis. There may be a high value resistor or low value capacitor between AC in ground and DC out negative, but that doesn't prevent using it as a negative rail supply.

Ungrounded power supplies (two wire line cord) are also quite common for low power (under 50W).
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Re: Simple Power Supply Question

by westfw on Thu Sep 23, 2010 3:57 pm

First, it's not actually a simple question, and what you're trying to do is relatively uncommon and relatively difficult. Depending on your budget, you are looking for a "DC to DC converter", with "about 12V" input and +/-12V (at 1.5A: almost 50W) as outputs. Here's one that ALMOST fits your needs (~$40)
http://www.jameco.com/webapp/wcs/stores ... tId=217701

It may be more economical to find a cheap "isolated" 12V converter that can be connect backwards (as other people have described) to provide the -12V.

It may be more economical to find an "off-the-shelf" computer or audio component power supply that actually provides other voltages that you don't need (for example, I found some inexpensive "ATX" computer supplies in the 100W range, but it loos like most of them have very tiny -12V outputs.)

It may be more economical to find an AC power supply that meets your needs, and power it from one of those common and inexpensive Automotive "AC Inverter" boxes.

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