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Old 29-05-2009, 14:34   #1
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5.2vdc from 12vdc?

I have an amplifier for my wifi antenna. It plugs into a small transformer that runs on AC power 120v. The transformer says 5.2 volts. How can I step down my 12v system so that I don't need to run an inverter up to 120v AC and then have that trnaformer drop the voltage back to 5.2v DC?
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Old 29-05-2009, 15:57   #2
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I did this with a series of Zener diodes. I have them somewhere and will try and take a photo.
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Old 29-05-2009, 15:59   #3
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Perhaps something like this?: http://focus.ti.com/lit/ds/symlink/pt78ht205.pdf Dc to DC converter... Digikey has a bunch of these in different Amps and voltages... http://search.digikey.com/scripts/Dk...e=296-20254-ND
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Old 29-05-2009, 16:11   #4
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Does the 120v transformer say "Output 5.2v DC" or AC ? ( is it a simple stepdown transformer or AC to DC rectifier) If the device runs on DC it is simple to convert your boat 12v DC to 5.2 DC depending if the load is variable or constant.
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Old 29-05-2009, 17:15   #5
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Christian's answer is bang on.

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(Retired electronics tech)
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Old 29-05-2009, 17:24   #6
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So what I'm looking for is a dc to dc converter. Is this worth the $$ DC/DC Converters, retail, wholesale, off-the-shelf and custom, DC/DC 5 watts to 1500 watts. It has the adapters that I might need variable outputs but I don't know about the efficeicy. But guess that is not such a big deal b/c it is less than a 1 amp draw
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Old 29-05-2009, 20:01   #7
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Heres what I did to run a laptop off 10v.

I have no idea what current you could draw, but I am sure enough for an aerial amp.

These are the diodes:

NTE 5814 PRV 400V 6A AXIAL Diode

I got them from radio shack I think. They are all in series. Polarity is obviously important

I seem to remember there should be a drop of 1.7v per diode?? but this is obviously only .4v.

Cheap and cheerful, your mileage may vary.
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Old 29-05-2009, 21:34   #8
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Forward biased silicon diodes (as seen in the above picture) will drop 0.7volts. A Shottky diode will drop quite a bit less, on the order of 0.3v. That's what you would want to use in series with a power source such as a solar panel if you didn't want backfeed loss.

The components linked by Christian are switching regulators which although more expensive, are much more efficient.

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Old 01-06-2009, 10:48   #9
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Not to hijack the thread, but I have a similar question - but different :

I have a Power over Ethernet (PoE) adapter for my external WiFi client bridge which uses 48Vdc. I've searched, admittedly not very hard, for a DC power converter which can step up 12-13.8V to 48V. Any ideas for a source? I'd like to eliminate the use of AC power as much as possible to increase efficiency.
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Old 01-06-2009, 11:02   #10
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I got a notebook DC 12 volt power adapter off eBay that can go from 9 volts to 22 volts in roughly 1.5 Volt stages. Had a variety of power tips with it as well. I think it was about 10 bucks and came from Hong Kong. Works fine for the notebook that I have that needs 19 volts. Not sure but there may be ones available that can go up to 48 volts.

Check close on the specs for that device, many PoE devices take a max of 48 volts but can actually work on less, one that I am looking at will operate on 9 to 48 volts DC.
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Old 01-06-2009, 12:33   #11
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Quote:
Originally Posted by Beausoleil View Post
Not to hijack the thread, but I have a similar question - but different :

I have a Power over Ethernet (PoE) adapter for my external WiFi client bridge which uses 48Vdc. I've searched, admittedly not very hard, for a DC power converter which can step up 12-13.8V to 48V. Any ideas for a source? I'd like to eliminate the use of AC power as much as possible to increase efficiency.
check out this site
DC/DC Converters, retail, wholesale, off-the-shelf and custom, DC/DC 5 watts to 1500 watts
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Old 01-06-2009, 13:05   #12
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Charlie,

Sirius Satelite radio car kits have a 12v cigeratte lighter plug that coverts to 5.2v dc .... You can buy one seperately or maybe get a similar one from Radio Shack.
12v Power Adapter for SPTK1/SPTK2/STRC1

Good luck, Bill A.
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Old 01-06-2009, 14:13   #13
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I found this 12/48V converter. My only concern is that it seems to be +48V, where I think my unit is supposedly standard -48Vdc PoE. The AC wall wart, a MeanWell ES18U48-P1J, says the output is "48V" and shows the little polarity diagram with the outer part of the connector as "-" and the inner as "+", but it doesn't say which side is really "ground". I'll check it out later today after my wife is finished with her online work...

And my other concern is the price: $94. If I can figure out how to get rid of an extra 499 of these units, I can get the volume discount of $41 each!
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Old 01-06-2009, 18:30   #14
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Beausoleil,
If you can ask a question of the seller, ask if there's a common ground between input and output. If there is no common ground, you can simply hook up the output to your amp with the proper polarity with no problems interfacing with any other equipment.

Steve B.
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Old 01-06-2009, 18:48   #15
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Beausoleil,
If you can ask a question of the seller, ask if there's a common ground between input and output. If there is no common ground, you can simply hook up the output to your amp with the proper polarity with no problems interfacing with any other equipment.

Steve B.
I think that's what they mean when they say "non-isolated" - the ground/negative is not isolated...

I wonder if there's a voltage regulator similar to the the Radio Shack LM317T which could handle 48V rather than just 37V... I could simply build one...
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