18V AC+ 12V AC to +/-15V DC?? - diyAudio
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Old 23rd July 2010, 08:37 PM   #1
Elbert is offline Elbert  Norway
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Default 18V AC+ 12V AC to +/-15V DC??

OK, here's an interresting one..

I have an amp project to be powered by a +/-38V AC transformer.
I allos need to power an active X-over that wants +/-15V DC.

Now, there is a pair of additional lower voltage windings on the trafo secondary side, one 18V AC and one 12V AC.. How can I use this to get the required+/-15V DC supply??
If both windings had been similar voltage, this would of course be a piece of cake, i.e. use any standard power supply solution.

The plan is to somehow rectify to DC and then use standard fixed voltage regulators to get +/-15V DC, the challenge is to design a power supply circuit that doesn't allow corrent to flow in the wrong directions due to the non symetry of the AC supply..

Any clever ideas anyone??
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Old 23rd July 2010, 09:26 PM   #2
AndrewT is offline AndrewT  Scotland
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try using half wave rectifier on the 18Vac tapping.
If you then half wave rectify for the opposite polarity you can use both outputs to feed your dual polarity regulators.
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Old 23rd July 2010, 09:37 PM   #3
Elbert is offline Elbert  Norway
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Hi again Andrew!

Good idea to use only half wave rectification, without having scetched up any schematic yet, it sounds like this can reduce the chance of current flowing in the wrong directions.

I don't follow you 100% with regards to this:
"If you then half wave rectify for the opposite polarity you can use both outputs to feed your dual polarity regulators. "

But then again, not allways easy to explain verbaly what is better shown with a schematic..

Anyway, I'll look in to half wave rectification for the time being!
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Old 23rd July 2010, 09:40 PM   #4
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What AndrewT is talking about is also called a voltage doubler.
File:Bridge voltage doubler.svg - Wikipedia, the free encyclopedia
If you then ground the middle point between the caps, you'll have a +/- 25V DC supply. Keep in mind half-wave rectification produces more ripple than other methods, although if you're regulating, that's less of a concern.
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Old 23rd July 2010, 09:49 PM   #5
Elbert is offline Elbert  Norway
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Ah, interresting!

Well, in this case, I should be able to use only one winding, e.g. the 18V winding, take 0V from between the capacitors, and feed it to a +/-15V regulated circuit made up from a L7815 and a L7915 regulator

https://www1.elfa.se/data1/wwwroot/a...x-L78xxC_e.pdf

Described on page 33, figure 25.

I hope that using regulators and perhaps some good size capacitors will smooth out the extra ripple?

Thanks for the clarification and the link!!
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Old 23rd July 2010, 09:57 PM   #6
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Yes, some good sized capacitors and regulators should smooth it out nicely. I recently built just such a circuit to power a microphone preamp and it's extremely quiet. I'm using 470uF, 22uF and 0.1uF film before the regulators and 10uF after.
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Old 23rd July 2010, 10:06 PM   #7
Elbert is offline Elbert  Norway
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Well, if it works for a microphone amp, then it should be no problem with a audio level signal circuit!

I can now progress with schematic and circuit board design with confidence!

Sure is good to have this forum, I know my basic electronics but boy do i get rusty between projects!! Then some nudges in the right direction is what saves the day!

Thanks!!
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Old 24th July 2010, 12:33 AM   #8
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Yes a voltage doubler will be fine, just don't try powering anything that uses over 15 watts with one.

I know they used to do that in <500W switchmode supplies to get them compatible with both 115/230v standards and it *sort of* worked, but it's far from optimal. Especially when you connect the power supply to 230v with the switch wired for 115... The caps go boom.
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Old 24th July 2010, 07:10 PM   #9
Elbert is offline Elbert  Norway
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Well, since I'm only powering an active x-over board, I guess this should be ok. I can see that this circuit would be far from optimal for a power application, both with respect to efficiency and ripple.
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