[power supply]-Triac to do 24VAC to 12VAC and then into DC?

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jsiei97
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Posted by jsiei97: Mon. Aug 10, 2009 - 03:45 PM
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Hi

Is it possible to use some kind of triac dimmer circuit as a first step in a power supply?

My idea is to have the triac chop of half of the sin wave (and half of the energy) and then rectify, filter and then let a normal 78XX clean up the rest.

The reason is that usually you just rectify the 24VAC and end up with appox 30VDC with is to much for most 78XX (and other regulators).
You end up with a big heat sink and it cant deliver any amp.

But how would that kind of dimmer circuit look like, I mean it had power it self...

What would happen if we start with something like this?

And then insert a rectifier (and the rest) where the "load" is?

Thanks
Johan

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Learning and Information, General Electronics
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alwelch
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Posted by alwelch: Mon. Aug 10, 2009 - 04:49 PM
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Look at Linear Technologys LT3972 - 33V, 3.5A, 2.4MHz Step-Down Switching Regulator with 75μA Quiescent Current

You can handle your high DC input and drive 3.5A on the load side.

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jsiei97
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Posted by jsiei97: Mon. Aug 10, 2009 - 06:11 PM
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alwelch wrote:
Look at Linear Technologys LT3972 - 33V, 3.5A, 2.4MHz Step-Down Switching Regulator with 75μA Quiescent Current

You can handle your high DC input and drive 3.5A on the load side.

It sure seems like a really good one, but the pricing at my local dealer was not humane :(

But is the "dimmer" idea really that bad?

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Plons
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Posted by Plons: Mon. Aug 10, 2009 - 06:20 PM
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Quote:
Is the dimmer idea so bad?
I am afraid that's an affirmative.

Low cost and widely available: MC34063
Use that "in front of" a 7805, or, if noise isn't an issue: just a MC34063 :)
You need a schottky diode like 1N5819, and an inductor of 330 uH

Good examples in the datasheet .... http://www.alldatasheet.com/

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tricia, and Ulyana. You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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alwelch
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Posted by alwelch: Mon. Aug 10, 2009 - 06:30 PM
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I have no idea but if you wanted to experiment I would put the triac on the primary side of your 120VAC to 12 VAC transformer and see if you can drop the secondary voltage by using the dimmer on the primary side. A typical dimmer from the hardware store will cost as much as the switcher part but you would have to build the switcher circuit up anyway with inductors, etc.

You might visit National Semi web site and play with their simple switcher program.

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jsiei97
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Posted by jsiei97: Mon. Aug 10, 2009 - 08:24 PM
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alwelch wrote:
I have no idea but if you wanted to experiment

Sometimes 3-4 components can do just as well in some cases, and if you just play safe all the time (you would never know).

alwelch wrote:
A typical dimmer from the hardware store will cost as much as the switcher part

I was talking about using a triac, a diac some resistors etc etc to create a "dimmer" and use that as part of a power supply. Who was talking about buying one from the hardware store?

Plons wrote:

Quote:
Is the dimmer idea so bad?

I am afraid that's an affirmative.

some ideas are good some are bad, and this seem to be a bad one :lol:

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uC-Harry
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Posted by uC-Harry: Tue. Aug 11, 2009 - 07:34 PM
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Hello,
a dimmer is not a good idea because for have of the power you still get the full voltage. It's half of the sine wave but not of the power line voltage. The way to have half of the power is done by the average of the lamp or the heater resistance.

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Jepael
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Posted by Jepael: Wed. Aug 12, 2009 - 06:57 AM
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In some applications, the AC waveform is passed through to next device only when the voltage is small enough.

Since you will be feeding a 78xx regulator from 24VAC, you could connect a transistor and some circuitry to pass voltages from 0-12V for example from the input to bulk capacitor, and disconnect the transistor after input is between 12V and peak AC. Just make the capacitor big enough so it can supply the regulator until AC input kicks in.

Some mains supply regulators work like this, but they are NOT isolated from mains, they only do it to get something very small powered, like rest of the switching power supply.

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ossi
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Posted by ossi: Wed. Aug 12, 2009 - 08:03 AM
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I remember that a similar curcuit was used in an old
Lab-Supply. I think it was a bridge-rectifier
behind the transformer and a SCR between bridge rectifier and "big capacitor" and then a linear postregulator.

So someone decided, that this idea was good.

added later:
http://www.seekic.com/forum/22_C...

See fig 14 here:

http://www.bgaudioclub.org/uploa...

I think a little more searching
for "SCR" and "preregulator" or similar will
give even better results.

like this: a complete schematic.

http://www.elektroautomatik.de/f...

The SCR is hard to find but its there !

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jsiei97
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Posted by jsiei97: Wed. Aug 12, 2009 - 12:04 PM
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ossi wrote:

See fig 14 here:
http://www.bgaudioclub.org/uploads/docs/HP-DC_Power_Supply_Handbook.pdf

And a lot more power supply info as well,
nice link that DC_Power_Supply_Handbook 8)

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