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Old 31st December 2003, 02:14 AM   #1
lgreen is offline lgreen  United States
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Default PS question- Need to get 20V AC from 60V AC - w/o Trafo

In my BOSOZ, I need to use about 600 mA from a DC power supply at 6V for some ancilary circuitry. The ancilary circuitry is essentially control circuitry, some digital some not. So I got me a 10V transformer and made a neato power supply out of a rectifier and an LM1084 regulator chip (max input to output differential is 29V) and thought I was set....until....

The problem is that I have run out of space and cannot fit another transformer. Ouch! so I am stuck with the one specified for BOSOZ, which is 60V AC center tapped. I cannot connect 60V AC to my rectifier which is connected to LM1084, since the max input to output differential of LM1084 is 29V and the output I need is 6 V, so i.e., 60 V (more like 80 when rectified) minus 6 V = 54 V which exceeds the LM1084 limit of 29 V differential.

My question- is there an easy way to reduce the 60V AC to 20V AC so that I can use my seperate rectifier and LM1084 regulator circuit to power the ancilary circuitry?

I tried putting a resistor in series with the 60V AC to drop the voltage, but as suspected, it limited the current to way below the 600 MA that I need.
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Old 31st December 2003, 11:57 AM   #2
Kermit is offline Kermit  Norway
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If you do it after the rectifier you could use this simple pre-regulator:
http://sound.westhost.com/project102.htm

Or perhaps you could try to add a extra winding to your transformer?
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Old 31st December 2003, 12:49 PM   #3
akb1212 is offline akb1212  Norway
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How about putting another secondary on your transformer???? If it is a toroid that should be quite easy, and I have done this myself.

The current you need means you can use a fairly thin wire (I have read somewhere that about 3 A/mm2 is a good rule of thumb for trafo wire, so you would need about .25 mm2), and the voltage suggests that you only needs a few turns. You should try some turns to figure out how many you need to get the voltage you need. And be sure to spread the windings properly around the core.

If your transformers aren't toroids....... to bad for you, it's not that easy to do that same trick on a non toroid trafo.

It's not the best way to do it. Large current draws from the main trafo will give you lower voltages. Lucky for you this is class A amps with at constant power consumption . But if you are only going to use it for digital circuits, and have a good regulator you should be safe.

It's even about the cheapest (possibly absolutely free... ) way around your problem

Anders
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Old 31st December 2003, 01:37 PM   #4
paulb is offline paulb  Canada
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An extra winding may be your only hope. Trying to reduce 80VDC to 6 at 600 mA will mean you need to dissipate almost 50 watts, just to drop the voltage. You could try to chop up the 60VAC with a triac or SCR, but that will probably make your supply noisy. You don't really want to run your control circuitry from the same supply as the audio circuitry.
How about a separate wallwart for the control circuitry?
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Old 31st December 2003, 11:29 PM   #5
Blues is offline Blues  United States
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You can build a separate ps in a separate box and cable it to your main box either Vac (if caps/regulator fit in your old chassis) or Vdc.
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Old 31st December 2003, 11:58 PM   #6
dhaen is offline dhaen  Europe
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Use a pre-regulator. Sit up the pre-reg's ground with a zener so it doesn't see the full 60v.
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Old 1st January 2004, 09:54 AM   #7
dhaen is offline dhaen  Europe
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Default A post-script

Dropping a lot of voltage with a regulator produces a surprising mount of heat. Do your calulations before embarking on this approach
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Old 1st January 2004, 04:27 PM   #8
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I once saw a circuit that used a mosfet, cap and a choke and a few resistors that made a simple swithmode regulator. Ia I recall, the gate was connected after the choke so once the voltage reached the deturmined voltage (at output) the mosfet shitched off and the flyback from the choke (through a diode to ground) continued to pump current to the cap until the output sagged low enough to turn the mosfet on again and repeat the cycle. You may need to do some more filtering after (and before), but this could be a good way to efficiently drop some voltage with a minimun of real estate.
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Old 2nd January 2004, 10:14 AM   #9
Vigier is offline Vigier  Netherlands
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Does anybody over here know what a Triac is, and what you can do with it?
Use the schematic of a dimmer for a lamp. Turn the potmeter until it has an output of 20V AC, measure the value of the potmeter, replace it with a resistor, and you have a VERY small supply!

For the people who don't know what I'm talking about:
http://home.howstuffworks.com/dimmer-switch3.htm

Grtz, Joris
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Old 2nd January 2004, 10:31 AM   #10
dhaen is offline dhaen  Europe
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I do indeed know what a triac is. Also, that triacs only work with AC.
Even its simple relative, the SCR needs a voltage "zero crossing" to work. It would be hard to implement one of these.
Another point is that these devices generate lots of noise.
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