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Old 12th April 2011, 06:22 PM   #1
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Default Dual use power transformer

I have a 150va ct transformer with primary taps at 100,120,220, and 240. with 120vac mains, At 100v tap its 55vac, at 120 it puts out 43vac, and so on.

Now with 120vac on 120 tap it puts out 56vdc after rec which is on the money for my lm3886 chipamp.com power supply. However i also need a power supply for my 31hz.com kit that needs 20-30vdc.

My question is, would it reduce the va of the transformer if i put 120vac on the 240v tap so the power supply outputs 28vdc, or would it reduce the va of the transformer if i just went ct to the positive rail to get 28vdc?
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Old 12th April 2011, 06:41 PM   #2
paulb is offline paulb  Canada
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To clarify: you have a transformer rated at 150VA, which has a centre-tapped secondary of 43V. Is this 43-0-43V or 21.5-0.21.5V?
Is the 56VDC for your chipamp actually a +/-28VDC supply?
Are you planning to use two separate transformers for your chipamp and 31Hz kit?
How much power does the 31Hz kit require?
Have you got specs on this transformer? That may be the only way to make sure.
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Old 12th April 2011, 08:06 PM   #3
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Sorry no specs on transformer, its 5lbs and was a pull from a 500 watt sub woofer.
centertap is 21.5-0-21.5

plan was to use a box with the transformer and power supply in it, then either connect the lm3886 based amp, or the 31hz based amp. The lm3886 would use the full +-28vdc, and the 31hz would use just the positive 28vdc rail. Since its a center tap, just taping the positive rail should get me full va, but i wanted to double check.

taping just the positive rail turns it into a full wave instead of a bridge, but it still uses both taps on the transformer vs if i just used 1 side of a dual secondaries setup, id loose half the va if it didnt parallel the windings.

so the 31hz would use this:
http://www.williamson-labs.com/image...ave-ct-304.gif

and the lm3386 would use this:
http://www.williamson-labs.com/image...y-dual-307.gif

If you notice, if you erase the left side of the bridge it turns into a full wave ct.





http://www.diyaudio.com/forums/attac...implepower.jpg
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Old 12th April 2011, 10:33 PM   #4
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At half line voltage, the VA rating would also be 1/2.

paul
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Old 13th April 2011, 05:12 AM   #5
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There are unpleasant negative side effects of useing a voltage below what the transformer winding was designed for....
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Old 13th April 2011, 11:43 AM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by digits View Post
There are unpleasant negative side effects of using a voltage below what the transformer winding was designed for....
eg.
runner much cooler than maximum design temperature, or
no risk of saturation when idling, or
lower power wastage of mains electricity.

There is no harm in running a transformer at half rated voltage.
Audio transformers are run this way for 99.99% of their lives. Maybe that should be six 9s, or seven 9s.

There are transformer designers and expert amateurs in our Membership. Have I got this right or wrong?
__________________
regards Andrew T.

Last edited by AndrewT; 13th April 2011 at 11:57 AM.
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Old 13th April 2011, 02:12 PM   #7
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Well here is what hammond says:
http://www.hammondmfg.com/pdf/5c007.pdf

Im not sure if that pertains to all transformers but their math lines up with mine on everything ive learned.

based on them, i would go from a .62% rating in current ability for a bridge, up to 100% current rating when taping just the positive rail. So my current ability goes up 50% if i go from a bridge to a full wave, in a cap loaded load.
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Old 13th April 2011, 05:00 PM   #8
macboy is offline macboy  Canada
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Quote:
Originally Posted by nightanole View Post
Well here is what hammond says:
http://www.hammondmfg.com/pdf/5c007.pdf

Im not sure if that pertains to all transformers but their math lines up with mine on everything ive learned.

based on them, i would go from a .62% rating in current ability for a bridge, up to 100% current rating when taping just the positive rail. So my current ability goes up 50% if i go from a bridge to a full wave, in a cap loaded load.
I think you are misunderstanding what you see here.
Did you notice the output voltage spec too? For the Full wave case the voltage V(Peak)DC is listed as 0.71 x Secondary VAC. For the Bridge case, the voltage V(Peak)DC is 1.41 x Secondary VAC. So the Volt-Amps (and real power) available from the transformer is a lot less in the full-wave case vs. the bridge case.
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Old 13th April 2011, 08:23 PM   #9
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Yes i noticed, full wave gives me half the voltage and 50% more current. So its no free lunch, i cant get half the voltage and double the current when cap loading vs full bridge.
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Old 17th April 2011, 08:13 PM   #10
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Well that that is a great link I will be sure to add that to my favorites. After a lot of checking of making sure my connections are right, I finally powered up the amp and its works perfect. After I get done with my other projects that I have to build for school, I think I will build a bridges tda2050 amp or a car audio amp for fun. Now I have to analyze each part of the circuit and see how it works all together. You have been a great help and I will for sure keep in contact with you (nightanole) in case I ever need more help. Also the other people on this board has been a great help.
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