Need help with center tap Xformer

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qdgjcl
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Need help with center tap Xformer

Post by qdgjcl » Wed Mar 09, 2005 9:33 am

I need help understanding how current flows in a center taped transformer. If the primary side has positive on the top and negative on the bottom then primary current would flow counterclockwise for this cycle. On the secondary side the top would be positive and the bottom would be negative. On the top half the center tap would be negative and current would flow counterclockwise. What direction and why is the current in the bottom half?
Thanks, Brad

kinneplastics
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Re: Need help with center tap Xformer

Post by kinneplastics » Wed Mar 09, 2005 9:47 am

Center taped transformers are mostly used in power supplys. The reason being that one end of the transformer is always positive compared to the center tap. Using diodes you can always have a positive output for less choppy DC current.<p>Also a center tapped transformer can be a sorce of two AC voltages, the center tap being half the voltage of the top tap.<p>I'm not sure this helps you but you question seemed as though you do not understand transformers or are way ahead of me in there understanding.<p>wkinne

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philba
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Re: Need help with center tap Xformer

Post by philba » Wed Mar 09, 2005 11:10 am

I could be wrong on this but I believe center tapped transformers are from a bygone era when diodes were expensive (like tubes). Basically its a way to get full wave rectification with only 2 diodes. These days, diodes are so cheap that a full bridge makes more sense than a ct transformer.
Image
The two secondaries are identical - current flows the same direction in both. Its how the diodes are used that makes it work. During the positive AC cycle, diode D1 conducts forward (D2 doesn't since it sees reverse current) and during the negative cycle, D2 conducts forward (D1 doesn't). <p>Sorry about the transformer symbol - eagle didn't seem to have a normal one in its library.<p>[ March 09, 2005: Message edited by: philba ]</p>

Engineer1138
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Re: Need help with center tap Xformer

Post by Engineer1138 » Wed Mar 09, 2005 12:22 pm

Another use for CT transformers is positive & negative supplies needed for opamps and the like. Cleaner and often cheaper than using voltage inverters for the same purpose.

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philba
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Re: Need help with center tap Xformer

Post by philba » Wed Mar 09, 2005 1:17 pm

yeah, thats true. Although, you can get + and - from a non-ct transformer. In general, it seems like transformers inside of equipment is a dying thing.

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Chris Smith
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Re: Need help with center tap Xformer

Post by Chris Smith » Wed Mar 09, 2005 7:45 pm

Alternating Current [AC], says it all. <p>Alternating current (AC) unlike Direct current (DC) flows first in one direction then in the opposite direction. Current amperage is function of time. <p>ONLY ...Direct current (DC) amperage is constant. <p>http://www.circuit-magic.com/sinusoidal ... alysis.htm

Enzo
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Re: Need help with center tap Xformer

Post by Enzo » Thu Mar 10, 2005 12:34 am

To get both +/- without a CT is more difficult. You need to reference them to ground somehow. The CT supply is simple, you don't need more than two diodes. I am not sure what advantage adding two more diodes brings to the mix. CT to ground and then you can easily make two full wave supplies, one per polarity.<p>You could use full bridge with two windings, then ground one side of the resulting DC to make one pos and one neg, but I see no advantage there.

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Re: Need help with center tap Xformer

Post by dyarker » Thu Mar 10, 2005 3:39 am

The current flow in the bottom half is also counter-clockwise. The center tap would be positive relative to bottom. The direction of center tap current would be opposite the current from the top.
So, center tap current equals top current minus bottom current. If top current equals bottom current, then center tap current is zero.<p>--------------------------------------------<p>"I am not sure what advantage adding two more diodes brings to the mix."<p>The same advantage you get in a single polarity supply using a bridge instead of one diode. Instead of charging the filter caps 60 times a second with large current spikes, the caps are charged 120 times a second with smaller current spikes. The caps can be smaller for the same ripple. Less need to worry about winding resistance and core saturation.<p>Ref Philba's drawing of a 2 diode single supply. The two halves of the secondary are each doing NOTHING on alternate half cycles. ALL the current goes through the resistance of half a winding. With a full-wave bridge the resistances are affectively paralleled. The temperature of the transformer is the same either way (assuming the core doesn't saturate), but the output voltage drops less under load with the full-wave bridge.<p>For a bipolar supply disconnect D2 from D1, and reverse D2. It is worse. Now every other half cycle the whole secondary AND the primary are doing NOTHING.
Dale Y

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philba
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Re: Need help with center tap Xformer

Post by philba » Thu Mar 10, 2005 9:07 am

Here how to get + and - from a single secondary. Its not wonderful as it needs a lot more filtering and a little higher input V for the regulators to do their thing properly but it does work. I even have a spice simulation...<p>Image<p>edit: sigh, fixed extreme bone-headed error in schematic. You'd think by now I'd learn to check...<p>[ March 10, 2005: Message edited by: philba ]</p>

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Chris Smith
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Re: Need help with center tap Xformer

Post by Chris Smith » Thu Mar 10, 2005 1:07 pm

The current flow is alternating. <p>That’s why its called AC.

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sofaspud
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Re: Need help with center tap Xformer

Post by sofaspud » Thu Mar 10, 2005 2:20 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>On the secondary side the top would be positive and the bottom would be negative. On the top half the center tap would be negative and current would flow counterclockwise. What direction and why is the current in the bottom half?<hr></blockquote>
On the bottom half the center tap would be positive, but current would flow the same direction as the top half. Why? Because all the current flows towards the most negative point, which is at the bottom of the secondary.

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Chris Smith
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Re: Need help with center tap Xformer

Post by Chris Smith » Thu Mar 10, 2005 2:37 pm

What part of the “Current Alternates”, is every one missing?

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philba
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Re: Need help with center tap Xformer

Post by philba » Thu Mar 10, 2005 2:47 pm

you tell us, oh wise one

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sofaspud
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Re: Need help with center tap Xformer

Post by sofaspud » Thu Mar 10, 2005 4:02 pm

I can't speak for everyone, but when Brad says "If the primary side has positive on the top and negative on the bottom then primary current would flow counterclockwise for this cycle" I'm assuming he means for that *half* cycle.

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Re: Need help with center tap Xformer

Post by Enzo » Thu Mar 10, 2005 4:26 pm

Dale, you explained the difference between full wave and half wave, but when I asked what I would get by adding two more diodes to the circuit Philba posted a picture of, what I had in mind was what is the advantage of usign a full bridge across a single widing over using a half bridge across a CT winding. Both result in a full wave rectification. SInce we usually need both polarities, the CT arrangement allows me to get both supplies off one winding rather than using two separate windings and two bridges. At that point I do indeed have a full bridge across the CT winding, but it is operating as a pair of half bridges.<p>I am not really concerned that each half of the winding is idle for a half cycle in the single polarity example. "They also serve who only sit and wait."

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