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11th February 2009, 09:38 PM  #1 
diyAudio Member
Join Date: Apr 2002
Location: Toronto, Canada

Power Trans Winding Parallel AC or DC?
Hi,
Have just picked up a 1kVA Toroidal power trans that has 4 secondary windings of 2x40VAC for my 2channel power amp. These secondary windings have identical voltage and amperage ratings, at least it appears so from the spec label. Since I'm building a 2channel amp I plan on paralleling the windings, two for the left and the other two for the right. My question is how you would parallel them up, would you do it on AC side or DC side? Would you just connect the leads of the windings together (in correct polarity of course) then feed the rectifier bridge, or would you get them through separate rectifier bridges (one per winding) then bring the output of the bridges together at the filter caps? The amp is only 2x125W into 8 ohm but I 'd like it to have good current output, therefore, an as stiff power supply as available. Thanks! Shaoyi 
12th February 2009, 01:18 AM  #2 
diyAudio Member
Join Date: Oct 2006
Location: Charlotte, NC

I don't know that it would make a huge difference either way, but my gut tells me you would have a better chance of sharing current through each winding equally by paralleling the windings, and using one rectifier bridge.
Pity we don't have three phase power to our houses; would make for a killer DC supply. 
14th February 2009, 08:00 PM  #3  
diyAudio Member
Join Date: May 2006

Quote:
If you use two sets of rectifiers, the diodes perform that function; if you parallel the windings, only the winding resistance (<<) does.
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WD 

14th February 2009, 10:14 PM  #4 
diyAudio Member
Join Date: Oct 2006
Location: Charlotte, NC

Perhaps you could show how a diode drop in each separate winding will ensure current sharing better than winding resistance? I am of the opinion in small transformers such as this, the delta V due to current mismatch is MUCH greater by resistance than it is by a PN junction. One of the features of a diode is a relatively constant voltage drop vs. current. This does very little to ensure current balancing. The winding resistance will have much greater ability to force current sharing than a fixed voltage drop in each diode.
Typical transformers exhibit regulations on the order of 5%15%. A diode does not produce this natural 'droop' in output voltage as load is increased. 
15th February 2009, 10:53 AM  #5 
diyAudio Member
Join Date: Jun 2004
Location: Warsaw

WD is right.

15th February 2009, 11:53 AM  #6  
diyAudio Member
Join Date: Jan 2007

Quote:
leakage inductance can help too, depending on the transformer construction. 

18th February 2009, 08:49 AM  #7 
diyAudio Member
Join Date: Dec 2004
Location: Cincinnati, Ohio

Leakage inductance probably wont help so much in a toroid.
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18th February 2009, 11:28 AM  #8 
diyAudio Member
Join Date: Nov 2004
Location: Florida

It would be fine to parallel the coils at the transformer, I'm sure that it was designed for parallel and series usage.

27th February 2009, 04:58 PM  #9 
diyAudio Member
Join Date: Apr 2002
Location: Toronto, Canada

Thanks to all for your input. I was leaning toward direct paralleling the windings so I did a couple of measurement to the secondary windings about the noload voltage and DC resistance, just trying to get a rough idea how much difference there are. To my surprise they are in fact very consistent.
To measure the noload output voltages I paired up two windings in series, headtohead, so that the meter would see the voltage differential. The reading was a consistent 0.000VAC on my Fluke 77 in any combinations of the winding pairup. To measure the resistance I hooked up all 4 windings together in series, and put through a known stable DC current of about 1A, then measure the voltage drop across each winding. They are within a deviation of 4%. So I guess I'll be fine with paralleling the windings directly, at least with this particular transformer. 
1st March 2009, 06:56 PM  #10  
diyAudio Member
Join Date: Oct 2006
Location: Charlotte, NC

For an interesting ponder, consider what happens when you parallel two secondary windings that are a little different either in turns ratio, stray inductance, winding resistance, or combination. Then leave this combined secondary open circuited (no load whatsoever). You will find that there is current flow in the secondary windings!
Why? The secondary terminal voltages are forced to be equal since they are paralleled. If the internal emfgenerated voltages are not equal (very difficult to get perfect balance), a current flows to force terminal voltage balance. This current is in the form of var flow, 90 degrees in quadrature to the voltage (with miniscule inphase current due to winding resistance). So this configuration is such that one winding is supplying vars, with the other winding absorbing vars. Net effect is a selfbalancing action that adds a teeny additional current to flow in the windings, while helping maintain balance over and above basic winding resistance corrective behavior. It is a mistake to consider a transformer a perfect turns ratio device with only winding resistance being significant for nonideal corrections. No different, really, than paralleling two generators; with improper field control, you can actually cause rated current to flow in each generator, even though there is no load connected to them, and essentially little real watt exchange is performed; all vars. There are a number of field control techniques used to minimize this circulating current; fundamentally, you adjust the field to control the internal voltages such that there is balance (net zero circulating current). This method is effective even when paralleling differently rated generators with different characteristics. Power system design can accomplish a similar correction with tap changers and parallel connected transformers of different size and/or impedance. Basic premise is to reduce circulating current, which guarantees good balance and optimal transformer utilization. Now, much of this effect is minimized when the paralleled windings are essentially the same and on the same core, such as your transformer, but the principle is the same. Allow this quadrature current to flow, and you have selfcorrection. So upon further reflection, use those diodes, and this automatic correction is in force only with regards to the winding resistance, so the self correction is less. Keep in mind that the quadrature current is most effective in causing voltage drop across the inductance of the winding, not the winding resistance. Back to the right triangle; vars on the imaginary axis, watts on the real. Quote:


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