EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: e100 on April 08, 2012, 05:24:11 am
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Wikipedia says
http://en.wikipedia.org/wiki/Center_tap#Common_applications_of_center-tapped_transformers (http://en.wikipedia.org/wiki/Center_tap#Common_applications_of_center-tapped_transformers)
"The center-tapped rectifiers are preferred to the full bridge rectifier when the output DC current is high and the output voltage is low."
It doesn't say if this is due to build cost or operating efficiency.
What constitutes low voltage and high current?
It saves a diode drop, but what about the I2 heating effect of having a split output winding with each half being idle for 50% of the time and hence peak currents twice that of a bridge rectified transformer?
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"The center-tapped rectifiers are preferred to the full bridge rectifier when the output DC current is high and the output voltage is low."
It doesn't say if this is due to build cost or operating efficiency.
Yeah, you lose voltage across the two active diodes with a full bridge, so if you can remove one of them it will increase efficiency when dealing with high current and low voltage, and its cheaper, so win/win.
But the transformer is a little less efficient in this configuration so its a tradeoff.
What constitutes low voltage and high current?
It saves a diode drop, but what about the I2 heating effect of having a split output winding with each half being idle for 50% of the time and hence peak currents twice that of a bridge rectified transformer?
Yeah, as above, it's a tradeoff.
You do the math and see which is better for your application.
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alternativly you buy a 4 leaded bridge rectifier package that has a voltage drop less than the diode you where already planning to use :/
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Wikipedia says
http://en.wikipedia.org/wiki/Center_tap#Common_applications_of_center-tapped_transformers (http://en.wikipedia.org/wiki/Center_tap#Common_applications_of_center-tapped_transformers)
"The center-tapped rectifiers are preferred to the full bridge rectifier when the output DC current is high and the output voltage is low."
It doesn't say if this is due to build cost or operating efficiency.
According to MCI, a transformer manufacturer, you only need a secondary rated at 1.2 times the DC output current for a center-tapped transformer, versus 1.8 times the output for a fullwave bridge. On the downside, the voltage rating needs to be 1.6 times the unregulated DC voltage, while the bridge rectifier only needs to be 0.8 times that voltage. Hence, the use when current is high and voltage is low. There's a lot of good information on basic PS design on their website:
http://mcitransformer.com/i_notes.html (http://mcitransformer.com/i_notes.html)
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A few other things to consider; you can go with just a single winding instead of a center tapped version. they're less labor intensive and you can often get a smaller transformer out of the deal. Sure the current requirements are higher which means you'll need a slightly larger gauge of wire for the windings but you don't need as many turns, and you can get away with a smaller core on average, so you're ahead of the game size wise. this can save you as much as a buck or two per unit, which can be substantial when you're ordering in quantities or having custom winds.
Another thing to consider is the duty cycle of the transformer. Is it going to be run for long periods of time near its rated output or will it spend most of it's time idling. This could mandate a somewhat larger transformer to prevent overheating and premature failure. This leads us into the possibility of A or B insulation which is rated to 105c and 130c respectively. And depending on the insulation, you may or may not be able to run the transformer a bit harder than the specs. Lots and lots of competing criteria. So just putting a few more things out there to think about during the selection process
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Hi @ampdoctor , your post is very informative .
So if i were to run a 50VA tranny at 40VA output , is that okay ? ( and it's not a name brand tranny , just a MIChina tranny )
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If it is a OneHungLow brand the 50VA would be an optimistic rating, one the best one they made first, and before the "Clost Leduction" came in and used fewer thicker laminates, less turns of copper and thinner wire. I would try, but look at the temperature rise. If it goes above 90C then it is overloaded. I have met some that did that even with no load. Some small sub 5W units would draw 10W at full load, and still the same at near no load.
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If it is a OneHungLow brand the 50VA would be an optimistic rating, one the best one they made first, and before the "Clost Leduction" came in and used fewer thicker laminates, less turns of copper and thinner wire. I would try, but look at the temperature rise. If it goes above 90C then it is overloaded. I have met some that did that even with no load. Some small sub 5W units would draw 10W at full load, and still the same at near no load.
I wouldn't be too sure , the windings look rather thick for a 50VA unit .