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Center Tapped Transformer Question.

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Patrick

Wed Mar 09 2016, 06:43AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I need help verifying my idea, so I don't start burning things down.

So I have a transformer, 120 to 25.2 center tapped. id like the secondary voltage to be 12.6 before rectification. so I'm thinking a half wave rectification with just two diodes right?

next, its rated for 2 amps for the full 25.2 vac output which is 50.4 VA. at 12.6 and 4 amps I also get 50.4 VA, so I can pull the full 4 amps at 12.6 vac right?

finally, I actually have two transformers, they're exactly the same manufacturer and identical. Id like to run these two transformers at the 12.6 VAC 8 amp configuration in parrallel. but can I use a single full-wave bridge rectifier, or do I have to use 4 separate diodes for each transformer leg ?

-Thank you

Wolfram

Wed Mar 09 2016, 08:43AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Patrick wrote ...

I need help verifying my idea, so I don't start burning things down.

So I have a transformer, 120 to 25.2 center tapped. id like the secondary voltage to be 12.6 before rectification. so I'm thinking a half wave rectification with just two diodes right?

This is actually full wave rectification, and it will give you an output voltage identical to if you used a bridge rectifier with a 12.6 V transformer.

next, its rated for 2 amps for the full 25.2 vac output which is 50.4 VA. at 12.6 and 4 amps I also get 50.4 VA, so I can pull the full 4 amps at 12.6 vac right?

With a center tapped half-wave rectifier, each secondary provides the full output current half of the time. Since resistance losses are proportional to current squared, you only get sqrt(2) times the rated current, 2.8 A in this case. This assumes you're not using any smoothing capacitor, if you add one then the current you can safely draw drops further, with a maximum value depending on the smoothing capacitance. With 1000 ÂµF of capacitance, you only get 1.5 A of output current without exceeding the winding current rating.

finally, I actually have two transformers, they're exactly the same manufacturer and identical. Id like to run these two transformers at the 12.6 VAC 8 amp configuration in parrallel. but can I use a single full-wave bridge rectifier, or do I have to use 4 separate diodes for each transformer leg ?

If they are perfectly identical, then you can directly parallel them. A separate rectifier on each should give you better margin for manufacturing tolerances though.

Proud Mary

Wed Mar 09 2016, 03:01PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Unlike the bridge rectifier which uses 100% of the transformer winding at all times, the full wave rectifier uses only half of the winding on each half cycle of the AC waveform. This leads to some additional losses, since the winding must have double the number of turns of that for a bridge rectifier. This means that the winding resistance is typically double that for a bridge rectifier, because the windings must be thinner so as not to occupy more area in the winding window. This leads to higher resistive losses.

radiotech

Thu Mar 10 2016, 08:25AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Beware of possible demons lurking. 25.2 V center tapped transformers may very well provide 12.6 per leg,
That is because the turns are the same per side. These transformers may be wound making the length of wire
different for the inner wind than the outer.

This additional resistance will make a center tapped two diode system have unequal voltage on the two
peaks that make up the 120 pulsation/second voltage.

Were you to parallel the transformers, 5 wires to 5 wires, and got the secondaries reversed, and phased
correctly on the primary there would be circulating currents under load

Patrick

Sat Mar 12 2016, 01:35AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Its generating 19.4 volts. I'm rectifying each transformer individually. now RMS rectified should be 17-ish volts. So I guess it will droop under load. I'm wanting 20 VDC, so I'm pretty happy if it stays towards 20V.

Proud Mary

Sat Mar 12 2016, 02:32AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Patrick wrote ...

Its generating 19.4 volts. I'm rectifying each transformer individually. now RMS rectified should be 17-ish volts. So I guess it will droop under load. I'm wanting 20 VDC, so I'm pretty happy if it stays towards 20V.

You might assume the transformer's output voltage would be highest with no load. It would then make sense that under loaded conditions the transformer's resistive and reactive components would cause the output voltage to drop below its no-load level. This may seem a logical assumption, but one that's not necessarily always so. Depending on the power factor of the load, the output full-load voltage can actually be larger than the no-load voltage.

Patrick

Sat Mar 12 2016, 09:00AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Proud Mary wrote ...

we'll see soon enough.

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