How are Center Tap transformers labeled? - Page 1
Forum section: electronics / beginners

author: rodcastler
author_rank: Regular Contributor
timestamp: October 25, 2023, 12:50:02 pm
content: I found this shorted transformer and I'm looking for a replacement.It belongs to a Paradigm PS-1000 subwoofer from the 90'sThere are two white wires on the secondary side, with a third, red wire for the center tap. The label reads:============BLK 230V 50-60HzWHT 56.5V 150VARED CENTER TAP============And that's it.So here's my confusion:1) Is it supposed to output 56v from the center tap to each of the white wires, totalling 113v if I measured the two extremes?or2) is it supposed to output 56v total at the two extremes and thus half of that from the center tap to each white wire?What is the general rule for labelling a center tapped transformer in terms of output voltage?Thank you.

author: Shonky
author_rank: Frequent Contributor
timestamp: October 25, 2023, 12:54:00 pm
content: Option 2 for sure. I think that's fairly clear.

author: CaptDon
author_rank: Super Contributor
timestamp: October 25, 2023, 01:18:41 pm
content: Generally, about 99% are marked with the end-to-end voltage. Usually something like 12.6VAC C.T. 2A or the old filament transformers were 6.3VAC C.T. 10A as examples. Your transformer would give around 40vdc after rectification and filtering if it is truly 56VAC C.T. or possibly + and - 40VDC if it were a 4 diode rectifier in place of a two diode rectifier and I would guess your subwoofer used the dual polarity supply. If it were 56VAC each side of center tap that would give around 80VDC after filtering with no load. That would require filter capacitors of around 100VDC rating which I doubt is what is in your unit.

author: rodcastler
author_rank: Regular Contributor
timestamp: October 25, 2023, 01:34:23 pm
content: Thank you! Crystal clear.It seemed fairly straightforward to me at first, but then after reading several forums on the topic, some mixed information came in and I ended up unsure.Thanks for the quick reply

author: 807
author_rank: Frequent Contributor
timestamp: October 25, 2023, 02:00:19 pm
content: The schematic does indeed show that its a centre tapped 56v transformer. Full wave rectified to give +/- 40v.

author: TimFox
author_rank: Super Contributor
timestamp: October 25, 2023, 03:30:46 pm
content: Sometimes, you will see center-tapped secondaries listed as "28-0-28V" for 56 V with CT, but 56 VCT is more common.The former notation was probably intended for use with full-wave, two-diode rectifiers, common during the vacuum tube days.

author: MrAl
author_rank: Super Contributor
timestamp: October 25, 2023, 03:48:27 pm
content: Hello,Be aware that the +40 and -40 are the unloaded voltages.The averages will be around 36v each, and as the transformer loads the voltage will go down more depending on the wire resistance and other characteristics of the transformer.

author: TimFox
author_rank: Super Contributor
timestamp: October 25, 2023, 10:10:10 pm
content: Also, as the load current increases, the filter capacitors discharge more between charging pulses and the mean voltage and minimum voltage decrease.The minimum voltage on the filter capacitor is important when using a voltage regulator (with dropout voltage) after the capacitor.