Understanding Center Tapped Transformers - Page 1
Forum section: electronics / beginners

author: queennikki1972
author_rank: Regular Contributor
timestamp: April 27, 2019, 02:24:52 pm
content: I tried to search for a thread on this before posting.I have a few questions regarding center tap transformers. Very amateur questions..First,If you have a center tapped transformer of say 12v-0-12v and is labeled 12v (2.5amp) - 0 - 12v (2.5 amp) and sold as a 5amp transformer 30VA total, does this mean you can acquire max of 24v at 2.5 amps across the two outside wires across the bridge rectifier before the regulation circuits?

author: madires
author_rank: Super Contributor
timestamp: April 27, 2019, 03:04:54 pm
content: Such a transformer would deliver (12V + 12V) * 2.5A = 60VA. For 5A you would need a transformer with two separate 12V @ 2.5A windings which you could parallel. Adding a bridge rectifier plus filter cap changes AC into DC and also changes the values. The 24V AC becomes nearly 34V DC (V_RMS * sqr(2)) and the current 1.7A (I_RMS / sqr(2)). These are the theoretical values, the real ones will be lower (voltage drop of diodes, ripple and so on).

author: iMo
author_rank: Super Contributor
timestamp: April 27, 2019, 03:07:36 pm
content: https://www.changpuak.ch/electronics/power_supply_design.php

author: mariush
author_rank: Super Contributor
timestamp: April 27, 2019, 03:21:34 pm
content: Your 12v - 0 - 12v 2.5A - can be used as a 24v 2.5A transformer ... you simple leave the center tap pin/wire disconnected, unused.Some transformers have a center tap which can be separated, to create two separate secondary windings. In this configuration, you can wire the secondary windings in parallel to get double the current, but half the voltage - so you get a transformer that outputs 12v AC at 5A.If the center tap can not be separated, you can still get higher current but low voltage but not using a bridge rectifier, see below :(i'll edit this message further in a few minutes)

author: queennikki1972
author_rank: Regular Contributor
timestamp: April 27, 2019, 09:36:25 pm
content: I understand that it would be 24v at 2.5 from the paragraph above, but I wonder why they advertise it as 5 amp.https://www.ebay.com/itm/12V-CT-5A-Power-Transformer-6-3V-0-6-3V/321565034079?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

author: queennikki1972
author_rank: Regular Contributor
timestamp: April 27, 2019, 09:39:54 pm
content: Sorry i posted link to a 6.3-0-6.3 by accident

author: ifrenide
author_rank: Regular Contributor
timestamp: April 27, 2019, 09:51:53 pm
content: the only reason they advertise it as 5A to indicate the total current that this transformer draw but its confusing little bit they tell you a truth but not the practical use of this 5A

author: mariush
author_rank: Super Contributor
timestamp: April 27, 2019, 09:54:44 pm
content: Well, look at the picture, you can see the label on the transformer .... the ebay guy probably just went with the text printed on transformer... 6v 5A = 30VA ... simple math.FYI 25$ for a transformer is not a very good price.You have distributors like Digikey or Mouser... here's for example Digikey :https://www.digikey.com/products/en/transformers/power-transformers/164Here's all transformers 20VA or higher, with 6v..9v secondary windings :https://www.digikey.com/short/pz7n5zA 36VA model is ~11$ plus shipping :https://www.digikey.com/product-detail/en/triad-magnetics/F12-2850-C2/237-1565-ND/3986340Here's all transformers 20VA or higher, with 10v..15v secondary windings :https://www.digikey.com/short/pz7nr3Cheapest 30VA model is 10$, and an 80VA model is less than 20$ :https://www.digikey.com/product-detail/en/triad-magnetics/VPS24-3300-B/VPS24-3300-B-ND/4746278

author: queennikki1972
author_rank: Regular Contributor
timestamp: April 27, 2019, 10:28:09 pm
content: Thank you..