Is it bad practice to leave a transformer secondary floating? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 04, 2019, 01:39:22 pm
content: Someone told me that it's bad practice to leave one half of a centre tapped secondary (6-0-6 V) floating. The transformer is a small PCB mount 0.5 Amp output 240 V mains one. I full wave bridge rectified one 6V winding and left the other one floating. Is this bad? Thanks. I am sure I have seen lots of transformers in things with unconnected secondary taps, so have doubts over this advice...

author: Ian.M
author_rank: Super Contributor
timestamp: July 04, 2019, 01:45:53 pm
content: Its not bad practice provided the load on the half-winding is well within its ratings. However if you are pushing its limits, it would be better to use two diodes from opposite ends with the center tap to provide full wave rectified DC with the I2R losses distributed over the whole winding and half the diode losses.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 04, 2019, 01:49:03 pm
content: If you use a full wave center tapped rectifier configuration instead of a bridge, you would save a couple of diodes....a few pennies.Perhaps most importantly you would save one diode drop, which in low voltage supplies will provide extra headroom to the regulator.Otherwise, just insulate the loose wire.

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 04, 2019, 02:14:49 pm
content: OK, understood and many thanks. I suppose I could parallel the two windings with due regard to phase and keep my single device FWB rectifier, so the current capability was doubled? Not that i am drawing more than 100mA from the bridge anyway.

author: cvanc
author_rank: Frequent Contributor
timestamp: July 04, 2019, 02:27:39 pm
content: Quote from: Chris Wilson on July 04, 2019, 02:14:49 pmWait, in the first msg you said it was a single center-tapped secondary. Here you say it's 2 independent secondaries... please clarify?

author: ArthurDent
author_rank: Super Contributor
timestamp: July 04, 2019, 02:40:03 pm
content: cvanc is spot on. Is this a 12 volt CT winding with only 3 leads or is it two 6 volt windings (4 leads) wired as 12 volt CT? If there are only 3 leads you can't do what you want and the best way would be to use 2 diodes for a full wave setup. This will slightly improve the no load to full load drop compared to the way you now have it wired using 1/2 the entire winding.

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 04, 2019, 02:41:37 pm
content: Hmm, I have to remind myself, I built it a while back, it's this transformer from RS:https://uk.rs-online.com/web/p/pcb-transformers/1739715/?relevancy-data=636F3D3126696E3D4931384E525353746F636B4E756D626572266C753D656E266D6D3D6D61746368616C6C26706D3D5E2828282872737C5253295B205D3F293F285C647B337D5B5C2D5C735D3F5C647B332C347D5B705061415D3F29297C283235285C647B387D7C5C647B317D5C2D5C647B377D2929292426706F3D3126736E3D592673723D2673743D52535F53544F434B5F4E554D4245522677633D4E4F4E45267573743D3137332D39373135267374613D3137333937313526&searchHistory=%7B%22enabled%22%3Atrue%7D

author: bdunham7
author_rank: Super Contributor
timestamp: July 04, 2019, 02:53:02 pm
content: It's two separate windings. If you were building your supply anew, you could parallel the windings with a bridge, or for a little more performance, you could connect them in a CT arrangement with two diodes for full-wave rectification. However, as you apparently have already built it and it works fine--your 100mA stated output is within the specs for 1 winding--I certainly wouldn't go back and redo it. The unused winding is of no consequence.

author: SG-1
author_rank: Regular Contributor
timestamp: July 06, 2019, 02:46:38 pm
content: The grounding advise applies to ac premises wiring in the United States under the NEC.Using a high impedance voltmeter measure the output to earth. Let this help you decide if it should be earthed or not.