Help with Toroidal Transformer please - Page 1
Forum section: electronics / beginners

author: stevetob67
author_rank: Contributor
timestamp: January 05, 2021, 05:47:45 pm
content: Hello,Firstly sorry if I am posting in the wrong area, still getting a feel for this forum.I have a Toroidal Transformer I salvaged from a Bauhn HTE-400 5.1 Home Theatre Amplifier that has 3 centre tap secondaries and 2 standard lower voltage secondaries. I also noticed a SCR winding on the transformer with 4 wires which I have included a photo with the pins connected to the front panel circuit board.I am wanting to use the transformer to make a double variable benchtop power supply also with dedicated 3.3v, 5v and 12v outlets separate to the variable side. I am assuming the transformer is a decent quality item because on the main boards they all use Topcon Caps as well as Toshiba Transistors and Samsung IC's.My question is what to do with the SCR winding, should I hook it up to something or leave it dangling in the wind? What does the SCR winding do for that matter, I googled a few things and it seems to do with the residual magnetical fields or if it is used on the incoming mains as a soft start but how will it affect me?Also the transformer gives no information with regards to the current I can expect from it, being from audio equipment I am sure it will give me at least 5amps which will be plenty for my needs. Below I have included a picture of the transformer and the SCR line. Any help would be gratefully appreciated.Stevehttps://drive.google.com/file/d/1an01ZhGj7I-rJwxsm5rigvGwzaPLU6ct/view?usp=sharinghttps://drive.google.com/file/d/1aeOIt7B_vxhb-AHrtCz2lmFtTMSJ2Mtf/view?usp=sharing

author: bob91343
author_rank: Super Contributor
timestamp: January 05, 2021, 10:53:54 pm
content: Those pics don't seem to want to load.

author: stevetob67
author_rank: Contributor
timestamp: January 06, 2021, 04:48:53 am
content: The second picture won't attach, I will try here

author: TimNJ
author_rank: Super Contributor
timestamp: January 06, 2021, 05:06:49 am
content: SCR is screen, I think. But, you said it's 4 wires?Screen is usually tied to chassis earth for these transformers. Its purpose is to reduce the effective primary-secondary capacitance, which reduces common-mode interference currents from passing through the transformer.

author: stevetob67
author_rank: Contributor
timestamp: January 06, 2021, 06:05:32 am
content: Quote from: TimNJ on January 06, 2021, 05:06:49 amYes it is 4 wires connected to the Front Panel Screen circuit board, the wired are a pair of Black and a pair of YellowConnected as followsBlack - 4vBlack - 0vYellow - GYellow - TEReducing noise being an Audio transformer makes sense so I guess that not connecting it on a Dual Variable Bench Power application with additional 3 - 3.3v, 5v and 2x 12v constant outputs (with on/off switches) should not make much of a difference. That is unless there is a simple circuit that can be connected to the 4 pin header to reduce noise.Filtering Caps will be connected to all power output lines.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 06, 2021, 06:36:48 am
content: Quote from: stevetob67 on January 06, 2021, 06:05:32 amSCR - this is one yellow wire.

author: dietert1
author_rank: Super Contributor
timestamp: January 06, 2021, 10:37:30 am
content: The second image shows two black and two yellow wires in a four way connector. Maybe that toroid has 2 shields. You would connect one shield to mains PE (don't know why it is labeled TE) and the other one to circuit Gnd (labeled G).In general toroids have bad capacitive coupling, sometimes several nF. A shield or two shields make sense.Regards, Dieter

author: stevetob67
author_rank: Contributor
timestamp: January 07, 2021, 02:59:52 am
content: The front panel does have an earth wire tied to the chassis so I take it that the two blacks form one pair and the two yellows form the second.The two blacks are labelled as 4v and 0v and the Yellow pair as G and TE (PE), is PE referring to Primary Earth?Could I get away with just using one shield using this transformer (G & PE) as a power supply as it will no longer be used for audio purposes?Sorry but I don't know much about Mains PE

author: dietert1
author_rank: Super Contributor
timestamp: January 07, 2021, 07:27:59 am
content: PE = protective earth. Here in Germany it is part of mains wiring and needs to be connected to the metal enclosure, if the appliance has metal parts that can be touched by the user. The idea is simple: In case something goes wrong inside the device that will connect mains AC voltage onto the metal enclosure, this will trigger the mains fuse to protect the user from the danger of electrocution.Regards, Dieter

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 07, 2021, 07:34:43 am
content: Everything is written on the transformer. If there are two yellow wires, then there are two screens or 2 outputs of the same screen - it is easy to check whether they have a connection. Using the screen is a good practice, but without the screen it will work exactly the same.As you can see, there was no person who unwound such a transformer and can not share knowledge. You need to work a little yourself and understand.

author: dietert1
author_rank: Super Contributor
timestamp: January 07, 2021, 08:11:30 am
content: Yes, it's a good idea to test with a multimeter whether the two yellow wires are connected inside the toroid. Pull that 4-way plug before doing that.If you have an LCR meter you can use that one to find out about the layer structure inside the toroid. No need to unwind the toroid. Measure the capacitance between primary and each of the two shields, and between secondary and each of the two shields. Probably one of them is near (= high capacitance) to the primary side, the other one near to the secondaries. Then the first one connects to PE, the second one to Gnd.Regards, Dieter

author: Gyro
author_rank: Super Contributor
timestamp: January 07, 2021, 10:43:48 am
content: Quote from: S. Petrukhin on January 07, 2021, 07:34:43 amThe label on the transformer shows the yellow wires as being a 29V winding, not screens (in addition toone, which is the screen).

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 07, 2021, 07:29:39 pm
content: Quote from: Gyro on January 07, 2021, 10:43:48 amTwo yellow together with black form a winding with a midpoint of 2x29V. Perhaps in the second photo they are connected to the connector. This is easy to check with a tester.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 07, 2021, 07:31:19 pm
content: The manufacturer of this transformer clearly has a problem with different colors of the wire and there is no possibility of other marking.

author: Gyro
author_rank: Super Contributor
timestamp: January 07, 2021, 07:46:06 pm
content: Yes, they do seem to have had a very poorly populated wire rack!

author: stevetob67
author_rank: Contributor
timestamp: January 08, 2021, 03:01:13 am
content: Yes they could have used different colours to identify each winding pair.All up on the secondaries there is a 6P header (Red, Black, Yellow) and 2x 2P headers, a Blue and a WhiteThe SCR is a 4P header (Yellow and Black) each colour has continuity so I assume that it is obvious that I can take any one of the two pairs and use between Mains Earth and Chassis Ground (I am assuming). Would I be making a mistake to do this, I have seen mains earth connected to chassis many times but am unsure where a transformer is involved.

author: stevetob67
author_rank: Contributor
timestamp: January 08, 2021, 08:37:51 am
content: Been doing some research on these and the Shield winding seems to be necessary in audio transformers to reduce or eliminate the humming. I guess maybe the reason there in two one this transformer is to have on on the primary and one on the secondary.Anyway, thank you everyone for helping me to fine tune my searching to get the answers I needed. Have a great new year!!