Tap on secondary of transformer - Page 1
Forum section: electronics / beginners

author: stolevole
author_rank: Newbie
timestamp: April 28, 2019, 06:57:58 am
content: I dissasembled transformer with burnt primary and good secondary coil.Secondary is tapped but i cant figure out is it center tap or not and how is it wound.It has three separate wires and it had three wires at output of secondary with two center wires soldered together.Which type of tap that is and how do i rewind it?

author: MarkF
author_rank: Super Contributor
timestamp: April 28, 2019, 07:58:59 am
content: It sounds like it had two secondaries wired in series. The voltage of each secondary would add in that case. You just need to determine the direction of each winding. Maybe each has a solid color and a striped wire.Picture?

author: MarkF
author_rank: Super Contributor
timestamp: April 28, 2019, 08:42:04 am
content: If your secondary has two solid color wires and one striped wire, the striped wire will be the center tap.You can also check the resistance. The two wires with the highest resistance will be the outside ends of the coil. The resistance between the third wire to either of the other two should be the same if the center tap is in the middle.

author: stolevole
author_rank: Newbie
timestamp: April 28, 2019, 08:53:41 am
content: If you meant photo of assembled secondary unfortunately i dont have it but heres photo of three coils i unwound i didnt look how they were placed because i thought transformer was unfixable but it turned out its fixable.http://oi65.tinypic.com/j0bq5d.jpgYou can see on picture where i cut stranded wires going out of transformer(number 2 and 3).I think number 1 was the wire that had two soldered solid wires coming out of transformer because theres solder on both ends.They arent unwound by any order how they were on transformer.Its taken out of car battery charger if that helps (its probably step down transformer).I think it had two white and one black stranded wire coming out of secondary.Maybe if its impossible to find out how it was before i can solder three wires in series and have untapped transformer?Thank you so much for your help.

author: Ian.M
author_rank: Super Contributor
timestamp: April 28, 2019, 01:18:09 pm
content: Its FUBARed. You don't know the turn counts, and although they can be roughly estimated from winding wire lengths and winding dimensions, I bet you didn't record the dimensions as you unwound each winding and probably don't have a length for the burnt primary. Also reusing salvaged once-wound magnet wire in a power transformer is a very bad idea - you'll never get the turns to lay neatly side by side as they were originally so each winding will take up more core window area, which means you'll probably run out of room to get all the windings back on. Also, the odds of a shorted turn (that will overload and probably burn your new primary) are much much higher as the insulation coating on the wire will be fatigued and probably cracked from the unwinding.Therefore, all you've got that *MAY* be usable is the core and any coil former that was present. so its design your own transformer from scratch time. The salvaged wire may be usable for single layer experimental RF coils, low voltage battery powered electromagnetism experiments etc. where the available energy is limited and the consequences of a coil failure are slight, but IMHO its barely above scrap grade.

author: stolevole
author_rank: Newbie
timestamp: April 28, 2019, 09:18:29 pm
content: Lol i learned new acronym today.Well then i guess i will reuse the core to wind new transformer.At least i didnt FUBAR EI sheets.I know number of turns in primary maybe i can use new wire and rewind primary (it has around 650 turns i didnt wind it all the way yet to count all the turns).Is there some calculation for finding out how many turns i need in secondary so it wont heat up?

author: Jwillis
author_rank: Super Contributor
timestamp: April 30, 2019, 12:12:38 am
content: The secondary winding is of less importance when your concerned about heat since that depends on the load . The primary is connected to your mains service and incorrect winding on the core can cause heating due to not creating enough magnetic field because of to few winding s or trying to exceed saturation with to many winding s which can also lead to heatingIt can become a complex balancing act to get a transformer to perform efficiently and effectively. This balancing relies on many variables such as service voltage and current, geometry of the core ,core material and secondary coil parameters.But its not impossible to rewire a core with some unknown variables such as the material used and core geometry. We take the best guess on the material used and measure the geometry.This will get in the ball park but the efficiency may be lacking a little.We use this formula to determine the number of turns for a primary coilN=(10000 * Vp) / (4.44 * f * B * A)4.44 is a constant RMS value of emf per turn on single phaseB is the Magnetic Flux Density measured in Tesla s Typically that will be 1.2 for laminated silicon steelA is the core area in cm squaredf is the Frequency or 50hz where you liveVp is the primary voltage which would be 230 Volts where you live .N is total turnsthe 10000 is a conversion since we measure the core area in cm square and not meters square.To determine the secondary winding we use ratios if you know three variables.Np / Ns = Vp / VsThe core area is measured length times width of the metal core inside the bobbin as viewed with the winding s going around .The maximum current will be determined by core area and the gauge of wire used.Also the higher the frequency the fewer the winding s .The larger the core the fewer the winding s