What to do with +/-26VAC center tapped transformer ? - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: September 05, 2018, 03:36:24 am
content: I have about a 100W transformer from a stereo. Its the only center tapped trans I have, and I wanted to make some signal generator circuits, including using a XR2206 but is Vmax is +/-13Vdc , this trans. would be about +/-35Vdc rectifiedNow I plan make a general linear PSU with some big transistors. But what would I even do with that much voltage? What bench stuff do people make for fun or for equipment that would need so much voltage ?

author: bitseeker
author_rank: Super Contributor
timestamp: September 05, 2018, 04:03:00 am
content: General purpose linear bench power supplies are commonly found with 30V/3A or 30V/5A output. It's not unusual for audio power amplifiers to have ±20 – 30V power rails.It's all relative, of course. Talk to the vacuum tube folks where power rails are 300 – 400V.

author: GadgetBoy
author_rank: Regular Contributor
timestamp: September 05, 2018, 04:05:09 am
content: I could definitely use that transformer myself. You could use it in a full wave rectified circuit for ~18V. Put a half-wave rectifier on each outer tap of the transformer, using the centre tap as ground. You'll end up with a full wave rectified signal, which you can then filter and smooth.Sent from my ONEPLUS A3000 using Tapatalk

author: Cliff Matthews
author_rank: Unknown
timestamp: September 05, 2018, 04:14:17 am
content: Is that 26vct unloaded? I recall 25.2v is a common voltage, somewhere way back. Of course you could always peel a small bit of the outer insulation where the CT wire comes out, and you'll find you can divide that secondary into 2 distinct windings. I've done this many times, soldered it back up and over-sprayed the solid-core wire with auto ignition spray. YMMV

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 05, 2018, 11:52:51 am
content: I'd use it for an audio amp

author: Benta
author_rank: Super Contributor
timestamp: September 05, 2018, 06:56:32 pm
content: It's perfect for a 30-0-30 V, 1.5 A linear bench supply (variable, of course).Or a 30 V, 3 A bench supply, either opening the center tap and paralleling the windings, or using two rectifiers with the center tap.

author: Cliff Matthews
author_rank: Unknown
timestamp: September 05, 2018, 08:18:58 pm
content: A little confused, the thread title says "+/-26VAC center tapped" but I fail to see where a XR2206 needs so much beef, when a wall wart and rail splitter would do...

author: bitseeker
author_rank: Super Contributor
timestamp: September 05, 2018, 08:29:19 pm
content: It doesn't. OP pulled the xfmr from a stereo system and wondered what to do with it since it's so beefy.

author: lordvader88
author_rank: Frequent Contributor
timestamp: September 05, 2018, 08:37:45 pm
content: The xr2206 can only handle +/-13Vdc rail to rail. Pretty much all ICs I have like LM723, opamps, etc can't use this transformer either, without lowering the voltage 1st.I'll look into making a linear PSU with big power transistors, I have lots of 2N3055 and heat sinks. I bet I'll need power resistors tho, and big current sense resistors.

author: Dr.Krieger
author_rank: Contributor
timestamp: September 06, 2018, 09:55:10 pm
content: Quote from: lordvader88 on September 05, 2018, 08:37:45 pmOf course you can use a LM723 for more than 30V, you just need to use a PNP Driver to power those 2N3055. For example look at theVelleman K7200 PSUto see how this is done, i made a clone of that that would go up to 70V (not the one attached).You just need a secondary supply voltage for the LM723 and any other OpAmps, nothing a Zener or a 7812 can't do if you don't have a dedicated Trafo or winding

author: Dr.Krieger
author_rank: Contributor
timestamp: September 06, 2018, 10:15:12 pm
content: You will only need two Diodes to make a full-wave rectification using your center-tapped "26Vac/100W" trafo, that would give you ~35Vdc so a nice 0-30V 0-3A Power supply!Come to think of it, even a simple LM338 in TO-3 could handle that, well maybe if you removed a couple of turns for a bit lower Vin...Oh and ofc don't try to get those 3A at a low output voltage, remember high Vout/Vin (or transistor'sVce*Ic) raise dissipation...