Some questions about a MultiTap transformer without datasheet - Page 1
Forum section: electronics / beginners

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: January 31, 2018, 09:32:26 pm
content: I have enough parts laying around know that I can build a pretty nice linear bench PSU without buying anything most likely, since I dont have one it sounds like a fun project. I am shooting to build a dual channel that can be arranged in bipolar mode. I found a nice multi tap transformer in my junk from an old onkyo receiver. I tested it the secondary output with a continuity test and it appears to have two center-tatped outputs. It has a set of brown, blue and yellow wires with a white and orange. It appears that the white wire is a center tap for the blue and brown sets and the orange is center tap for yellow.Here is an imagehttps://www.dropbox.com/s/gj1dhf4sbutge9y/psuStuff.jpg?dl=0Testing the voltage I getBrown to Brown - 56vACBlue to Blue - 31.5vACBrown to White - 28vACBlue to White - 15.7vACYellow to Yellow - 35.75Yellow to Orange -17.88So what I need to know is if I use a connection like White to Orange which gives 7 volts, or since these are on what appears to be on different output coils is this not a good idea? Secondly how do I go about testing the amount of current the transformer can safety deliver? The model number is HAWK h86-002A, I cant find any data on the thing, im wondering if 002A means 2 amps. Im not interested in my PSU being high current, but pushing at least 2A on each channel simultaneously would be great,Somewhat off topic but may as well bring it up here, when you look at most bench psu's online they all use a voltage ref with an opamp and pass transistors, what exactly is the benefit of using a self made regulator vs a current limiting circuit followed by a plain old adjustable regulator, I dont see very many adjustable supplie schematics using a 3 pin regulator and I am curious why.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 01, 2018, 12:01:53 am
content: Did you continuity check between the windings? The voltage you measured between the Yellow/Orange winding and the other will not support any current. To determine VA rating, do you have the Onkyo model number? If not, weight and dimensions could help estimate by comparing it with catalogs from manufacturers like Triad Magnetics or Hammond, etc..

author: Cliff Matthews
author_rank: Unknown
timestamp: February 01, 2018, 12:29:22 am
content: FWIW, the size doesn't look too much above 140VA so don't expect much more than 2.5 amps. If it were mine, I'd electrically split it down the center by opening-up both center taps to get 4 windings in total. (I test everything before soldering-in new high-temp wires and spray over joints with dielectric insulation). BTW, that looks like a nice line filter.- 2 dual output high current windings for 2 output voltage ranges (blue's up to ~16v and brown's up to ~32v regulated)- 2 independent (likely <1 amp) windings for other circuitry(like op-amps, metering, voltage references or output/tap relay control)

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: February 01, 2018, 04:38:26 am
content: I don't know the model of onkyo receiver I got this out of, I bought it for 5 dollars broken at a swap meet and tore it down for parts years ago. The transformer weights about 5lbs or 2.27Kg that weight seems to correspond to about 120VA when it comes to E I cores. That would mean I could most likely get away with 4 amps total at 30v. or dual 2 amp 30v channels, which is good enough. The thing is the transformer has these two separate output coils, I dont even need the second output coil really, it doesn't have enough tapping options, but if I cut the wires and leave it unused Im sure I wont get a full 120VA will I?I would like to try loading it until the voltage drops by 10%, while monitoring the temperature, but I don't have cash for a 200w electronic load or 200w power resistors for that matter. Is there a cheap easy way to make something like a 55 and 110 ohm load that can handle 200 watts?

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: February 01, 2018, 10:43:26 am
content: @cliffI think your post just hit me after re rereading. Your saying the blue/brown taps will most likely give me about 2 amps a piece and the yellow/orange CT will give me around an amp independently to run the actual psu circuitry?I'd love to hear it down and rewind the secondary but I have no idea how to get a transformer core apart especially this one it's built like a tank.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 01, 2018, 11:18:50 am
content: It should be easy to open after you cut off the sheet metal wrap (it won't be required anyway). Knowing "Wonkyo" they likely had it too close to a phono preamp.. After you open it, you'll see the wire gauges used in both winding's. BTW, nichrome wire cores from old hair dryers can be used as a makeshift load. In North America each of the 2 heating elements is about 21 ohms, so in parallel you'll get 11 ohms. I've found in free air you can use them at 50 watts without tripping the bi-metal overheat switch (in the center), so use a small fan to cool it.*edit - Last year I got about 30 brand new hair-dryer element inserts from an old repair guy and made a drawing to wire 8 of them together with terminal blocks and simple switches to get some flexibility out of them and 4 50ohm power resistors. (attached as food for thought).