TWO 400mA adapters versus ONE center tapped transformer of unknown mA? - Page 1
Forum section: electronics / beginners

author: dentaku
author_rank: Frequent Contributor
timestamp: June 18, 2014, 04:03:14 pm
content: I have two identicalinput:120VAC 60Hz 9W - output:15VAC 400mAadapters that I can connect together, then to a bridge rectifier and use exactly like I would a regular center tapped transformer. It works very well.My multimeter measures them as 18VAC and the scope shows a waveform that's roughly 26V P-P. That's seems logical because 26 * 0.707 = 18.382I have a transformer that seems to work fine at supplying the right voltage for an LM317/337 power supply I've been messing with (it outputs around +/- 20V) AND using the two wallwarts/plugpacks together instead of that big transformer also seems to work well. In fact is gives me a max DC voltage on the output the regulator slightly higher than the transformer.QUESTIONS:would I be correct in assuming that this unknown, salvaged transformer is capable of supplying more current than the TWO 400mA wallwarts together?Is two 400mA wallwarts equivalent to one center tapped transformer rated for a max output of 800mA? OR are two together still just 400mA?I like the idea of using the two wallwarts because it saves allot of space in my enclosure but if the transformer can better supply larger current (LM317/337 maxes at 1.5A I think) then I guess I can live with the giant lump inside the enclosure.Here's a photo of the transformer.https://www.eevblog.com/forum/beginners/resistance-of-different-transformer-windings/msg404658/#msg404658

author: HalfSpace
author_rank: Regular Contributor
timestamp: June 19, 2014, 11:22:08 pm
content: Hi dentaku,The salvaged transformer looks to me about 40-50 watts. If you can post some dimensions of the transformers metal bracket, but don’t include the mounting tabs, I will be able to give you a better idea of its power rating.If the secondary side of the two plug packs is connected in series you will get 30 Volts AC @ 400mA and if connected in parallel you will get 15 Volts AC @ 800mA (See attached picture).But the two secondary’s have to be in phase when wired in parallel or you could destroy the plug packs.To find the phasing of the plug packs secondaries connect them inseriesas in the attached picture, power them up and measure with a multimeter set to AC volts the voltage between points 1, 2 and then between 2, 3 they should both read 15 volts AC each. Now measure between points 1, 3 if it reads almost zero volts then one of the plug packs is out of phase. Reverse the secondary wires on one of the plug packs and measure between points 1, 3 again you should now get around 30 volts AC. The plug packs are now in phase and by marking the wires indicated by the “X” in the attached picture you will know the phase of each plug pack. Now they can be wired them in parallel with the wires that are marked connected together see attached picture.What is the project that you are making? If it’s a lab power supply then the salvaged transformer could be the better option as you could use the isolated windings (red and blue) as maybe fixed 12v DC supplies that are completely isolated from the LM317/LM337 supply.HalfSpace.

author: dentaku
author_rank: Frequent Contributor
timestamp: June 20, 2014, 11:18:12 am
content: OK.I never thought about connecting them in parallel but that wouldn't be useful for me because I was trying to create something that's like a center tapped transformer anyway.It's good you mentioned phase because I was wondering about that since you see phase indicators on transformers in software simulations.I just did your experiment and it works like you described. In series I was getting almost zero volts connected one way and over 30V when I reversed one of them.The way that works for me is to have the two center pins of the barrel jacks connected together. Of course it also depends on which way I have the packs plugged into the power strip too.Anyway...It seems that while this works, it's nowhere near as capable of supplying a decent amount of current compared to a regular transformer so I guess I'll stick with that.The metal casing of this transformer is 77mm wide X 43mm deep X 65mm tall (or 3"X1.75"X2.5") and weighs 1.56kgI have another similarly sized one that has useful information printed on it and it's rated for 2A but unfortunately it doesn't put out enough voltage for the power supply I want to build.

author: HalfSpace
author_rank: Regular Contributor
timestamp: June 23, 2014, 12:29:44 am
content: Hi dentaku,By the dimensions I would say around 50 watts. To find out what the transformer capabilities are, try loading your power supply with a 500mA load on both the LM317 and LM337 outputs and see how warm the transformer gets after 1 hour. If it’s only just warm to the touch (30C) try increasing the load to 1 Amp and wait another hour. Keep an eye on its temperature during the hour just in case it gets too hot. At this point increases of 250mA would be best as the temperature increase may be steeper once you get closer to the transformers limits. Just warm is about 30C, warm 40C, hot 50C and very hot 60C. This is the point where it is uncomfortable to touch and shouldn’t be taken any higher.HalfSpace

author: dentaku
author_rank: Frequent Contributor
timestamp: June 23, 2014, 03:16:57 pm
content: Quote from: HalfSpace on June 23, 2014, 12:29:44 amOK, thanks for the info.