Transformer question - Page 1
Forum section: electronics / general

author: shebu18
author_rank: Frequent Contributor
timestamp: August 19, 2012, 11:27:18 am
content: Hello all. I have found at home a transformer that would fit my PSU. It is a big thing. It has labeled on one side 0 and 380V and the outputs are 0v/24V/28V/E. I put 220V on the input and measured the output. Used 0V for the negative lead and got 15.5V/18V/10.7V.Now my question is how can this be working? As far as i know you need 2 wires on the output to make the widing, 3 for a center tapped. I have 3 of which the firs 3 have a common point tested with the continuity function and the 4 one is on its one. Now i am asking can i use 3 rectifiers where one lead is common on all? I want to use the 15.5V for the PSU, 18V for a soldering Iron and the 10,7V for the 5V,12V regulator.

author: szhighstar
author_rank: Contributor
timestamp: August 19, 2012, 12:48:36 pm
content: there are three output voltage in the transformer, so you can proceed three applications.

author: Architect_1077
author_rank: Regular Contributor
timestamp: August 19, 2012, 02:36:29 pm
content: Could give us a picture of the transformer?I'm not sure if I've misunderstood anything you said, but seems to me you have an auto-transformer (as opposed to an isolated transformer). Anyway, a picture would help in that regard. Or maybe someone who knows more than I do

author: shebu18
author_rank: Frequent Contributor
timestamp: August 19, 2012, 05:57:27 pm
content: Here are the pics.

author: bilko
author_rank: Frequent Contributor
timestamp: August 19, 2012, 06:25:38 pm
content: It looks like a 380V to 24V control transformer, in poor condition.Check resistance between primary and secondary windings, it looks like it is an isolation transformer.The alternative tappings on the AC side are if the supply voltage is out of tolerance, on the DC side often connect 28V to allow for voltage drop on longer cables.If it was mine, I would check insulation with a megger before I put it into service.

author: Gall
author_rank: Frequent Contributor
timestamp: August 19, 2012, 06:34:42 pm
content: This is not unlike the transformer used for lighting on heavy machinery. 24 V and 28 V are voltages for incadescent bulbs or something. The "E" wire is for some special purpose. It may be separate winding sharing connected at the end, not a tap from the 0-24-28 winding.The 24 pin is most likely a tap in the middle of 0-28 winding. The E pin may or may be not a tap. So it may be like that:0---E---24-28or like that:0------24-28\---Eor even like that:E---0---24-28Try to measure voltage and resistance between E and 28 to figure it out.Since it was originally used for AC only, there was no problkem using tapped windings at all. If you want to make 3 separate voltages, the easiest thing you can do is to use 3 bridge rectifiers. But DO NOT connect their outputs. A rectifier from 0-24 and a soldering iron connected to 18VAC are Ok. A second rectifier from 0-E is Ok too as long as its output is not connected to the PSU output. If you want the PSU and the 5-12V regulator share the same ground, you should use half-wave rectifiers - just one diode at 24, one diode at E and 0 directly connected to the output ground. I guess the soldering iron does not need a rectifier at all and would work fine at AC. If it has some electronics in its controller, it is Ok to power it from a full-wave rectifier since it does not connect to anything else.

author: SeanB
author_rank: Super Contributor
timestamp: August 19, 2012, 06:39:16 pm
content: The 10.7V is measured between 0V and E, which is the interwinding shield winding. It is meant to be connected to the chassis ground, to provide isolation from high frequency noise between primary and secondary. Try measuring again with a 1K resistor in parallel with the probes, you will see that the first 2 voltages are the same, but the last will be almost zero.

author: shebu18
author_rank: Frequent Contributor
timestamp: August 19, 2012, 06:43:36 pm
content: I use it for 220V AC.between 0 and 24 i get 1.8ohm0 and 28 i get 1.5ohmE and 28 nothingE and 24 nothing24 and 28 1.6ohmI wnated to use 3 rectifiers with common gnd. 0-24 for PSU, 0-28 for the iron and 0-E for the logic of the PSU. I could use the voltage for the logic from the 0-24V tap, so the PSU has its own part.LE: With a 2,2k resistor between 0 and E i get 0V and i get 15V oin the 24V line and 18V on the 28V line.

author: shebu18
author_rank: Frequent Contributor
timestamp: August 20, 2012, 08:20:18 am
content: What would be the voltage if i use a half way rectifier? for the full wave i know that i have to multiply the ac output voltage with 1,4.

author: Gall
author_rank: Frequent Contributor
timestamp: August 20, 2012, 12:32:06 pm
content: Ok, then E is shield for sure. "E" stands for "Earth". Connect it to the green-yellow wire of mains outlet.Only two outputs, 24 V and 28 V then.Are you sure 0-28 resistance is less than 0-24 resistance? Try to calibrate your multimeter first.Half-wave rectifier gives the same voltage as full-wave one. Exactlier:Full-wave = 1.41*in - 2*0.7, where 0.7 is diode forward voltageHalf-wave = 1.41*in - 1*0.7However the voltage is less under load. To reflect this, one uses 1.2 factor for full-wacve and something like 1.1 factor for half-wave under full load. The main disadvantage of half-wave rectifier is increased ripple level and some loss of power due to DC magnetization of the core.As there's no need to connect PSU and soldering iron together, I suggest to use two completely separate full-wave rectifiers. 0-24V for PSU with control logic and 0-28V for soldering iron control logic. The heater of soldering iron would work fine at 28VAC, no rectifier is needed here. Use a triac if you want to control power.There are low-cost step-down switch mode converters to provide 5V or 12V from 24 or 28V with high efficiency. Some are available on eBay as readily-made modules.

author: shebu18
author_rank: Frequent Contributor
timestamp: August 20, 2012, 01:34:45 pm
content: Than i will use 2 full-wave rectifiers. The voltage for the Iron will be send through a cable into the housing of the iron and there it will be converted to 5V and i will use 24V DC for powering the iron. The transformer does not output 24 and 28V but rather 15 and 18V because i use 220V not 380V.

author: Gall
author_rank: Frequent Contributor
timestamp: August 20, 2012, 02:42:33 pm
content: Oh sorry, I forgot it is rated 380 VAC.2 full-wave rectifiers would work just fine. BTW, why do you want DC power for the iron? AC power with a light duty rectifier for control circuit only would be just fine. Do you already have 18VDC soldering iron with control circuit?

author: shebu18
author_rank: Frequent Contributor
timestamp: August 20, 2012, 03:58:45 pm
content: I have the blue one:http://www.tme.eu/html/RO/piese-de-schimb-pt-statii-de-lipit-pensol-serii-sl10-sl20-sl30-cu-element-incalzire-standard/ramka_2715_RO_pelny.htmlIt uses 24V AC, 2Amps 48W. If i use 24V DC and 2Amps i still get the 48W. 18VAC and 2 amps does not get me the 48W. I do the development of the control circuit.

author: bilko
author_rank: Frequent Contributor
timestamp: August 20, 2012, 04:47:56 pm
content: You won't get 2 Amps at 18V, the soldering iron element is resistive, about 12 ohms. At 18V you'll get around 27W max.

author: shebu18
author_rank: Frequent Contributor
timestamp: August 20, 2012, 05:45:02 pm
content: That if i use ac but dc is another storry. Worst case scenario i would need to add a litle step up smps.Sent from my GT-I5800 using Tapatalk

author: bilko
author_rank: Frequent Contributor
timestamp: August 20, 2012, 06:32:33 pm
content: Quote from: shebu18 on August 20, 2012, 05:45:02 pmJust referencing your previous postQuote from: shebu18Of course you can use DC but you said 18V AC and 2 amps does not get me the 48W, that is because you do not get 2A at 18VAC.Anyway, its a total bodge, can't you get hold of a proper 220V to 24V AC transformer and do the job properly ?

author: shebu18
author_rank: Frequent Contributor
timestamp: August 20, 2012, 07:09:58 pm
content: I could but why pay 7 euros when i could use this transformer. I do not use the psu and the iron at the same time. 48W are for maximum temperature, if use 250-350* i do not need 48W, right?

author: Monkeh
author_rank: Super Contributor
timestamp: August 20, 2012, 07:18:06 pm
content: Quote from: shebu18 on August 20, 2012, 07:09:58 pmBecause this transformer isn't really appropriate?QuoteYou'll regret the lack of power very quickly when attempting to rework lead-free or working on large terminals or components.

author: bilko
author_rank: Frequent Contributor
timestamp: August 20, 2012, 07:20:23 pm
content: Quote from: shebu18 on August 20, 2012, 07:09:58 pmIt just seems a lot of hard work, surely the boost converter (with heatsink) is going to take most of the 7 euro you were planning on saving.The 48W is very useful if you need to solder to boards with huge ground planes, low wattage irons just won't do it.Challenge ... Try to desolder a capacitor from a PC motherboard with a 25W iron

author: bilko
author_rank: Frequent Contributor
timestamp: August 20, 2012, 07:21:36 pm
content: Quote from: Monkeh on August 20, 2012, 07:18:06 pmYou beat me to it, with your reply, I was still typing mine

author: Monkeh
author_rank: Super Contributor
timestamp: August 20, 2012, 07:23:11 pm
content: Quote from: bilko on August 20, 2012, 07:21:36 pmI'm quick like that.

author: shebu18
author_rank: Frequent Contributor
timestamp: August 20, 2012, 07:24:14 pm
content: You are right. I will buy a separate transformer for the iron.For the psu i will use only the 24V(15V) connection. I think that is the power lane.