Main Toroid Primary and Secondary Connections -noob question - Page 1
Forum section: electronics / beginners

author: medical-nerd
author_rank: Regular Contributor
timestamp: April 18, 2017, 12:16:11 pm
content: Apologies for this very very basic question regarding mains toroid transformers.If I have a toroid transformer with dual 120V primaries and want to use it on UK 240V and:the wires are e.g. labelled 0-120V red orange and 0-120V black green,do I wire the transformer as red, orange-black, greenor red, orange-green, black???The same question regarding dual secondaries 0-27V 0-27V where I want to build a dual rail power supply.I have done the usual searches on google and forums but can't find a straight answer - other than possibly use a light bulb to check that phases are 'correct' but no details on this.Any help would be much appreciated.CheersSteve

author: cowana
author_rank: Frequent Contributor
timestamp: April 18, 2017, 12:59:29 pm
content: On most labels, the phase is consistent - so red/orange will have the same phasing as black/green.As such, wiring it as red, orange-black, green will give the correct configuration for 240v. Going with orange, red-green, black would give the same effect.Switching the polarity of one of those phases would cause the windings to 'fight' against each other, leading to excessive current draw, overheating, and no output.

author: Seekonk
author_rank: Super Contributor
timestamp: April 18, 2017, 01:14:41 pm
content: That said, you only get one chance before it goes up in smoke. Use the lamp in series with the primry to verify everything is correct.

author: tkuhmone
author_rank: Regular Contributor
timestamp: April 18, 2017, 01:32:53 pm
content: Thanks for info.... I do have similar case. One ring core transformer (BSAB Electronic, C-RKT 160/2.110/2.22) with dual 110V primaries. The color names are in German, I can identify two of these (sw-or, ws-vi). With multimeter I can measure both primaries with similar resistance, and both primaries are isolated from each other.I will need to setup also the lamp system. I quess the lamp could flash for short while, in the peak current when connecting transformer to AC mains? (in case of correct polarity). My transformer has rating of 160VA - so that should not take as big startup current as bigger transformers, like 500VA or more.EDIT: Transformer above (BSAB Electronic) does not have any phasing dots....

author: ady.price
author_rank: Contributor
timestamp: April 18, 2017, 01:38:21 pm
content: if you really want to connect the primaries in series you need to connect the wires of the primary which are closest to each other or farthest from each other <strong>on the toroid</strong>. if you can't see that, and you get it wrong, you will get 0V on each of the secondaries, since the directions of the magnetic fields will be opposite each other and become neutralized, which also means that you will effectively <strong>short your primary</strong>(since there is almost nothing left to oppose the current anymore) so have a resistor >=10k rated @ >= 10W or a peak mains voltage* rated 100n capacitor in series with it. although a capacitor can be problematic.* peak for 240V mains is ~340V

author: Ian.M
author_rank: Super Contributor
timestamp: April 18, 2017, 04:33:02 pm
content: WTF? Apart from the Tapatalk mangling, there's just so much *wrong* with that post.Assuming you have enough data to be 100% certain which wires ate the primary windings and that the transformer is designed for dual voltage operation, then the easiest safest method is to apply approximately 115V, current limited, to ONE primary winding, wit5h the other one that you have previously confirmed is separate by resistance checks in series with it. If you've got the phasing correct, the ends of the two windings that are not directly connected will have approximately double the voltage you applied across them. If you've got it wrong, they will have little voltage between them, typically less than 10% of the applied voltage.For current limiting, use a mains voltageincandescentbulb in series with the transformer under test - 60W is a good choice, but its fairly non-critical as long as its wattage at least 10% of the transformer VA rating and not more than double the VA rating.Outside North America, to get 115V you've got three options - use a Variac if you have one, or a step-down autotransformer (make sure its real iron and copper, not an electronic voltage 'converter'), or instead of the single series current limiting bulb, put two identical bulbs in series across your mains supply and tap off half the supply voltage from in-between them.

author: ady.price
author_rank: Contributor
timestamp: April 18, 2017, 06:08:06 pm
content: Quote from: Ian.M on April 18, 2017, 04:33:02 pmcan you explain what's wrong with my post?Sent from my Nexus 6 using Tapatalk

author: Gyro
author_rank: Super Contributor
timestamp: April 18, 2017, 06:18:40 pm
content: Quote from: Ian.M on April 18, 2017, 04:33:02 pmI think what he's trying to say is that you can use a highish value resistor or a lowish value capacitor as a series current limiter while sorting out the phasing of the dual primary and secondary windings. He's right in that you don't need a lamp current limiter to test an unloaded transformer, a small current will do, you don't need to apply full voltage for phasing tests.I would add the caution that a 1/4W resistor doesn't have sufficent voltage rating for mains use, you'd need a larger body size. A capacitor would also need to be rated for peak AC voltage (preferably an X cap). Probably quicker to rig up for a quick test than a lamp holder though.

author: Ian.M
author_rank: Super Contributor
timestamp: April 18, 2017, 06:33:53 pm
content: Quote from: ady.price on April 18, 2017, 06:08:06 pm<strong> corrected to BBcode bold tags. Red hiliting added.Quote from: ady.price on April 18, 2017, 01:38:21 pmYou cant determine winding phasing on a toroid by visual inspection, unless the enamelled winding wire is brought out as the lead and you can see the direction it is wound, and compare that with the other windings. If there is a flex lead attached to the actual winding, or the winding has a taped over loop to anchor it, or there is heavy outer insulation over the windings, you cant tell.In addition to Gyro's caution regarding its voltage rating, your proposed resistor >=1Meg, wont allow more than 0.24mA to flow, which will be far less than the normal magnetising current of the transformer. The voltage on the other windings will b e very low and it will be easy to misidentify the phasing. The capacitor suggestion is somewhat better, as that will allow 7.5mA to flow, however it needs to be a class X capacitor for safety, and you have to worry about series resonance (a primary inductance near 100H would be troublesome - primary inductances that high are rare but not impossible).A mains voltage bulb is merely a conveniently packaged high power PTC thermistor. Its nearly purely resistive at the timescale of the supply frequency, tand also provides an immediate visual indication if the current through it is much higher than expected.As the UK uses bayonet base bulbs, if you don't have a lampholder handy you can just solder to the two contact pips on the base of the bulb. I dont recommend that for Edison screw based bulbs as too much live metal is exposed, the screw thread may not be easily solderable, and you don't want solder on the thread if you ever reuse the bulb in a socket.

author: ady.price
author_rank: Contributor
timestamp: April 18, 2017, 06:46:49 pm
content: thank you gyro and Ian for clarifying.Sent from my Nexus 6 using Tapatalk

author: medical-nerd
author_rank: Regular Contributor
timestamp: April 18, 2017, 11:08:38 pm
content: Many thanks to everyone for the information.It is a dual voltage transformer with the two primary and two secondary windings identified on the outside which I have brought new from MAPLIN for a power supply project.I will use the suggestions to confirm that I have the windings in phase.CheersSteve

author: rdl
author_rank: Super Contributor
timestamp: April 18, 2017, 11:35:46 pm
content: Are you sure the phasing information is not on the transformer label? Usually it will just be phase dots on the hook up diagram, and easy to overlook if you don't already know what it means.

author: james_s
author_rank: Super Contributor
timestamp: April 18, 2017, 11:40:51 pm
content: The method I usually use for working out the connections of a transformer is to use a small transformer that puts out low voltage AC and use that to feed one of the windings on the mystery transformer then measure the others. When you have the two primary windings in series the right way around you'll get a significant voltage, the wrong way around will be nearly zero. Something common like a small 6.3 or 12.6V transformer is fine and won't deliver enough current to burn anything up in the few minutes you spend testing.

author: rdl
author_rank: Super Contributor
timestamp: April 19, 2017, 12:07:54 am
content: Sorry, I was not clear. This is apparently a commercial product purchased from a reputable dealer. The connection information should be provided and not need to be "worked out". I have bought several such transformers and every one had the phasing dots on the wiring diagram section of the labeling.

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2017, 12:32:39 am
content: I've seen some that had color coded wires and required looking at the datasheet for hookup info. It depends, but it's usually not too difficult.

author: Ian.M
author_rank: Super Contributor
timestamp: April 19, 2017, 01:00:39 am
content: Of course the label and the datasheet *should* give the phasing clearly, without ambiguity, but if they don't, or you aren't 100% certain you've got it right, powering up for the first time with an incandescent bulb as a current limiter in series with how you've connected the primaries, *before* making the final connection to parallel any secondaries if that's your intention, and before connecting any secondary loads only takes a few minutes extra. Its cheap and easy insurance against getting it wrong and possibly fusing a fine wire primary, or blowing a buried thermal fuse.

author: rdl
author_rank: Super Contributor
timestamp: April 19, 2017, 01:08:22 am
content: Sure, it's a good idea to check, particularly if you don't trust the source. I'd be surprised if a reputable manufacturer didn't label their toroidal transformers with the basic information though.