A few questions about two transformers that I have. - Page 1
Forum section: electronics / beginners

author: Mint.
author_rank: Frequent Contributor
timestamp: June 17, 2012, 09:17:12 am
content: So I have a few transformers that I have salvaged and today I decided to play around with them, but I really couldn't because I don't know much about them. In the attached images below I have an image of two transformers, one is a standard transformer nothing special, however the second one is really versatile on the voltages it accepts on the inputs. My questions are as follows:How can I find out the primary of each of the transformers?For the second one, how can I find out which voltage is selected?Is there any danger in connecting the secondary to mains, e.g. the other way around on how its supposed to be?Any transformer safety measures to be taken?Is it safe to measure the output with my scope?- Thanks in advance!

author: rr100
author_rank: Frequent Contributor
timestamp: June 17, 2012, 10:43:18 am
content: If you can even for a second consider connecting the secondary to mains I strongly advice to just stop playing with mains, completely.Now if you really want to continue start just with a known (isolated, "safe", maybe fused) transformer and use it as AC low-voltage (5-20V) power supply.Feed this AC to the primary of your transformer and see what you get.

author: SeanB
author_rank: Super Contributor
timestamp: June 17, 2012, 11:08:19 am
content: With the one on the left whichever winding has the thinnest wire is the primary. It will also be the one with a thermal fuse in series with the winding, probably well hidden under the tape. I would hazard that the brown wires are the primary, as the wire on the winding with the white is a little too thick for winding the thousands of turns needed to keep that small core from saturation.The other one has basically a primary winding tapped at 100VAC, 120VAC, 220VAC and 240VAC, with the switch selecting which winding tap will be connected to the white wire, the black being the common. Take a multimeter and check the DC resistance of the windings as the switch is rotated to each of the 4 settings, the highest resistance is the 240VAC selection.To check the output voltage simply use a plug in wall wart that gives 9VAC ( old modem power supply will do) and apply to the one secondary winding, and measure the primary carefully with a multimeter set to the highest AC voltage range, and be careful not to contact the bare leads, use a clip to attach the lead to the wire and insulate it. Measure the voltage on both input and output and then you will be able to work out the voltage ration and thus the design voltage rating.If you are going to connect them direct to the mains use an in line 1A HRC fuse in case you get it wrong.

author: T4P
author_rank: Super Contributor
timestamp: June 17, 2012, 11:10:12 am
content: Use a small isolated transformer ( AC Plug pack , the ones that gives a low voltage AC )All transformers are versatile on the inputs, except that they give less voltageI do notice a black thing on the second tranny which i assume is a bridge rectifier

author: IanB
author_rank: Super Contributor
timestamp: June 17, 2012, 03:35:27 pm
content: Quote from: Mint. on June 17, 2012, 09:17:12 amIn addition to what others have said, the answer is definitely yes!Several bad things may happen:1. The winding may draw too much current and blow the mains fuse/trip the breaker2. The winding may draw too much current and overheat rapidly, causing damage3. The unconnected primary will generate far too much voltage and the insulation will break down, causing arc faults and permanent internal damageWhen you connect unknown things to the mains, a variac is a good idea so you can wind up the voltage slowly. If you don't have a variac, put a bulb (not a CFL) temporarily in series with the mains supply to limit any fault currents.

author: T4P
author_rank: Super Contributor
timestamp: June 17, 2012, 05:51:19 pm
content: Or you can build this and power the circuit with a current limited (1A) lab DC supply and compare how much voltage you are getting on the outputHere's some common weights and ratings15 VA for 500gm, 30VA for 1kgm 50VA for 2kgm and and 100VA for 2.5kgm

author: mariush
author_rank: Super Contributor
timestamp: June 18, 2012, 12:08:31 pm
content: Or you can just get any DC adapter, and desolder the diodes inside which rectify the voltage and remove the capacitor (unless you want it to pop inside)There should be four diodes inside and a capacitor:You have two traces on the pcb coming from the transformer, the ones on the picture with ~ sign is the AC. Remove the diodes and solder one of the ~ wires to the + and the other ~ to the - and you got your self a AC adapter.Remove the capacitor if there's any because it won't like AC signal at all.Now you can use this 3-12v AC to text your transformers. Be careful because depending on how you connect that transformer, you'll have either much smaller voltage coming out, or quite a lot more voltage, so test it with the multimeter on 750v AC and be careful how you touch the wires with the meter probes.

author: rr100
author_rank: Frequent Contributor
timestamp: June 18, 2012, 12:26:58 pm
content: I have to say I cringed a little when I saw the oscillator above , everything just to get some AC for test purpose...

author: SeanB
author_rank: Super Contributor
timestamp: June 18, 2012, 03:54:47 pm
content: I did a similar design to provide a quick and cheap 26VAC 400Hz power supply, using a 555 driving the transformer directly from pin3. Worked well for many years, just had to decouple the 12V input rail with a 4.7uH inductor and 2 100uF 25V solid slug wet tantalums. Nothing got too hot after I placed a small heatsink on the TO100 can 555. I used this to run one of the hour meters i had, it whined a little, but worked well on 24VAC, after adding around 0.22uF capacitor across the secondary of the transformer to make it look vaguely like a sine wave.

author: Mint.
author_rank: Frequent Contributor
timestamp: June 18, 2012, 10:50:21 pm
content: Hey fellasThank you everybody who commented, much appreciatedI've got a 9VAC plug pack and I can use that to test my transformersAlso I still don't understand if I can measure the output with my Rigol DS1052E oscilloscope?

author: vk6zgo
author_rank: Super Contributor
timestamp: June 19, 2012, 05:15:03 am
content: Quote from: Mint. on June 18, 2012, 10:50:21 pmYes,you can,as you are isolated by the plug pack--just keep the poor old Rigol away the mains!One thing to be aware of,is that if you put 9V AC across the 9Vsecondary,the primary will have 240V across it--transformers work both ways around!!EEEK!If you put 9V AC across a 6V secondary,you will have 360v on the primary!Double EEEK!Because this can happen,do not touch ANY of the windings when the thing is active!--let your Rigol do the touching!(It probably wouldn't kill you,but it would certainly make you feel a bit sorry for yourself!)

author: Mint.
author_rank: Frequent Contributor
timestamp: June 19, 2012, 05:54:28 am
content: Quote from: vk6zgo on June 19, 2012, 05:15:03 amI remember that one of the transformers (the first one in the image) gives out 30VAC at one end, so I don't think it will be too much of a problem just for that bit, however second one I am not sure. Thanks for the warning, I never realised that the voltage can become so high when I test it with just 9VAC.

author: T4P
author_rank: Super Contributor
timestamp: June 19, 2012, 06:34:34 am
content: The first method is to always check the resistancesThe finer wounded primary (usually for stepdown's) has higher resistance then the thicker wounded secondary or else it's shot ( I would call it blown/shorted turns )For stepup's its the other way round