Most good transformers I pull are "shorted", whats going on? - Page 1
Forum section: electronics / beginners

author: Syllith
author_rank: Regular Contributor
timestamp: November 27, 2019, 05:04:46 pm
content: Okay, dumb question time. Let's say I go to an electronics store and pick up some transformers. I can read the resistance across the primary and I'll get like 50-200 ohms. Seems normal. Hook it up, it works fine. But, I repair a lot of rectifiers and converters at work and sometimes I'll pull some transformers just to play around with them. But, literally every single one I pull, I'll check every combination I can think of, and I get less than .5 ohms on every wire. And of course, when you hook this up, it shorts out (I use a variac and slowly ramp the power up for safety). I can tell its shorting because the variac has some resistance in the knob and you can hear the coil whine get louder. So I stop before I do any damage. Do this same thing on a transformer I bought, it works fine. What's going on here? These were pulled out of known working units. Why is there no resistance and how was it working to begin with?I have 2 theories. Either these are designed to have pulses sent through them (which is evident by me checking it with the scope while its hooked up), rather than a straight sine wave, and that's preventing me from hooking it up to the variac. Second theory is something someone else suggested and they said there needs to be a load on the other end so the transformer doesn't act as the load itself. I personally don't think this is the case but I could be wrong. Transformers should be able to operate perfectly fine without a load right? Not really sure whats going on but hopefully someone can shed some light. Thanks

author: MagicSmoker
author_rank: Super Contributor
timestamp: November 27, 2019, 05:12:53 pm
content: A picture is worth a 1000 words and all that, but it sounds like you are trying to apply 60Hz AC to a switchmode power supply (aka SMPS) transformer.Mains transformer (ie - those designed for 50/60Hz power) tend to require lots of turns per volt so their primaries exhibit a substantial resistance reading; SMPS transformers, on the other hand, tend to have few turns per volt so trying to identify which winding is the primary with just a DMM is all but impossible.

author: Syllith
author_rank: Regular Contributor
timestamp: November 27, 2019, 05:36:48 pm
content: I see. So my first theory was correct. I did take one apart one time and it had very few turns. Like 2 or something. And these transformers are in fact coming out of SMPS's. So just to be clear, you said you can't identify which is the primary because of the little turns it has. Does this also mean there's no way to use these as a regular transformer because it has so little turns? I'd have to send a pulsed signal to it?

author: MagicSmoker
author_rank: Super Contributor
timestamp: November 27, 2019, 05:43:18 pm
content: Quote from: Syllith on November 27, 2019, 05:36:48 pmYeah, trying to discriminate between, say, a 10 turn winding and a 2 turn one with just a DMM is pretty much impossible because they simply don't read resistance under 1 ohm reliably. That said, you can't really reuse SMPS transformers, anyway, unless you design the rest of the converter around it... And it has a turns ratio somewhat close to what you need.

author: magic
author_rank: Super Contributor
timestamp: November 27, 2019, 06:00:48 pm
content: It's not even about pulsed vs sinewaves, it's about frequency. They are intended for very high frequencies and therefore they can get away with small size, few turns and low primary inductance.It's the same reason you can't use 60Hz transformers on 50Hz mains, but taken to another level.

author: Syllith
author_rank: Regular Contributor
timestamp: November 27, 2019, 06:36:30 pm
content: I see. I've been wanting to build a SMPS myself for a long time. It's easy enough to fix them when we have thousands at our disposal and have all the parts, but I'm really curious on the details of how they work. I asked a question on here a while back with a crappy schematic I made and someone flat out told me "Don't even attempt to build that". Looking back, it makes a lot of sense why he said that. Literally didn't even have any kind of PWM. So I actually started with this schematic (photo attached). Works pretty well actually. Not optimal, but a good place to start fiddling around. I may want to start a new thread about this but maybe you guys will know. This schematic outputs 5v given you provide 12 or so going to it. My long term plan is to make a standard 24v linear power supply to power this thing (instead of the bench supply im using now) and I want you to be able to adjust the output. So to keep things simple, I'll start with this question. How would I change my output voltage from 5v to 12v based on this schematic? Also long term goal, I don't want the output voltage to change if the input goes up or down, so I think I'll need a kind of buck-boost system going on. Any suggestions on how to integrate this into the existing schematic? I can look into the details myself, but maybe someone can point me in the right direction?Also, heres a quick little video of it working, just to get a better idea of what i'm talking about.http://box.endurehosting.com/contents/public/20191126_135616.mp4

author: Bud
author_rank: Super Contributor
timestamp: November 27, 2019, 06:40:56 pm
content: Strictly to identify windings you can use an inductance meter. Windings with more turns will have larger inductance.

author: Syllith
author_rank: Regular Contributor
timestamp: November 27, 2019, 06:43:56 pm
content: Good idea, I need to pick one of those up. I did this thing where I had to use the scope and a resistor+inductor and a little math to figure out a ballpark of what the inductance is. It worked, but it's a hassle.

author: Buriedcode
author_rank: Super Contributor
timestamp: November 27, 2019, 09:18:42 pm
content: Many smaller SMPS (<90W) are flyback converters - the transformer inside is a coupled inductor, the current doesn't flow in both windings at the same time, so energy is stored in the core (more specifically the gap). This is why many of them have gapped cores. And as others have mentioned, the switching frequency of these coupled inductors tends to be >100kHz because of their size. Or rather the frequency is chosen to be high to reduce the size/weight.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 28, 2019, 01:50:17 am
content: Quote from: magic on November 27, 2019, 06:00:48 pmWell, actually, youcanuse 60Hz transformers on 50Hz -----it's done all the time!Ideally, manufacturers would design & fit special transformers for each frequency standard, & indeed, some do, but many just have two 120v primaries to cover the 230v/120v problem, slightly over design the transformer, & call it good.SMPS transformers are a totally different kettle of fish, not only do they have less primary (& secondary) turns because of the high frequency of operation, but the core materials they use are usually not suitable for 50/60Hz.

author: perdrix
author_rank: Frequent Contributor
timestamp: November 28, 2019, 03:22:31 am
content: 60Hz transformers used on 50Hz will run hotter at the same voltage because they will pull more current.Why - the inductance is the same but the current I = V/Z will be higher as Z will be lower (Z = 2πfL).So if you need to use the same transformer for both 50Hz and 60Hz, you shouuld size the windings etc. for use at 50Hz. It should then be OK to use at 60Hz (subject to saturation).David

author: Zero999
author_rank: Super Contributor
timestamp: November 28, 2019, 10:52:08 am
content: Small 60Hz transformer work fine on 50Hz because they have a high enough resistance to limit the extra current and they're also designed for a slightly higher voltage, than the rating. Note that the power rating will be halved, since the maximum current will remain the same. Incidentally, it's fine to run a transformer at a much lower frequency, as long as the voltage is reduced proportionally, so a 240V 50Hz transformer will happily run at 25Hz, but at a maximum voltage of 120V. It's also possible to run at a higher voltage, than the rating, if the frequency is increased, but only up to a certain point, when hysteresis and eddy losses or the maximum voltage rating of the insulation become limiting factors.In this case, a switched mode power supply transformer will be designed for operation at >50kHz, so won't be able to be used for any serious amount of power at 50Hz.

author: Gyro
author_rank: Super Contributor
timestamp: November 28, 2019, 06:31:43 pm
content: I remember (or rather don't remember!) an HP or Tek scope that was prone to transformer failure due to saturation on 50Hz in Europe.As I say, I can't remember the model, but it showed that even the 'big boys' could sail a bit too close to the wind at times.