Electronics > Beginners
Dumb question
TimFox:
Remember that the voltage or emf induced in a transformer winding is the time derivative of the flux linked in the winding by the core. At a fixed frequency, the emf is (2 pi)x(f)x(peak flux). The linked flux is the cross-section area multiplied by the (flux density = B field), multiplied by the number of turns. For typical iron alloys, the B value at core saturation is about 2 Tesla. That limits the AC voltage on the winding: if the transformer is designed to handle 120 V on a primary winding (5 x N turns), it cannot handle 120 V on a winding of only N turns.
I wish it were easier to enter equations here.
jmelson:
--- Quote from: GadgetBoy on September 25, 2018, 01:53:11 am ---If I take a transformer that takes 120v in and puts 24v out and put the mains on the secondary instead, will it output 600v on the primary?
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No. Transformers are made with minimal iron in them. This means that the volts/turn of any winding has an upper limit. So, the transformer you describe has 5 times as many turns on the primary as on the secondary. And, the iron was sized so that it has, at best, a 10% margin before the iron saturates. Transformers have a V*T rating, that's Volts * time. If you raise the frequency by a factor of 5, then the time is 1/5th as much per cycle, and you can get roughly 5 X more voltage before saturations starts. Core losses will increase, however. But, at the same mains frequency, you can't get more voltage.
Jon
ArthurDent:
GadgetBoy-“Well, aren't you an arrogant twat. I'd already assumed the answer to be no, but I like confirming my conclusions on occasion.”
Well I beg to differ with you on your well thought out logical retort. What I posted wasn’t a snap judgement. I have seen other threads you’ve started and there have been quite a few posters who have given you advice similar to mine. I apologize if you misinterpreted my suggesting that you should refrain from doing something that might kill you. Read back over your previous posts and you will see a pattern. One of the posts suggesting your knowledge is limited is your own. I stand behind what I posted and strongly suggest you stay away from any line or higher voltages until you really understand more.
“Should you try this as a practical experiment? Here's where the answers become much more clear .... NO.”
GadgetBoy-“If I put multiple schottky diodes in parallel, will their voltage rating add (i.e. put three 40v schottky diodes in parallel for 120v rating)?”
GadgetBoy-“Would the capacitive dropper keep the mains side of the transformer from melting itself?”
GadgetBoy-“To my knowledge (which is admittedly limited), this *should* work, but it isn't.”
GadgetBoy:
--- Quote from: ArthurDent on September 25, 2018, 06:59:22 pm ---GadgetBoy-“Well, aren't you an arrogant twat. I'd already assumed the answer to be no, but I like confirming my conclusions on occasion.”
Well I beg to differ with you on your well thought out logical retort. What I posted wasn’t a snap judgement. I have seen other threads you’ve started and there have been quite a few posters who have given you advice similar to mine. I apologize if you misinterpreted my suggesting that you should refrain from doing something that might kill you. Read back over your previous posts and you will see a pattern. One of the posts suggesting your knowledge is limited is your own. I stand behind what I posted and strongly suggest you stay away from any line or higher voltages until you really understand more.
“Should you try this as a practical experiment? Here's where the answers become much more clear .... NO.”
GadgetBoy-“If I put multiple schottky diodes in parallel, will their voltage rating add (i.e. put three 40v schottky diodes in parallel for 120v rating)?”
GadgetBoy-“Would the capacitive dropper keep the mains side of the transformer from melting itself?”
GadgetBoy-“To my knowledge (which is admittedly limited), this *should* work, but it isn't.”
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I called you an arrogant twat, and stand by the assertion, because you've made a huge leap of logic in order to assume my level of knowledge based on a few posts.
Most of my expertise is focused on CMOS logic, PLC programming, and industrial motor control. I know how to be safe around high voltage (a necessary skill when working with three phase power), it's just that in my classes, transformer saturation wasn't something that was discussed - critical gap in my knowledge for sure on THIS particular subject, but it doesn't indicate that I'm incompetent or unsafe.
I also know that most enameled wire has an insulation rating of at least 600v (on paper), so I wasn't tremendously concerned about arcing. I have custom power cables with built in fuses that will break a circuit long before the breakers in the house would even start to warm up. I typically have a 250mA fuse in them unless I anticipate pulling more power than that (which is rare given my current area of experimentation).
My work bench has rubber pads under the legs, and so does my wood stool. I have esd rated work boots, and if I'm feeling particularly nervous about something I'm working on, I also have insulated gloves rated for 1kV.
Electronics is a HUGE field, and it is the absolute height of arrogance for you to assume that I'm incompetent or dumb because I display a lack of knowledge on a few areas. Even further, you made an assumption on my level of knowledge based on my previous posts - of course I will come here to ask questions about something I'm unsure about - why would I ask questions to something I already know the answer to?
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HB9EVI:
--- Quote from: GadgetBoy on September 25, 2018, 06:02:06 pm ---
I've attached a picture of the eBay thing I tried to link earlier.
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This inverter is working quite well, but, like you maybe can conclude from the transformer size, its operation frequency is ~42kHz. That's no problem, if you want to feed for example a mains LED lamp or any other device, where the AC is rectified again to a DC, but you cannot use it on any device with a 50/60Hz mains transformer
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