Dumb question

[GadgetBoy]

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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[ArthurDent]

Absolutely not!!

[CatalinaWOW]

Absolutely not!!

Technically the answer is yes.  What you really should have done is explain why he should absolutely not do this. 

All other things being equal the turns ratio will end up giving about 600 volts when hooked up in reverse.  BUT!  The primary side of this transformer is wound to give a reasonable current when connected to the mains, and in fact to give that current with the design reflected load from the secondary.  The secondary is not wound appropriately to give a reasonable current when connected to the mains, nor would the wire normally be sized appropriately for this use.  What you will have created is not a high voltage transformer, but a fire starter.

If you had a high impedance source (NOT THE MAINS) you could avoid the fire starter problem, but there would still be potential for dangerous problems.  The primary windings have insulation designed for the mains voltage with appropriate safety margin.  There is real doubt as to its adequacy of the insulation at five to six times its rated voltage.  The same problems would occur in the secondary, which is design for 24 volts with some safety margin.

These types of concerns are not deeply hidden dangers.  It is well worth thinking through what could go wrong when using a device in a manner different than its intended application.  Think about why the part is designed the way it is for it's intended usage.  Then think about how your usage changes those assumptions.  It isn't always wrong to use a part differently than originally designed.  Sometimes it is brilliant (look into the use of a light bulb in the original Hewlett-Packard audio oscillator.)

[Nusa]

Probably not, since while the math might look good for an IDEAL transformer, such things don't exist in the real world. The turns on one side are probably adjusted slightly to allow for efficiency losses, which would mean the theoretical 5:1 winding ratio is actually a bit higher. Which would work against you in a step-up application.

Also, in the real world, keep voltages on each winding within the specified ranges for safety reasons. Wire insulation that works fine at 120V may fail badly at 600V.

[helius]

To be specific, the windings are not sufficiently insulated to be safely used in that way. The primary and secondary must be insulated from each other, and insulation for 120V is not sufficient for 600V. The turns in each winding must also be insulated from one another, and the same problem exists there. Finally, the windings must be insulated from the core and chassis, and that may also be insufficient.

[GadgetBoy]

So, while it looks good on paper, in practice probably not a good idea. I've been trying to come up with a way of stepping up voltage for inputting into a Cockroft Walton multiplier, but I don't have a 240 or 320v transformer on hand.

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[Brumby]

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?

As said above - for an IDEAL transformer, the answer would be "Yes".  So if this is just a question you have asked to confirm your understanding - then you are thinking in the right direction.

HOWEVER - would you get this in a practical experiment?  Probably not.

Should you try this as a practical experiment?  Here's where the answers become much more clear .... NO.

There a several reasons for not attempting this.  First is that you will be putting 5 times the voltage on the 24V winding - so just think of the current that will flow and what that alone would do to the transformer.  Then we have insulation issues with windings operating at voltages that they were not designed for - and we haven't even ventured into the magnetic field issues.


As a thought experiment - on basic principles - then this is quite OK to ask ... but do not try it in practice.

[Brumby]

So, while it looks good on paper, in practice probably not a good idea.

Change the word "probably" to "definitely".

I've been trying to come up with a way of stepping up voltage for inputting into a Cockroft Walton multiplier, but I don't have a 240 or 320v transformer on hand.

Look for a transformer designed to work at the voltages involved.  It will not only pay dividends with reliability - but with safety, too.

Don't try and take short cuts when it comes to high voltages.  Be careful.  Be thoughtful.  Do it properly.  Stay safe.


Maybe try something with an automotive ignition coil - but I would feed it something significantly less than 12V.

[vk6zgo]

So, while it looks good on paper, in practice probably not a good idea. I've been trying to come up with a hway of stepping up voltage for inputting into a Cockroft Walton multiplier, but I don't have a 240 or 320v transformer on hand.

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Some old tube equipment uses a full wave rectifier with just two diodes.
This requires a secondary with a centre tap.
The beauty of this, is that, say a "200 volt per side " secondary as needed for a DC rail of around 200v, will be able to deliver 400volts ac across, the whole secondary, with the centre tap not used.

In Australia, I would say, "Look for an old AM Broadcast radio", but, in North America, post WW2,those were mainly Transformerless designs, so that limits the range of things to either, very old (1930s) radios, Tube "HiFi" amplifiers, communications receivers, test gear, etc.

I wouldn't mess with the 1930s transformers, as these had  quite horrendous secondary voltages, like 570 volts, & 770volts, corresponding to  "285 volts a side" & "385 volts a side respectively.
It is quite likely that 80 years has taken a toll on the insulation, so they may be "dead" or "not long for this
world".
As for the other stuff, it would be a sin to butcher a nice old HF radio or test instrument----I don't care about the "HiFi"!

Of course, if you could get someone in Oz to find you an early tube type TV power transformer, you would be laughing, as they commonly were 300v or 350v "a side" corresponding to 600 volts & 700 volts across the whole secondary.
As these were designed for a 240v primary, they would deliver you half those voltages at the secondary when used on.120 volts, & the insulation would be overkill for the job.

I had one, but it ended up being dumped---sorry!
Also, freight may cost too much to make it viable.

[james_s]

The winding insulation is the least of the problems, what will happen if you connect a 24V winding to 120V is the core will saturate. The output voltage will stop rising as the core reaches saturation, the waveform will get really ugly, the magnetizing current will go through the roof and the transformer will get very hot.

[GadgetBoy]

What about something like this? A Walton Cockroft ionizer needs very little power, so I'm wondering if a mains step-up transformer is a bit overkill. I just don't know if the waveform on this will work for a WC multiplier (or if waveform even matters much).

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F252419857902

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[CatalinaWOW]

The link doesn't work for me, but I don't understand your question.  If you are asking if you can use a Cockcroft-Walton multiplier (commonly called a voltage multiplier) to convert mains voltage to low current 600 volt DC then the answer is yes.  If you think you need 600 volts as the first step in generation of a much higher voltage then I recommend that you sit back and think some more.  You should understand the basic concept, insulation, spark gaps, field concentration and a number of other concepts better than you seem to before proceeding.

[ArthurDent]

The explanation by james_s in reply #9 is the main reason why reversing the transformer won't work backwards and is very dangerous. However the level of understanding about basic electronics indicates you should not try anything that involves any high voltage until you gain the necessary knowledge.

[GadgetBoy]

The explanation by james_s in reply #9 is the main reason why reversing the transformer won't work backwards and is very dangerous. However the level of understanding about basic electronics indicates you should not try anything that involves any high voltage until you gain the necessary knowledge.

Well, aren't you an arrogant twat. I'd already assumed the answer to be no, but I like confirming my conclusions on occasion.

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[GadgetBoy]

The link doesn't work for me, but I don't understand your question.  If you are asking if you can use a Cockcroft-Walton multiplier (commonly called a voltage multiplier) to convert mains voltage to low current 600 volt DC then the answer is yes.  If you think you need 600 volts as the first step in generation of a much higher voltage then I recommend that you sit back and think some more.  You should understand the basic concept, insulation, spark gaps, field concentration and a number of other concepts better than you seem to before proceeding.

Well, I live in Canada. North American voltage isn't high enough to make a WC ionizer work, even with an abundance of stages. I've been trying to come up with an economical way of doubling or more the mains voltage before putting it into the multiplier.

I've attached a picture of the eBay thing I tried to link earlier.

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[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]

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]

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.”

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]

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

[GadgetBoy]

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

Was hoping the output would work on a WC ionizer.

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[Macbeth]

Back to back transformers, it's actually a good ol' boy way of isolating the mains without expensive isolation transformers.

[Macbeth]

So, while it looks good on paper, in practice probably not a good idea. I've been trying to come up with a way of stepping up voltage for inputting into a Cockroft Walton multiplier, but I don't have a 240 or 320v transformer on hand.

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Ever heard of MOT's? (Microwave Oven Transformers) - freely available providing your local council junk yard inspector is not being an arsehole.

[GadgetBoy]

So, while it looks good on paper, in practice probably not a good idea. I've been trying to come up with a way of stepping up voltage for inputting into a Cockroft Walton multiplier, but I don't have a 240 or 320v transformer on hand.

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Ever heard of MOT's? (Microwave Oven Transformers) - freely available providing your local council junk yard inspector is not being an arsehole.

Any time I hear of someone playing with a microwave transformer, I swear I can hear Darwin cackling with glee.

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[james_s]

Any time I hear of someone playing with a microwave transformer, I swear I can hear Darwin cackling with glee.

I've been playing with them for many years, they are not any more dangerous than any number of other things people play with. Flammable materials, firearms, cars, power tools, etc. It's simply a matter of understanding the risks and using a bit of common sense. The primary side DC bus in just about any SMPS can kill you just as dead if you aren't careful.