So you are saying the 115v windings have such low resistance that they will cause too much current drawn when used on 230v, and therefore overheat even without load. I am not so certain of this, always thought of transformers as work mules, as long as you calculate and make sure load and supply effect with special consideration to current compared to wire thickness, the voltage can vary greatly. The limit is insulation type and thickness
I'm tempted to try, just don't know where I can find a possible sacrifice. Do not have any 115v winding trafo's here. Certainly not going to buy a new one for breaking.
I still believe the transformer would break the circuit before it broke itself.
the 115Vac winding is already saturating as 127Vac is approached. The equivalent values in the UK are 230Vac & 253Vac.
Measure the primary current of your 115Vac transformer with a variable primary voltage. Start at 0Vac and step in ~10Vac increments until you get to ~100Vac, then try using 5Vac increments (or smaller) to 130Vac.
Plot the Ip vs Vp and see the saturation becoming a problem.
Now imagine what would happen if Vp were 200Vac or 220Vac or 240Vac and finally 253Vac.
The primary insulation will almost instantly be damaged beyond saving.
This is where the Mains Bulb Tester is our saviour for wiring errors.
Connecting a dual primary with one winding phase inverted will present only the primary resistance to oppose the mains voltage.
Measure the primary current of your 115Vac transformer with a variable primary voltage. Start at 0Vac and step in ~10Vac increments until you get to ~100Vac, then try using 5Vac increments (or smaller) to 130Vac.
Plot the Ip vs Vp and see the saturation becoming a problem.
Now imagine what would happen if Vp were 200Vac or 220Vac or 240Vac and finally 253Vac.
The primary insulation will almost instantly be damaged beyond saving.
This is where the Mains Bulb Tester is our saviour for wiring errors.
Connecting a dual primary with one winding phase inverted will present only the primary resistance to oppose the mains voltage.
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The issue is that a 115V primary given 230V will saturate the core, so inductance will drop severely and current flow will be limited mainly by winding resistance. This could be a few hundred ohms or some tens of ohms. Many amps will flow so perhaps kW will be dissipated in the winding. If you are really lucky a fuse or breaker will trip. If you are really unlucky your house may burn down. Are you lucky?
Learn about core saturation. Never, ever, apply 230V to a 115V winding. Always use a lamp limiter.
Learn about core saturation. Never, ever, apply 230V to a 115V winding. Always use a lamp limiter.
Isn't core saturation mainly decided by current?
As I stated before, you'd have to be careful with current, the dissipated total power would possibly double because of the voltage increase, and the current should roughly be restricted to 1/3rd, but I am certain it would work. Voltage is decided by windings, current is decided by wire and core.
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I never mentioned using both primary windings!
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Yes, and current is decided by voltage. Most mains transformer can't cope with an increase in primary voltage of more than about 10-20%, so doubling it is asking for trouble. Do some reading about transformers before you use them.KaffiMann said:
I don't understand what you are saying here. I suspect you don't either! Have you heard of inductance?
I can see that you are more interested in absolute rules, and, at least in the context of this thread, that you have little interest in the general functionality of electronic components. I completely understand what you are saying and I respect your opinion.
However, all I said was: "Possibly not the transformer (it's just a block of copper wire, over amping would cook it and melt the insulation then it short circuits, over volting MIGHT work fine, pending on insulation used), but certainly the amplifier or the psu board (caps exploding, circuits frying)."
I merely argued that there might be a possibillity the transformer itself survived, but the voltage increase on the secondaries would cause components to break, and maybe as a further result of components breaking, the circuits themselves. The amp load may or may not be high enough to destroy the transformer, that would depend on several factors, such as transformer size, wire thickness, number of windings, what was destroyed in the circuit, if anything in the circuit shorted as a result of components breaking and so on.
All I'm saying is, the transformer does not know anything, it is not intelligent. If you make a primary winding of, say 50 turns, and a secondary winding of 25 turns you will, if you did everything else right, get near half voltage on the secondary. It is very simple. You will have to calculate core/wire thickness/length/windings to accommodate the desired voltage and load, but that is it. It's the same with speakers, some are 4 ohm some are 8 ohm, you have some 16 ohm and 2 ohm stuff going about as well, it's just the same as any transformer. wire thickness and lenght = resistance, core saturation and thermal power compression are not complete opposites. It's like you are saying a specific amplifier made to work optimally with 8 ohm speakers will not work at all with 2 ohm speakers. That is not 100% correct, it will work to a given point, but it will have much stricter limits to adhere to in order to avoid overload.
However, all I said was: "Possibly not the transformer (it's just a block of copper wire, over amping would cook it and melt the insulation then it short circuits, over volting MIGHT work fine, pending on insulation used), but certainly the amplifier or the psu board (caps exploding, circuits frying)."
I merely argued that there might be a possibillity the transformer itself survived, but the voltage increase on the secondaries would cause components to break, and maybe as a further result of components breaking, the circuits themselves. The amp load may or may not be high enough to destroy the transformer, that would depend on several factors, such as transformer size, wire thickness, number of windings, what was destroyed in the circuit, if anything in the circuit shorted as a result of components breaking and so on.
All I'm saying is, the transformer does not know anything, it is not intelligent. If you make a primary winding of, say 50 turns, and a secondary winding of 25 turns you will, if you did everything else right, get near half voltage on the secondary. It is very simple. You will have to calculate core/wire thickness/length/windings to accommodate the desired voltage and load, but that is it. It's the same with speakers, some are 4 ohm some are 8 ohm, you have some 16 ohm and 2 ohm stuff going about as well, it's just the same as any transformer. wire thickness and lenght = resistance, core saturation and thermal power compression are not complete opposites. It's like you are saying a specific amplifier made to work optimally with 8 ohm speakers will not work at all with 2 ohm speakers. That is not 100% correct, it will work to a given point, but it will have much stricter limits to adhere to in order to avoid overload.
Do you understand? I suspect not. Unfortunately for the 'free thinkers' around here, transformers always obey the "absolute rules" of physics. These "rules" dictate that applying too high a voltage to a power transformer primary will result in very high primary current due to core saturation. You seem to doubt this, thus indicating that you are ignorant of the "rules".
Now of course, the other stuff connected to the secondaries might not survive either. A transformer is not intelligent; it merely follows physics, and hopes its user appreciates this.
This is just one of these things I will have to test for myself, and I will. But I am moving house these days and do not have the opportunity to "play around" so much right now. I've wanted a good variac since the days of going to school and heating hot-dogs in series (in the buns ofcourse, so they got hot too), and this seems like a good excuse to get one.
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I do actually understand what you mean, no need to puff air and smoke. Like I said, I should test this myself.
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KaffiMan, it seems to me that your comments and replies show you do not understand the problem and certainly don't recognise the consequences of connecting 110/120Vac Primary/ies to a 220/240Vac supply.
Just test a universal 230Vac transformer (with the secondaries open circuit and insulated for safety) using a Variac from 0Vac to 253Vac to see the primary current problem due to the onset of core saturation.
I suggested this (for 0Vac to 127Vac) back in post22 and you seem to have missed the significance.
Just test a universal 230Vac transformer (with the secondaries open circuit and insulated for safety) using a Variac from 0Vac to 253Vac to see the primary current problem due to the onset of core saturation.
I suggested this (for 0Vac to 127Vac) back in post22 and you seem to have missed the significance.
It's likely that it's your transformer that's going to do the puffing and smoking. And before you test it, I suggest you read paragraph 12.1 of this article in which a transformer is subjected to under- and overvoltages.
Spoiler: at just 50 V overvoltage, a 240 V (nominal) transformer already cooks itself, and that is without any load connected to it.
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Yes you did! And like I've mentioned twice already: this is something I am going to try myself! It's just that I am moving house right now, and I do not have a Variac, 115v primary transformer to sacrifice or time to play around with those things at the moment, but it's very high on the to-do list.
I've made transformers proffessionaly before, admittedly this was just a temporary job while preparing to move on a previous occasion, but I've made many transformers, ranging from 200kVA up to 4000kVA, and I got very good feedback on the stuff I did. Contrary to your belief I have some, albeit little knowledge of how transformers work.
I really do understand what you and DF96 are saying is correct, all I said was: the transformer MIGHT survive, read the words in the post!
The transformer will be operating well out of it's limit's ofcourse, but if all else is correct: you have a properly dimensioned fuse for the application already connected prefferably in the mains socket, the transformer is connected to the psu board and the psu board is connected to some adjustable heat dump load. When you switch on the power, the current will be higher than anticipated, which will break first? The fuse, the circuit or the transformer? And if the circuit breaks first, what will break next?
This is what I would like to try, but as already mentioned: it will be at the very top of my to do list, when I get enough time and the equipment is available.
/
Yes it will! But will it fail before anything else? It is my assumption that the results will vary greatly pending on all parts involved.
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I completely accept that all you guys just think I'm silly for not just plain 'ol understanding "simple facts". But i've just never tried this before, and I really want to.
Yes it will! But will it fail before anything else? It is my assumption that the results will vary greatly pending on all parts involved.
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I completely accept that all you guys just think I'm silly for not just plain 'ol understanding "simple facts". But i've just never tried this before, and I really want to.
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And that would be a correct assumption, the weakest link will go first as I found out first hand.
When I was finding out about electricity, a long time ago, I hooked up two tiny transformers on the secondaries to see if a low voltage would be transformed back up again. It did, my multimeter told me so.
Then I hooked up a 220 V bulb to see if it would light... It did! But after a few seconds one of the transformers blew with a puff of smoke coming from inside... A 60 W bulb was a bit too much for a < 1 VA transformer... oops.
No better teacher than hands on experience, right? I won't encourage you to go on with your experiment (I agree with the others here), but should you choose to do so, please take the neccessary precautions to minimize shock and fire hazard.
Sorry for resurrecting this old thread but I learned a lot from it.
I have been considering buying a couple of the many 480v primary 120v secondary (~1kva) transformers available on ebay for dirt cheap and putting 120V across the primary which would give me about 30v from the secondary. From the article referenced in this thread it appears that putting 120v across the primary designed for 480v most likely won't sufficiently magnetize the core and the resulting 30v from the secondary is going to be a very rough wave instead of the smooth sine wave I want for a quiet amp?
Do I understand this correctly?
If I buy transformers with 120v primaries and 40v secondaries then limit the primary voltage with a variac (To lets say 90v or 60v input) am I going to run into the same problem with a noisy output voltage from the secondary coil?
It sounds like if I want a good variable power supply for experimenting with various Class A amplifier designs I should just buy a couple of HP lab variable power supplies? From what I have read many HP lab power supplies have trace function which allows one power supply to automatically follow the voltage of the other ps?
If I am looking at used HP lab PS's on ebay I was thinking of a pair in the range of about 0-60v and 5a? My current bench PS is only 0-30v @ 1a x 2 outputs. Should I just try starting with this bench PS?
I am interested in playing with some of the KSA 50 class A boards which require dual +/- voltages. The KSA 50 appears to require the higher voltage for control side of the amplifier and lower voltage for the power transistors which is where I would want higher current capabilities? So I almost need 4 lab power supplies to play with the KSA clone amps?
I assume a high current capable (5a) pair of lab power supplies would be sufficient for 2 channels of Class A amp? I.e run left and right channels from the same pair of lab PS's?
Thanks!
I have been considering buying a couple of the many 480v primary 120v secondary (~1kva) transformers available on ebay for dirt cheap and putting 120V across the primary which would give me about 30v from the secondary. From the article referenced in this thread it appears that putting 120v across the primary designed for 480v most likely won't sufficiently magnetize the core and the resulting 30v from the secondary is going to be a very rough wave instead of the smooth sine wave I want for a quiet amp?
Do I understand this correctly?
If I buy transformers with 120v primaries and 40v secondaries then limit the primary voltage with a variac (To lets say 90v or 60v input) am I going to run into the same problem with a noisy output voltage from the secondary coil?
It sounds like if I want a good variable power supply for experimenting with various Class A amplifier designs I should just buy a couple of HP lab variable power supplies? From what I have read many HP lab power supplies have trace function which allows one power supply to automatically follow the voltage of the other ps?
If I am looking at used HP lab PS's on ebay I was thinking of a pair in the range of about 0-60v and 5a? My current bench PS is only 0-30v @ 1a x 2 outputs. Should I just try starting with this bench PS?
I am interested in playing with some of the KSA 50 class A boards which require dual +/- voltages. The KSA 50 appears to require the higher voltage for control side of the amplifier and lower voltage for the power transistors which is where I would want higher current capabilities? So I almost need 4 lab power supplies to play with the KSA clone amps?
I assume a high current capable (5a) pair of lab power supplies would be sufficient for 2 channels of Class A amp? I.e run left and right channels from the same pair of lab PS's?
Thanks!
your 1kVA 480:120Vac transformer has maximum current values of ~2.1Aac for the Primary and ~8.3Aac for the secondary.
These do not change if you change the supply voltage.
Your 1kVA effectively becomes a 250VA transformer when you change the supply from 480Vac to 120Vac.
Otherwise it operates as normally as any mains transformer.
The output AC waveform will still be a close replica of the AC waveform that you input.
The waveforms through the rectifier will still be the same.
The waveforms across the smoothing capacitors will still be the same.
These do not change if you change the supply voltage.
Your 1kVA effectively becomes a 250VA transformer when you change the supply from 480Vac to 120Vac.
Otherwise it operates as normally as any mains transformer.
The output AC waveform will still be a close replica of the AC waveform that you input.
The waveforms through the rectifier will still be the same.
The waveforms across the smoothing capacitors will still be the same.
Thank you for the information Andrew! That makes sense about transformers although I had not thought of it in those terms. I equate it to halving the voltage in house wiring halves the amount of power the wire will carry. I.e. 5hp over 12 gauge @ 220v = no problem. 5hp over 12 gauge @110v = over heating the wire insulation.
Decided? Its an unsuitable antropomorph term here. Saturation is not a mental process. The flux is accumulating (integrating) and causes magnetisation current. When flux density reach saturation the magnetisation current starts to increase very fast, and shorts the primary loop, until only DCR and leakage limits the current, which will be tipically 10-20 times higher than the nominal. The transition is very fast, about 10...200 us.
And this is typically true. Even though the current is very high, power loss is extremely high (100...400 times the nominal) the thermal mass is big enough to withstand the heat for some seconds, which is more than enough to blow a right sized fuse.
Still better to learn physics first...
Andrew is right, overvoltage is a very severe condition. The basic of the operation of transformers is flux changing. Since we almost always work with forced voltage, this is the very first rule. u(t)=dphi/dt. In other form: phi(t)=integral(u(t))dt. This is a linear equation, but phi is limited by saturation, so at a given waveform and frequency u(t) is also limited. Everything else comes after you ensured avoiding saturation.
In practice amplitude of operational flux is choosed very close to the possible maximum. As close as can be with maintaining the headroom for the specified variability of line voltage, so about 0.8...0.9 times of the saturation flux.
The key is "if you did everything else right". But if you double the voltage, then this is a very strong violation of "everything else". To be exact the voltage will really be double for the first quarter period, and then suddenly saturation will decrease it to 0 for the 2nd quarter, and will release it to the nominal voltage for the 3rd quarter, and short it for the 4th, etc... So basically your quess is right, it can kill the supplied circuit very easily.
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