EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: robsims on May 25, 2024, 02:36:24 pm
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Hi, i'm a beginner.
Mains voltage is 127 volts. I have a transformer with primary side input of 127 volts and secondary side output of 12 volts. When i connected the line side directly to mains, the transformer worked perfect. Yesterday I connected a bridge rectifier (TS10K60S) AC input directly to mains and i connected the DC output of the bridge rectifier to the primary AC line input of the transformer. After 5 seconds the primary side of the transformer felt hot, it smoked and was damaged. The DC voltage i measure at the output of the bridge rectifier is 116 volts. Can someone explain what went wrong. Here is a link of the datasheet of the bridge rectifier https://pdf1.alldatasheet.com/datasheet-pdf/view/561671/TSC/TS10K60.html Many thanks
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Why in the hell did you apply D.C. to the transformer????? Of course it F--king burned up!!!
Transformers run on A.C.!!!!! ONLY!!!!
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In my opinion i don't think so CaptDon. Transformers do not work on AC only. A transformer works because of a change in magnetic field. A change in magnetic field does not automatically mean a AC signal only. It can also be a chopped up DC signal. Switched mode powersupplies with small transformers work with chopped up DC voltages. So my question is did my DMM measured the pulsating DC voltage correctly at the output of the Bridge rectifier. Maybe the pulsating DC voltage is higher than the 116 volts the DMM measured?
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transformer core saturation
transformer core reset
transformer flux walking
some terms for you to research
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Too funny.
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CaptDon: This is the beginners section. If you can't avoid cursing at people and making fun of them, please don't even come here.
To the OP: Transformers only work on AC. Ignoring the other windings and the load, a transformer primary is basically a giant indicator. Inductors resist a change in current. If you apply a DC voltage the current will steadily rise until it is limited only by the small resistance of the wires. With AC voltage, the voltage keeps reversing polarity before the voltage can build up to damaging levels.
On the secondary side, it takes a change in magnetic field to generate a voltage. A steady magnetic field won't do anything. So again using an AC drive gives a magnetic field that keeps changing and inducing voltages in the secondary.
You put the rectifier on the secondary side to get a DC voltage for your end use.
Working with AC mains voltage is not difficult but it can be dangerous. I strongly recommend you don't do anything touching AC wiring yet. You can get pre made power supplies that will suffice for basically any electronics. If you want to control AC powered appliances you can get pre packaged relays that have AC in and out and take digital control inputs and have the necessary isolation.
Once you have some basic experience, doing simple mains working such as connecting a transformer or power supply is easy but needs to be done correctly.
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: This is the beginners section.
To the OP:
shouldn't we be trying to help beginners,instead of possible causing them confusion further down the line,the carte blanche statement that Transformers only work on AC is wrong,as stated above
A transformer works because of a change in magnetic field
a 6v battery and dc motor wired in series with the secondry of a bell transformer will proof this,and make you jump when you grab the primary output
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shouldn't we be trying to help beginners,instead of possible causing them confusion further down the line,the carte blanche statement that Transformers only work on AC is wrong,as stated above
In the beginners section, it is not wrong. In the beginners section transformers work only on AC. This means pure AC, not rectified AC, not pulsating DC. It means AC, of the expected frequency!
In the first post, the person connected rectified mains DC to the primary of a transformer that was designed for an AC mains supply, and this means the transformer naturally passed more current than it was supposed to, and burned up.
In my opinion i don't think so CaptDon. Transformers do not work on AC only. A transformer works because of a change in magnetic field. A change in magnetic field does not automatically mean a AC signal only. It can also be a chopped up DC signal.
It is not sensible to contradict knowledgeable people who are giving you answers. Ask more questions by all means, but don't tell your teachers they are wrong. You said, "In my opinion". But the facts are that you destroyed a transformer by connecting it to a DC supply instead of the expected AC supply. Facts are facts. Facts beat opinion here. There is no point in holding an opinion contrary to the facts.
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Full Wave Rectified A.C. does have an 'average D.C. value' which is the D.C. value that burned your transformer. For what possible reason did the O.P. put a bridge rectifier on the mains feeding the primary of the transformer???? That was senseless! Full Wave Rectified D.C. IS NOT CHOPPED D.C.! In my wildest dreams and in real life I have never seen anyone put 'Rectified A.C. directly onto a transformer unless it was also connected to a push pull driver circuit or a PWM circuit either of which could be the topology of a SMPS. You don't exactly apply 'chopped D.C.' to a transformer either but more generally you would apply D.C. to one side of the transformer or possibly the center tap and then 'switch', 'chop', or 'pulse' the other end or ends. I am even more baffled by a clueless newbie amateur monkeying with MAINS!!!!! Would have been real sweet if that bridge rectifier had burned internally instead of the transformer and had burst in an exploding ball of epoxy pieces flying around. I am clueless, so I hooked up a bunch of stuff into a circuit I don't understand and plugged it into the wall outlet! I keep the Emergency Responders number on speed dial for all of my experiments. Texting you from the local burn ward, or something like that! READ and LEARN FIRST, especially if you are crazy enough to plug your experiment into the mains without regard for fusing, isolation or GFCI protection!! Go, man, go! Knock yourself right on out.
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So i've read some articles about transformer core saturation and core reset as suggested to me by oPossum. What happened was that the transformer core saturated by feeding the primary side a pulsating DC (rectified mains) voltage. Therefore the transformer primary windings let an almost constant high current through which burnt up the transformer. So IanB was right. Thermadippy and i are right too. The theory is that a change of magnetic flux will cause a current flow in a coil. It does not necessarily means that the direction of the flux has to change. Examples are pulse transformers which work with DC pulses. They have slightly different characteristics than normal transformers
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Hi, i'm a beginner.
In my opinion i don't think so CaptDon.
Your opinion doesn't count. See above. :palm:
You did something really, really dumb. What were you trying to do and where did you get the idea you could do what you did? :-//
You are lucky not to be uninjured. :scared:
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CaptDon you are right. It would be way better if i had lost the rectifier. Sadly my beloved transformer is gone. Yes but now i know. The rectified voltage was too high
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The rectified voltage was too high
NO!
There was a DC component that the transformer was never designed for. I suggest you do some study before you plug things into the wall!
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BillyO, what i was trying to do originally was connecting the bridge rectifier to the primary side of my welding transformer to get DC on the secondary side. The circuit breaker kept tripping so i decided to connect the bridge rectifier to a smaller transformer to see what would happen. So the circuit breaker didn't trip this time but my small transformer sadly went up in smoke
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You connect a bridge rectifier of sufficient size to the secondary of the transformer, not the primary.
(https://i0.wp.com/dcaclab.com/blog/wp-content/uploads/2019/08/transformer.jpg?w=772&ssl=1)
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BillyO, what i was trying to do originally was connecting the bridge rectifier to the primary side of my welding transformer to get DC on the secondary side. The circuit breaker kept tripping so i decided to connect the bridge rectifier to a smaller transformer to see what would happen. So the circuit breaker didn't trip this time but my small transformer sadly went up in smoke
And you didn't have rectifiers that could handle the welding currents so you thought, why not rectify the input to the transformer with lower current rectifiers?
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But okay, i learned from my mistake. I want DC output from my transformer welder. Can anyone of you suggest the components. I know i need a rectifier, capacitors, but what are the ratings i have to look for. Many thanks
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BillyO, what i was trying to do originally was connecting the bridge rectifier to the primary side of my welding transformer to get DC on the secondary side. The circuit breaker kept tripping so i decided to connect the bridge rectifier to a smaller transformer to see what would happen. So the circuit breaker didn't trip this time but my small transformer sadly went up in smoke
And you didn't have rectifiers that could handle the welding currents so you thought, why not rectify the input to the transformer with lower current rectifiers?
Right
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You need to provide more details.
Before getting into that, why do you want to try and rectify the welder's output? Are you wanting to use it for something other than welding?
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You need to provide more details.
Before getting into that, why do you want to try and rectify the welder's output? Are you wanting to use it for something other than welding?
I want to use it for welding. DC current gives a better weld than AC
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Any idea what the transformer's output open circuit volt and short circuit current is?
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It may be marked on the welder or transformer.
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Open circuit voltage is 42V and short circuit current is 80 amps
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Any idea what the transformer's output open circuit volt and short circuit current is?
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It may be marked on the welder or transformer.
Open circuit voltage is 42V and short circuit current is 80 amps
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The direction of the applied voltage DOES need to change so that volt-seconds do not accumulate in only one direction where the core will saturate and the primary magnetizing inductance goes to around zero.
Placing the rectifier on the secondary instead of the primary is the proper way.
I kind of doubt it will weld very good though. Lettuce know if you do get that to work.
boB
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For welding one usually does not need perfect DC. So one may get away without a filter capacitor and use a current smoothing inductor instead.
At high power an inductor can be more attractive, as is also stabilitzes the current and tends to improve the power factor, where as a filter capacitor would reduce the power factor and thus the current that can be drawn without overloading things.
For welding the current is usually quite high, like in the 100-400 A range. So this would need quite large diodes with cooling.
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If I use Digi-key and search for say 80V 100A diodes, there are several listed. You may need a way to cool them.
Looking at the costs for cheap welders in this range, it may be cheaper to buy a new/used welder with the features you want rather than trying to modify yours.
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If I use Digi-key and search for say 80V 100A diodes, there are several listed. You may need a way to cool them.
Looking at the costs for cheap welders in this range, it may be cheaper to buy a new/used welder with the features you want rather than trying to modify yours.
Many thanks joeqsmith. Think i found a good rectifier at amazon
https://www.amazon.com/Baomain-Bridge-Rectifier-MDQ-200A-Module/dp/B08CN639BS/ref=sr_1_1?dib=eyJ2IjoiMSJ9.rPrc-a6zQJnZDgBb2KZIZkj6mXgxrjZ42GpN0JnGVy04h0lPgjBXEGlr-ryiGDwQz3kZsWRc3x-DxegmxmimPellKeXq9vCjdtr6Xd7yUDfI5BAxwUkUkhXhdBVk5azl2McApxHrRYh1wOpIt4TwoKjShV5UTlBbBZx4kM7w3b-xjr5Jm921SOykHL1Pzl0guza5d0_3k30nH4bVWg0ib02LK82NmSAZl15SSDGtFWOGK6FByXjpUB0skrj21C6UWRb5jBpzRxCVzZZpCdscls8hirD75GWsdUsCBtwK0m8.iA7aFbBhky91EFHwBNI779FVYHsJsP38qwOYMMpdAMs&dib_tag=se&keywords=200+amp+bridge+rectifier&qid=1716664856&refinements=p_72%3A1248921011&rnid=1248919011&s=industrial&sr=1-1#customerReviews (https://www.amazon.com/Baomain-Bridge-Rectifier-MDQ-200A-Module/dp/B08CN639BS/ref=sr_1_1?dib=eyJ2IjoiMSJ9.rPrc-a6zQJnZDgBb2KZIZkj6mXgxrjZ42GpN0JnGVy04h0lPgjBXEGlr-ryiGDwQz3kZsWRc3x-DxegmxmimPellKeXq9vCjdtr6Xd7yUDfI5BAxwUkUkhXhdBVk5azl2McApxHrRYh1wOpIt4TwoKjShV5UTlBbBZx4kM7w3b-xjr5Jm921SOykHL1Pzl0guza5d0_3k30nH4bVWg0ib02LK82NmSAZl15SSDGtFWOGK6FByXjpUB0skrj21C6UWRb5jBpzRxCVzZZpCdscls8hirD75GWsdUsCBtwK0m8.iA7aFbBhky91EFHwBNI779FVYHsJsP38qwOYMMpdAMs&dib_tag=se&keywords=200+amp+bridge+rectifier&qid=1716664856&refinements=p_72%3A1248921011&rnid=1248919011&s=industrial&sr=1-1#customerReviews)
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Found a good one at amazon.
https://www.amazon.com/Baomain-Bridge-Rectifier-MDQ-200A-Module/dp/B08CN639BS/ref=sr_1_1?dib=eyJ2IjoiMSJ9.rPrc-a6zQJnZDgBb2KZIZkj6mXgxrjZ42GpN0JnGVy04h0lPgjBXEGlr-ryiGDwQz3kZsWRc3x-DxegmxmimPellKeXq9vCjdtr6Xd7yUDfI5BAxwUkUkhXhdBVk5azl2McApxHrRYh1wOpIt4TwoKjShV5UTlBbBZx4kM7w3b-xjr5Jm921SOykHL1Pzl0guza5d0_3k30nH4bVWg0ib02LK82NmSAZl15SSDGtFWOGK6FByXjpUB0skrj21C6UWRb5jBpzRxCVzZZpCdscls8hirD75GWsdUsCBtwK0m8.iA7aFbBhky91EFHwBNI779FVYHsJsP38qwOYMMpdAMs&dib_tag=se&keywords=200+amp+bridge+rectifier&qid=1716664856&refinements=p_72%3A1248921011&rnid=1248919011&s=industrial&sr=1-1#customerReviews (https://www.amazon.com/Baomain-Bridge-Rectifier-MDQ-200A-Module/dp/B08CN639BS/ref=sr_1_1?dib=eyJ2IjoiMSJ9.rPrc-a6zQJnZDgBb2KZIZkj6mXgxrjZ42GpN0JnGVy04h0lPgjBXEGlr-ryiGDwQz3kZsWRc3x-DxegmxmimPellKeXq9vCjdtr6Xd7yUDfI5BAxwUkUkhXhdBVk5azl2McApxHrRYh1wOpIt4TwoKjShV5UTlBbBZx4kM7w3b-xjr5Jm921SOykHL1Pzl0guza5d0_3k30nH4bVWg0ib02LK82NmSAZl15SSDGtFWOGK6FByXjpUB0skrj21C6UWRb5jBpzRxCVzZZpCdscls8hirD75GWsdUsCBtwK0m8.iA7aFbBhky91EFHwBNI779FVYHsJsP38qwOYMMpdAMs&dib_tag=se&keywords=200+amp+bridge+rectifier&qid=1716664856&refinements=p_72%3A1248921011&rnid=1248919011&s=industrial&sr=1-1#customerReviews)
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Doing a quick search on YT. Found a guy attempting it and showing results.
https://www.youtube.com/watch?v=ytVAOlzytcg (https://www.youtube.com/watch?v=ytVAOlzytcg)
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Saw that video too joeqsmith. The man did a great job. A big thanks to you and all the others who gave their opinion. You all pointed me into the right direction. mMany many thanks again guys
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Two methods are common for D.C. welders. They usually involve very large stud mounted rectifiers on heat sinks. Sometimes they have a normal type diode where the cathode is the stud mount end and it is paired with a 'reversed' diode where the anode is the stud mount end. This allows two diodes to form part of a full wave rectifier without needing insulating washers. Another method uses large stud mounted SCR's and varies the firing angle to vary the output current. This way the open circuit voltage is always high enough to maintain good arc length even at low current like for welding sheet metal. Many modern welders have gone away from the 'buzz box' days and now use high frequency inverters taking advantage of smaller transformers, lighter weight, and sometimes lower cost. Inverter welders can also be gentler to the A.C. power line with less inductive reflections and transients being backfed onto the mains.
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That is so CaptDon, but the buzzboxes are still the most reliable welders. They are great for home/hobby use. They don't break like the inverter welders do
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For the higher than primary secondary current rectification, you could also use synchronous rectification and get rid of a lot of that power dissipation.
boB
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For the higher than primary secondary current rectification, you could also use synchronous rectification and get rid of a lot of that power dissipation.
boB
I wish it is this easy. Above about 100A diodes start to catch up.
Remember that resistive dissipation scales with square of the current while diode dissipation is linear (almost).
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For the higher than primary secondary current rectification, you could also use synchronous rectification and get rid of a lot of that power dissipation.
boB
I wish it is this easy. Above about 100A diodes start to catch up.
Remember that resistive dissipation scales with square of the current while diode dissipation is linear (almost).
I see what you mean. But you can add more FETS to reduce the dissipation (lower RdsOn) but a diode drop is still a voltage drop and adding more diodes spreads out the dissipation.
I had a 250V FET with RdsOn of around 20 milliOhms in mind which might be worse at around 100A
So yeah, I guess it depends on what you have to work with and how long you want to spend on it.