I have a 120v-36v transformer hooked up to a bridge rectifier composed of 4x IN4004 diodes, 2 filter capacitors and a 100k resistor paralleling them as a bleeder resistor. The rectifier puts out the full 50v DC I expect it to, but at the same time the transformer faintly vibrates (you can only really tell if you touch it), and over time it gets quite hot. Why is this? I have checked over my connections quite a few times and I am 99.9% sure its all connected properly.
Either you have a bad diode or capacitor or the transformer has an internal turn-to-turn short. Remove the rectifier and filter and see if the transformer still heats up. If not, add the rectifier. Then one capacitor at a time. By then you should know where the problem is. -- Regards, John Popelish
Since you are getting the correct voltage from the transformer+rectifier+filter, the most likely reason the transformer is heating is that it is has been designed with the least amount of wire that can possibly do the job, and thus the resistive losses are fairly high in the primary winding (the winding connected to 120 V). If you will be drawing a lot of power from the supply, you should ensure adequate ventilation for the transformer for cooling. Most transformers have a strap that can be mounted to a metal case to help dissipate some of the waste heat. If the excess heat is a problem, you might consider upgrading to a higher wattage transformer. Be sure to check the output voltage under load - it won't be nearly as high as when there is no load connected. The vibration may be due to low quality lamination in the metal transformer core, or loose windings, or both. (The transformer core is made up of thin strips of iron that are insulated from each other, usually with a thin lacquer coating. Quality transformer cores will be very tightly laminated.) The vibration/noise is consistent with the apparent low quality of the transformer, but should not be a problem.
As Bill R says for working it out. As to how to answer in an exam, First write what is known, and what is wanted down. Write the appropriate equation(s) down and substitute values/simplify equation down as far as possible, you haven't been given all the information required to get that answer in the question. If you don't have all the data provided, and aren't expected to make assumptions then I'd argue the correct answer is actually the formulae produced when you've correctly put all that known in. But quite often you ARE expected to make reasonable assumptions. Sometimes an exam might have a cover page with assumptions to be made (if you don't make them you'll not get the answer right) sometimes you've just got to know how to make reasonable assumptions without being told. as it's 60Hz mains you're probably in the US. I'm in the UK so would it be reasonable for me to assume 50Hz mains if I were sitting the exam here? In cases where you have to make assumptions, it's usually a good idea to then state you are making an assumption. I suggest showing the difference which can happen if f is at least two different values eg finish with assuming f50Hz as mains is in the UK, then C needs to be C1followed on the next line by assuming f60Hz as mains is in the US, then C needs to be C2If you were feeling a little annoyed at those who wrote the exam, and wanted to make your point extra clear you might even throw in the case of f 120Hz. I believe many aircraft alternators generate AC at that f. Ask your teacher/lecturer about what kind of assumptions you might be expected to make. If they skirt around the issue maybe quote some Charles Babbage, and hope they laugh at the sarcasm ' On two occasions I have been asked,—Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out? I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.' —Charles Babbage, Passages from the Life of a Philosopher
The transformer may be vibrating because the winding inside have loosened up and vibrating due to the varying magnetic field. But it is quite possible it may have a shorted winding this will definitely make it heat up and vibrate. Disconnect the filter capacitors and plug it back in if the transformer stops vibrating and/ or the transformer doesn't overheat you have a leaky filter capacitor.
