| Electronics > Beginners |
| Choosing the right transformer for a power supply |
| (1/3) > >> |
| Paul92:
I just started finally building a little electronics toolbox for my projects and the time has come for a power supply. I got a kit for a 0-30V 5mA to 3A power supply (https://www.amazon.co.uk/gp/product/B071F4M96P/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1). The question is what transformer to choose? The kit says that it requires a 24V AC input source. I found an affordable transformer at farnell (I am going to connect it to an European 230V socket): http://uk.farnell.com/myrra/74032/transformer-flyback/dp/1501048?st=24v%20transformer Now, the questions: - From my poor knowledge of how transformers work, they are simply 2 coils on a core that resonate, and the parameters of the transformer are tuned by the number of twists in the coil, wire gauge, core properties. How can a transformer have a variable input voltage and guarantee a steady output voltage? I would have expected to be some sort of proportionality between the input and the output. - The transformer from the link I found is rated at only 1.5A. The power supply handles up to 3A. This leads to 2 questions: if I try to draw more than 1.5 from the transformer, will it simply not handle it or will release the magical smoke? Also, if I try to add 2 such transformers in parallel, will I get 3A? - Should I add anything between the transformer and the power supply and/or before transformer? I am thinking more about protection (some limiting resistor if the transformer gets fried if I try to draw too much? may some circuit breakers, rated at the maximum transformer current?). |
| james_s:
The transformer you linked is a ferrite core transformer for a high frequency switchmode power supply. What you need for your project is an iron core transformer that will work on 50Hz, this will be a rather large chunky thing. The power supply will actually draw somewhat more current from the transformer than it delivers on the output, without going into too much detail this is because when you rectify and filter AC into DC the resulting DC is the peak voltage of the AC which is higher than the RMS value, you don't get something for nothing so the current has to be higher. Yes you can parallel multiple identical transformers in most cases to increase the current capacity. If you overload a transformer it will overheat and eventually burn up. |
| Audioguru:
I agree that you need a big 50Hz transformer not a tiny 100kHz transformer. Guess what? The cheap power supply kit is a Chinese copy of a 15 years old defective Greek kit that is not reliable and does not work properly because many of its parts are overloaded. I helped fix the original Greek kit at www.electronics-lab.com where there are a few long threads about it. ebay and Banggood stopped selling this copy of the defective cheap kit. The 24V 50Hz or 60Hz transformer has a voltage too low for an output from the kit of 30VDC at 3A but is too high for the 36V opamps used. My fixed circuit uses a 28V transformer and opamps rated for a 44V max supply. The 28VAC has a peak of 28V x pi/2= 39.6V which at an output of 3A is a power of 39.6V x 3A= 118.8VA. Therefore I recommend that at least a 120VA transformer is used. 120VA/28VAC= 4.3A, not 3A. |
| IanB:
--- Quote from: Paul92 on May 29, 2018, 06:14:36 pm ---Now, the questions: - From my poor knowledge of how transformers work, they are simply 2 coils on a core that resonate, and the parameters of the transformer are tuned by the number of twists in the coil, wire gauge, core properties. How can a transformer have a variable input voltage and guarantee a steady output voltage? I would have expected to be some sort of proportionality between the input and the output. --- End quote --- Correct. The output voltage will vary in proportion to the input voltage. So if the mains input voltage varies between 230 V and 240 V the output will go up and down correspondingly. If you live in the UK you should try to obtain a 240 V transformer (not a 230 V or 220 V transformer). --- Quote ---- The transformer from the link I found is rated at only 1.5A. The power supply handles up to 3A. This leads to 2 questions: if I try to draw more than 1.5 from the transformer, will it simply not handle it or will release the magical smoke? Also, if I try to add 2 such transformers in parallel, will I get 3A? --- End quote --- It is not obvious that that power supply can actually output 3 A (at least not without a heat sink on the regulator and a fan, and even then it's doubtful). I would suggest setting your sights lower and settling for 1 A. --- Quote ---- Should I add anything between the transformer and the power supply and/or before transformer? I am thinking more about protection (some limiting resistor if the transformer gets fried if I try to draw too much? may some circuit breakers, rated at the maximum transformer current?). --- End quote --- Probably not worth it. A fuse between the transformer and the power supply might be a good idea, but at such low power levels I would not bother. |
| capt bullshot:
Audioguru pointed it out, but even before reading his post I knew from looking at the picture of the kit you've chosen that this thing will most probably end up in flames or smoke once you use it at more than 33% of its ratings. That's an experience a beginner has to make once or more than once in his career, but it can be really frustrating. And todays money making driven market makes it even more frustrating since the specifiation of this kit is nothing but a bunch of lies. As a beginner one cannot jugde this because the lack of experience, but one may be blinded by the really low price tag. A decent transformer to supply this kit according to its ratings would cost a multiple of that. Alas, I can't give a recommendation for a decent kit suitable for a beginner. Such a kit should contain not only the bare electronics, but also the the transformer, heat sink, enclosure and other stuff to complete the project. I've built a few kits when I was a beginner some decades ago, and since they were good and complete, I was able to build it and get it working as expected. Later on, I've learned how to design my own circuits and what the correct component choices would be. So far remembering the good old times. Still don't have a recommendation for you, sorry. |
| Navigation |
| Message Index |
| Next page |