Transformers - Page 1
Forum section: electronics / general

author: LarsT
author_rank: Contributor
timestamp: June 20, 2016, 02:09:19 pm
content: Ok, so I get the basic principles of transformers, for instance two coils around a common core where the output voltage is proportional with the ratio of turns on the coils. But how many turns is nescessary? If I made a transformer to transform for instance 24V down to 12V (AC) and I used only 4 turns on the 24V side and 2 turns on the other side I'm pretty sure all I would get is smoke and tears. Why is that?

author: johansen
author_rank: Super Contributor
timestamp: June 20, 2016, 02:30:38 pm
content: the core of a transformer is there to increase the inductance of the coil.. that's it. iron for instance has a high permeability, but once the flux density exceeds 1.3T the iron begins to saturate and its permeability decreases. cheap E cores can be used up to about 1.5T, but they can be pushed to 1.7T with forced air cooling.Tape wound cores can run at 1.7T, some as high as 1.9T. 4$ per pound tape wound cores available from a certain manufacture have a core loss of about 1 watt per kilogram at 60hz at 1.7T, cheap E cores are on the order of 10 times that loss. still makes for an expensive transformer though.. at 4$ a pound, plus 7-9$/lb for the copper.so a core with 1 square inch of core area, you get .25 volts per turn at 1.5T flux density.any more than that and the core will waste excessive heat.. and the amps needed to maintain that 1.5T flux density will be excessive.i have a transformer design spreadsheet that is unlike any other i've found online.. it takes some getting used to.www.johansense.com/bulk/spreadsheets/

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 20, 2016, 02:50:13 pm
content: Some high frequency switching PSUs can indeed have just 2 turns for a 12V output.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: June 20, 2016, 09:05:50 pm
content: Quote from: johansen on June 20, 2016, 02:30:38 pmThanks! That is a very interesting spreadsheet. I am working through a design where one of the options is to use a flyback switcher. The trouble is I would be using it in a couple unusual ways, so I need much more data than is provided by the manufacturers. This let's me explore the behavior of a custom transformer.Now all I have to do is learn how transformers work...

author: uncle_bob
author_rank: Unknown
timestamp: June 20, 2016, 10:31:16 pm
content: HiOne simple way to look at an AC power transformer:When there is no load on the secondary, and it it hooked up.... all you have is the inductance of the primary. The smaller that inductance, the more current flows when nothing it going on. In some very small transformers, there can be a surprising amount of current moving in this condition.That's not the only constraint on the transformer design. It may be one of them.Bob

author: Tomorokoshi
author_rank: Super Contributor
timestamp: June 21, 2016, 02:14:40 am
content: One of the design goals is to use the transformer to limit power, with some combination of impedance protection or going into saturation. It's the saturation information that is difficult to find in data sheets. It seems that most manufacturers are expecting people to stay within the nominal ratings.There are other requirements that would likely involve a custom design.

author: 10101
author_rank: Contributor
timestamp: June 21, 2016, 03:33:45 am
content: how do you come up with the "rules of thumb" (about the magnetic fluxes) that johansen said ?

author: johansen
author_rank: Super Contributor
timestamp: June 21, 2016, 09:01:52 pm
content: Quote from: 10101 on June 21, 2016, 03:33:45 amthose are not rules of thumb. at 1.5T, .25 volts per turn per square inch of core area. that's a mathematical formula that is predictable and accurate.a rule of thumb might be that 1 watt per 10 square inches of surface area will have a temperature rise of 40C. i can't recall if that's accurate though.high frequency transformer design requires that you calculate the eddy current losses in the conductors. at 60Hz and less than 1 cm^2 this current is usually negligible. at 60Khz, it is not. my spreadsheet doesn't even consider eddy current losses in the conductors.