How to spec a mains transformer - Page 1
Forum section: electronics / projects

author: nukie
author_rank: Frequent Contributor
timestamp: August 06, 2011, 03:21:48 am
content: AC is one of the things I rarely get involved, I am not good at it at all.Often, I would just hack a pre-made 5V switchmode phone charger as a powersupply for my projects. It works damn well because most of my micro powered designs are 5V and below. Then come another 0-30v power supply project which specifies 24V 3A 72VA mains transformer. that's all good and nice.So what do you do when you are given the input AC voltage as the only requirement?I have a device which specifies 15VAC input. I don't know how much current it will consume so I wired up the DC circuit and found it consuming 40mA. So I sourced a 2.8VA 2x15VAC transformer. I thought if I screwup I could wire the two secondary winding in parallel to increase the current. A single secondary would supply 15VAC 1.4VA 93mA.Unfortunately for me, I didn't take in account unloaded voltage. When I wired up the transformer to the mains, I am getting about 26VAC unloaded. I connected the device and measured the dc voltage after rectifier diodes it's 30VDC. You might think this is all cool and normal but there's a 78L15 regulator on the board and it's dissipating some amount of power, it's quite warm actually.So I should find a transformer with a lower VA? How do I spec a transformer for situation like this?

author: IanB
author_rank: Super Contributor
timestamp: August 06, 2011, 04:06:38 am
content: Something is wrong here. If a transformer winding is specified at 15 V RMS then an AC measurement should show close to 15 V, not 26 V. Well designed transformers have a degree of self regulation so the voltage doesn't jump about wildly. (Just imagine if your mains voltage jumped up from 240 V to 416 V when your neighbours are not using any power!)So you need to double check your 26 V AC measurement. Either your voltmeter is crap or your transformer is crap.If you rectify the 15 V AC and feed it to a smoothing capacitor then the DC voltage on the capacitor without load should be 15 x sqroot(2) = 21.2 V.Nothing is cool and normal about any of this. It is all wrong, terribly wrong...

author: jahonen
author_rank: Super Contributor
timestamp: August 06, 2011, 08:59:39 am
content: I think that your measurements are nothing out of ordinary. Small transformers have quite large voltage swing between full load and idle (tens of %'s). For larger transformers, situation improves significantly (we get around 5% or so). One way to solve this is to use transformer with a lower voltage secondary, but it might take some trial and error to find the right one. And of course if the current consumption of your device will vary too much, there might be trouble at times when the current consumption is high.Regards,Janne

author: Bloch
author_rank: Unknown
timestamp: August 06, 2011, 09:13:36 am
content: Quote from: jahonen on August 06, 2011, 08:59:39 amTrueHere are an example 6VAhttp://produkt.dantrafo.dk/product.aspx?m=prodlist&viewtype=4&catid=135&lang=uk15VAC = 100% load21VAC = 0% loadDC = SQR(2) * 21 = 29.69V at 0% loadTry add a small permanent load at maybe 2-3mA And then Measure the Voltage

author: nukie
author_rank: Frequent Contributor
timestamp: August 06, 2011, 02:03:24 pm
content: I hooked up a 265ohm resistor(translate to about 80mAC load current) across the output, the AC voltage drop to about 20.5v. The transformer is rated 93mA.Maybe this is mislabeled, I have 4 of these transformer, I will test them later.

author: Bloch
author_rank: Unknown
timestamp: August 06, 2011, 02:28:23 pm
content: Quote from: nukie on August 06, 2011, 02:03:24 pmNo it works fineThen try at 2*90mA load or the or 2 * a 265ohm resistorI am 99.99% sure that it will give 15VACRemember also to measure the Pri. 230V

author: nukie
author_rank: Frequent Contributor
timestamp: August 06, 2011, 03:23:08 pm
content: I hook up resistors to both the secondary winding to almost full load, lowest voltage I saw was 20.1VAC. I tried a normal 9V e core transformer, unload voltage is around 10.9v, 50% load is slighty above 10v.It seems the German like their voltage high? I'll email the manufacturer and see what they say.

author: Rufus
author_rank: Super Contributor
timestamp: August 06, 2011, 03:42:47 pm
content: The data sheet specifies a no-load factor of 1.31. Your mains is probably higher than 230 - measure it.Something isn't right. With 250v mains and no load you shouldn't see much more than 21v from a 15v winding.

author: nukie
author_rank: Frequent Contributor
timestamp: August 07, 2011, 01:01:22 pm
content: I'm beginning to suspect these transformer are out of spec. Unfortunately all 4 of them are out of the same batch so there's no way to tell if they are manufacturing problem.My voltage is 246vac. Calculated ratio is about 1:15.3 it's still way off from the 230VAC->15VAC when loaded.Like I say before, I tried a cheap 9V E core transformer it performs as it should up to specification. It certainly does not behave like the PCB transformers. I will update again if I get any news from the manufacturer.

author: nukie
author_rank: Frequent Contributor
timestamp: August 10, 2011, 05:21:07 am
content: Hello guys,I received a reply! hmmmmm.....Quote

author: Rufus
author_rank: Super Contributor
timestamp: August 10, 2011, 09:04:39 pm
content: Quote from: nukie on August 10, 2011, 05:21:07 amSo either they are talking crap or their specifications are crap. Being a 'short-circuit proof' transformer could account for abysmal load regulation but the specification should reflect that in the no-load voltage factor.I just checked a 240v 1.2VA transformer from my junk box, the measured no-load voltage factor was 1.24 which I suspect is typical for normal transformers of that size.

author: Zero999
author_rank: Super Contributor
timestamp: August 11, 2011, 07:24:05 pm
content: Lots of small transformer (<3VA) are inherently short circuit proof because they have a high DC resistance and a crappy efficiently.

author: nukie
author_rank: Frequent Contributor
timestamp: August 11, 2011, 11:10:04 pm
content: I won't trust the datasheet from this manufacturer in the future. Safe for use yes but no on my equipment, too close to maximum input of 78L15.

author: ciccio
author_rank: Frequent Contributor
timestamp: August 12, 2011, 07:04:31 am
content: I've used similar transformers, because of the small size and lots of approvals, but their low efficiency was annoying.They are inherently short-circuit proof, and that means that the winding impedance is high.This will save you a fuse on the secondary side, but the transformer will, in some applications, dissipate more heat than the voltage regulator.I had a small company custom manufacture for me a pi-to-pin replacement, not short-proof (I actually did not need this feature) but with really better characteristics.When they disassembled the original transformer (just because they were curious) they told me that the wires were too thin for their standards. Thin wire > high resistance > low efficiency > high no-load voltage.Lets do some simple (too simplified, in effect) math (I hope that nobody will be annoyed):Have a look at the specs on the manufacturer's site: they state a 57% efficiency.That means that 100 - 57 = 43 % of input power will be dissipated inside the transformer.If we model the secondary winding as a "perfect" ac generator with a series resistance, a full load of 2.8 VA at 15V (190 mA = 0.19 A) will result in2.8*43/57 = 2.11 VA dissipated by the internal resistance.These 2.11 VA result in 2.11 VA /0.19 A = 11.12 V that is the voltage loss from no load to full load.The no load voltage will thus be 15 + 11.12 = 26.12 V.Increase this value to account for the higher mains voltage, this will result in 26.12 * 246/230 = 27.94 V, that is the value that nukie has measured (27.22).I think that the data sheet is correct in terms of efficiency, but the no-load voltage is wrong.In any case, I've learned by experience that a 5 volt linear regulator must be supplied by a 9 volt transformer. 15 V is too much, in any case.

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2011, 04:48:30 pm
content: IT's possible to make a transformer which is both short circuit proof and thermally protected of any size by integrating PTC resistor into the winding. I wish most manufactures would do this, it's not expensive or difficult to do.

author: ciccio
author_rank: Frequent Contributor
timestamp: August 12, 2011, 07:39:45 pm
content: Quote from: Hero999 on August 12, 2011, 04:48:30 pmNo, from my experience this it is not practically feasible in such small units: the transformer that nukie was testing is 22.7 mm x 22.0 mm x 15.0 mmThat's less than 7.5 cm3, or about 2/3 of a cubic inch!There is no place for a thermal fuse: it will need some pins inside for soldering it . Really not practical.Thermal protection, in such small sizes, is entrusted by it being intrinsically short-circuit proof.

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2011, 07:53:44 pm
content: I'm talking about a PTC resistor, not a thermal fuse.PTC resistors are available in SMT packages which are tiny compared to the dimensions you've given.http://docs-europe.electrocomponents.com/webdocs/060d/0900766b8060d31d.pdf

author: ciccio
author_rank: Frequent Contributor
timestamp: August 12, 2011, 08:28:57 pm
content: Quote from: Hero999 on August 12, 2011, 07:53:44 pmYes, there is the space for such a small component, but it will be a really hard task to solder it to the pins and the wires.Do you know some transformers manufacturer that uses them?In effect, the Raychem data sheet does not mention transformers in the applications list (only a mention to chargers, that have a PCB inside).Regards

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2011, 09:05:06 pm
content: I've seen a bi-metal strip fuse inside a wall wart transformer before but I admit it wasn't as small as that, it was around 4VA from memory.And so what? Just because no one's done it before, it doesn't mean it isn't possible.

author: nukie
author_rank: Frequent Contributor
timestamp: August 13, 2011, 02:22:28 am
content: Quote from: ciccio on August 12, 2011, 07:04:31 amIf you mean my case, I said 78L15.Protection fuse in small transformers is not uncommon there are readily available.