Measuring mains via a transformer - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: November 11, 2020, 04:17:22 pm
content: Rather than using a string of resistors directly across L,N - is it viable to use a small transformer to both step down & isolate mains fo this purpose?Possible issues are accuracy/repeatability of transformer, obtaining a small but reliable model, effects of loading.Is there a well known model or type of transformer geared towards monitoring mains potential?I want to be sure the value is inside 207, 253V as per wiring regs 18th Ed, 722.411.4.1, (iv)

author: Kleinstein
author_rank: Super Contributor
timestamp: November 11, 2020, 04:50:54 pm
content: One can use a transformer. However the small ones are not very accurate, when used at there full rated voltage. Things are a little easier in 110 country as one could use a 230 V transformer and this way avoid high magnetizing current.If one is lucky one finds a transformer for 400 V in, but these are rare at low power. Special low standby transformers are also a viable option. These are essentially the same as a higher voltage transformer, just down-rated to get low loss and this way lower distortion.Otherwise I would go for something like 2 transformers for 230 V in series, like 3 VA rating and preferable toroid typ.The difficulty is getting it small, as the small (e.g. < 1 VA) transformer are far from ideal.

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 11, 2020, 05:15:27 pm
content: Rule #1:Resist the temptation to use the same voltage sensing transformer to also provide power to the circuit.EDIT;A small transformer has a quite large equivalent series resistance and the peak capacitive charging currents will significantly distort the wave form.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 15, 2020, 05:57:53 pm
content: The resistive divider has two major drawbacks: the lack of galvanic isolation and temperature instability. When using two resistors in a divider with a very significant difference in the shoulders, when the temperature changes, the resistance druif in the shoulders will be very different and the division coefficient accordingly.Finding a miniature transformer at 50 Hz and 120/240V voltage is not easy and they are expensive.I plan to use a current transformer. Its essence is that the primary winding is connected through a high-resistance resistor, and the secondary has a load resistor. Look datasheet for this one:https://lcsc.com/product-detail/Current-Transformers_Qingxian-Zeming-Langxi-Elec-ZMPT107-1_C125191.html

author: drussell
author_rank: Super Contributor
timestamp: November 15, 2020, 06:12:17 pm
content: Small torroids are much better than the typical E-I core transformer as far as frequency range and actually representing what is going on on the primary side. They're also not really very expensive.But yes, as noted above, do not actually power anything from the secondary of your sensing transformer....

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 16, 2020, 12:42:43 pm
content: In the past I've build a capacitive voltage divider for 0 cross detection.The capacitors on the mains voltage side were home build using a regular power cable and heat shrinking some wires near the location of the conductors in the cable, and I used an opamp to compare the differential voltage between the two sense wires. Beware that opamp inputs always need a defined DC path, so couplig can not be 100% capacitive.If you want to modify this for actually measuring AC voltage, then use real capacitors for both sense wires and put the opamp in a differential amplification configuration.If you want to use a transformer based solution, then put the primary windings of two identical transformers in series to lower the peak voltage and stay far away from saturation of the transformer cores.For higher accuracy, it's probably better to use a resistive divider, digitize the signal, and use some kind of digital isolation barrier to go to the rest of the circuit.

author: 741
author_rank: Frequent Contributor
timestamp: November 17, 2020, 10:16:06 am
content: The ZMPT107 looks like a nice solution - thanks.

author: Yansi
author_rank: Super Contributor
timestamp: November 17, 2020, 10:24:03 am
content: Quote from: S. Petrukhin on November 15, 2020, 05:57:53 pmDisagree strongly. PCB mounted potted mains transformers are readily available from like 0.35 VA up. And they are cheap, well under 4 yankee bucks.But sure, they are not a metrology grade transformers, that's why you need to have the nominal primary voltage over-rated as much as possible, to not get significant voltage error just by the input magnetizing current that makes voltage drop on the winding resistance itself. And that is the reason metrology grade transformers are quite large for their rated power, as they are significantly over-engineered to draw least amount of magnetizing current and have least amount of leakage inductance and good coupling factor.See: 400V primary 0.35VA PCB mount transformer:https://www.tme.eu/cz/details/bv2020167/transformatory-pro-pcb/hahn/bv-202-0167/

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 17, 2020, 11:19:42 am
content: Quote from: Yansi on November 17, 2020, 10:24:03 amUnfortunately, I do not find them at an affordable price with reliable delivery to Russia. The ZMPT107-1 costs $0.5 and is available on LCSC. The fact is that I do not make a hobby to buy myself one or two, I make projects that are then produced and have to select components according to their availability, among other things.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 17, 2020, 02:27:31 pm
content: I would avoid those super small transformers: there efficiency is horrible and they run quite hot with something like 1.2 W of no load loss for a 0.35 VA transformer. Under load the power consumption may actually be lower.So with this small size a 400 V nominal rating may not be enough to give an accurate picture of 230 V mains.

author: poorchava
author_rank: Super Contributor
timestamp: November 17, 2020, 08:59:32 pm
content: Being professionally in T&M business and dealing predominantly with gear for measuring various parameters of mains networks let me tell you: don't use a transformer unless you can live with +/- 10V accuracy and/or can reverse map output to input using a uC.Differential amplifier is the way to go and is simple to implement. 3V accuracy over 400VAC range (.75%) can be achieved using almost any fet/cmos precision opamp. My go-to part for such stuff is OPA2376. Better results can be achieved with auto zero and chopper amplifiers.Small mains transformers have only bad qualities, besides being small and generating isolated voltage between 50 and 300% of their rated voltage. Each unit has completely different parasitics and they also drift with temperature as hell, which is a problem because they heat up on their own with no load to something like 50*C in open air. Due to very thin wire being used their source impedance is very high and therefore they have no load regulation to speak of. Loading such 12V trafi with a few megaohms already affects measurement results significantly. If you measure for example a 230V to 24V trafo with an LCR bridge, you'll get winding ratio of about 6...7 rather than expected 10, as they need to compensate for ESR to get the specified output voltage at rated load.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 17, 2020, 10:25:42 pm
content: Quote from: poorchava on November 17, 2020, 08:59:32 pmI didn't really understand how the temperature drift of active resistance of the winding conductor affects the transformation coefficient. No one thinks of loading an LCR measuring transformer with a circuit. The output of the transformer will be loaded by the OP.At the expense of parasites due to the winding in bulk of a huge number of turns - completely agree.However, I prefer current transformers that do not have such problems. They introduce a phase shift, which is bad in some cases, but it can be compensated for programmatically during signal processing. Temperature drift in current transformers, whose windings are loaded with low resistance and are the arm of dividers, is compensated by the same number of turns of both windings 1:1, they drift in parallel. The spread of parameters is easily compensated programmatically for calibration by a simple function y=a+x, according to my observations. Although, I put the function y=a+bx in the calibration.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 18, 2020, 08:38:38 am
content: The small transformers below some 1-2 VA are no longer close to an ideal transformer. The magnetizing current gets a very significant factor. So a significant part (this could be some 30% maybe even 50% also under no load) of the voltage is already lost from the ohmic resistance of the primary winding. As a 50Hz transformer at this small size is difficult they drive the core to near saturation and this way get some voltage stabilization - it is kind of a desperate measure in a lost battle, to get the best out of poor starting conditions.Larger transformers can get much better. Already 5 VA is a lot better. Another point already noted is using less than the rated voltage, so that the core is no where near saturation and in the more linear part of the curve. A problem one has to keep in mind is however a possible DC offset, that can effect especially larger transformers.If it has to be small, a resistive divider (possibly with parallel capacitors) and an isolation amplifier is definitely the way to go. A resistive divider has to keep heating and parasitic capacitance in mind - so it is more than just 2 simple resistors.If really needed for precision measurements there are special transformers with dual windings and a split core: one core and set of winding does most of the work and the 2nd set does corrections only with very little magnetizing current. However these are special designed ones, not commercial power transformers. This way transformers in precision bridges can work to the sub ppm level, better than resistive dividers.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 18, 2020, 09:40:33 am
content: Quote from: Kleinstein on November 18, 2020, 08:38:38 amI probably don't fully understand the terminology in English... But how can there be magnetization at AC? Do I understand correctly that you are talking about the residual magnetism of the core when there is no applied voltage to the winding?

author: Kleinstein
author_rank: Super Contributor
timestamp: November 18, 2020, 01:02:57 pm
content: Quote from: S. Petrukhin on November 18, 2020, 09:40:33 amNo, it is about the current to build up the AC magnetic field. For the transformer to work the magnetization changes from some 1.2 T in one direction to 1.2 T in the other direction wth every period. In the linear approximation this is from the inductance not being so large that the current is very small. With larger transformer this current is still there, but with a relatively small winding resistance it is not a problem and nearly 90 deg. out of phase. With the very small transformers this no load current is relatively large, no longer negligible to the current at nominal load - it can be even larger.

author: merport
author_rank: Newbie
timestamp: November 18, 2020, 07:11:52 pm
content: I was wondering if an isolation amplifier would be useful for this application. Such as the TI ISO224.https://www.ti.com/product/ISO224

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 18, 2020, 10:14:00 pm
content: But low-power 1-5W transformers are cheap enough now, so I can think TS may buy it and check. I am 90% sure it will work fine up to rated voltage at least, and up to 10-15 % higher. If there isn't very strict accuracy demand they will work fine.And the same will work for power transformer used to power the device itself - it can be used too. It is very convenient and efficient way. The only drawback is of cause is a bit worse accuracy.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 19, 2020, 08:07:12 am
content: I have some doubt, a small transformer will work accurate up to the rated voltage. At least not all will work, especially at less than 3 VA. Quite a few of the small transformers drive the core relatively close to saturation, and thus cause nonlinear effects. The nominal voltage is also not such a fixed value: the same transformer could be specified as something like 2 VA at 230 V with low no load loss or as 3 VA 400 V with more normal no load loss. To get an accurate picture of the primary voltage one needs a relatively low no load current - at high voltage the effect is not just linear. Approaching saturation the current goes up quite fast and thus can cause quite some trouble.Those transformers with a relatively small size for the given rating tend to be less suitable, unless they use a ring core and are smaller for this reason.In theory there would be a way to also measure the current with a separate current transformer and than correct the drop due to the ohmic resistance at least approximately. With this correction chances are good to get reasonable accuracy even when powering the circuit through the transformer (though only at a fraction of the rated power).