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    Primary resistance of mains transformer

    Hello
    Do you know what it is for this transformer...

    https://uk.farnell.com/myrra/44090/t...24v/dp/1689052

    the datasheet doesnt say.
    It matters to us, because if its large then we cant also use this transformer to power our micro etc in this mains voltage data logger circuit

    - - - Updated - - -

    Also, it says that its only for 230VAC.....if the mains goes up to 265vac then the primary magnetising current will increase...this may saturate the transformer (or rather move it more into saturation) but insufficient details are available in the datasheet to be able to evaluate this...
    do you know where we can find out?

    All we want this transformer for is to isolate and drop down the mains so that we can measure it for monitoring purposes....we dont even need the step down as we can drop it with a resistive divider in the secondary.
    Last edited by treez; 9th March 2019 at 22:41.

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    Re: Primary resistance of mains transformer

    Maybe use a dropping resistor in the primary instead of secondary. That gives almost the same effect (there will be some additional phase shifting which may not be important to you) and buys you a degree of transformer protection as well.

    As a pure guess (I have one of these in a unit but it is packed away so I can't check easily) the DC resistance will be around 200 Ohms.

    Brian.
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    Re: Primary resistance of mains transformer

    It matters to us, because if its large then we cant also use this transformer to power our micro etc in this mains voltage data logger circuit
    These baby transformers are more resistive than inductive. You can easily measure the DC resistance of the input side with a regular multimeter.

    The size does matter: but if you do not need that much of output power, you can put a power resistor in series with the input. In case of a catastrophe, the resistor will be martyred.


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    Re: Primary resistance of mains transformer

    An old thread about small transformer parameters. Among other topics, it shines a light on the rather distorted magnetizing current. It may be interesting to estimate the errors when using a small transformers for measurement purposes.
    https://www.edaboard.com/showthread....mer-Inductance

    I generally doubt that a small transformer like the discussed 1.5VA is suited for quality measurement purposes, although we didn't get a specification regarding accuracy, bandwidth, linearity and phase error.

    If some uA leakage current can be accepted, I would refer to a non-isolated measurement with differential voltage divider as implemented in the below shown transducer module.

    Click image for larger version. 

Name:	100_1 voltage transducer.PNG 
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ID:	151618


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    Re: Primary resistance of mains transformer

    Thanks FvM,
    That looks good, and we would like to connect the pin 5 (negative supply) of the INA826 to earth ground as we are not interested in the negative going peak. We would then connect live to P1, neutral to P2, and add a 1N4148 diode across R303.
    (though looking at the INA826 datasheet it appears that the “V-“ pin can be connected to a voltage 40V below the voltage at “V-“, which is impressive, so we wouldn’t really need the diode across R303.)
    Also, I like the 0.1% resistor accuracy, but in our rough-and-ready case, I think we would just use 1% resistors throughout as we will calibrate it…ie look at what output voltage the INA826 gives at 240VAC and then we would know from that whether the mains had gone up or down thereafter, and roughly by how much to the nearest 2V or so.
    We are indeed not worried about leakage current to earth, and I think we will make the divider 1MEG/10K instead of 10MEG/1K….since I believe it makes it slightly less noise prone….and we are never going to sell this product, its purely for in-house use.
    We will have to remember not to feed the INA826 inputs with the output of one of our 1kVA mains isolation transformers. (we would be unlikely to do this anyway as we need that for other uses)
    Thanks Again



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    Re: Primary resistance of mains transformer

    I don't like the idea of operating the instrumentation amplifier input in saturation. You can better connect the divider foot point and difference amplifier reference to a virtual ground if you don't have a bipolar supply.



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    Re: Primary resistance of mains transformer

    Thanks, i see your point, though we are just interested in sampling the positive going peak every 200us with a micro ADC......and we will store the maximum value every second into a data logger.........when the IN amp input is in saturation it is bad, but we dont care, it will soon recover, we feel, when the inputs reverse polarity again and the output starts going positive for the next 10ms of the mains half cycle.
    Yes you are right, we dont want to bother with a negative supply.

    - - - Updated - - -

    Also , i believe we will use the INA126 instead of the INA826 because the INA126 comes in DIP packaging.

    INA126 datasheet
    http://www.ti.com/lit/ds/symlink/ina126.pdf

    Having said that, maybe we will operate it with +/- supply after all, as we dont want phase reversal happening and such unwanted malaise.
    Last edited by treez; 10th March 2019 at 13:50.



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    Re: Primary resistance of mains transformer

    If you are going to monitor voltage and spikes, you will need to define cutoffs and also provide a timestamp. Some spikes that can be much shorter than 200us (and that can do real damage to electronics) but does not bother fans and lamps (mostly).

    I have stopped running radios on the wall adapter because something or the other is sensitive enough to get damaged by a tall and slim spike!

    This is not a new concept: I have seen chart recorders running 24X7 recording line voltages in computer rooms (and other places with sensitive electronic instruments).

    If you are going to use this output as an evidence, make sure that it is solid.



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    Re: Primary resistance of mains transformer

    Thanks, you are right, we would also liek to monitor spikes, but or now will just have to settle for monitoring the general level of the mains voltage.
    The spike detection is another project.
    (We have a cheap spike-detector which is just a diode bridge followed by divider which goes into a FET gate which is connected to shunt the HV bus when the spike happens because the spike will overvoltage VGS of the fet via the divider......the fet then blows up and blows the fuse in its Drain connection.......and thus turns the led off)



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    Re: Primary resistance of mains transformer

    Thanks FvM for your schem in post #4 above.
    Its interesting, because the INAMP in our own schem (attached) will be on a GND that is isolated from the primary side live and neutral……(well not quite truly isolated because of the 1 MEG resistors but “as near as dam it” isolated)
    Our schem is as attached here.
    The current in your R301 must obviously flow in a loop….that loop is through R301, then through R303, then along the “GND”, then up through R304, then back through R302, then presumably through the X2 capacitor which presumably exists between live and neutral somewhere upstream of your schematic?….if no X2 capacitor exists then the loop will eventually be made presumably in the secondary of the distribution transformer wherever that is……this is a very large loop of current. What would you say about noise issues with such an enormous loop of current may I please ask?

    I think there could be more issues if the “GND” of your schematic was connected to Earth. This is because there is then another loop of current for the current in R301… this involves R301 -> R303 -> back along Earth….then back through the mains generator and back up into the LIVE line.
    I therefore presume you would not recommend earthing the “GND” in your schematic?

    In our schem you can see that the loop of current involving the current in the input resistances of the INAMP involves going through the primary of the small mains transformer
    Last edited by treez; 12th March 2019 at 21:51.



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    Re: Primary resistance of mains transformer

    I'm expecting circuit ground connected to protective earth. But floating ground is also an option. With single phase input, the circuit ground floats to 115 V.

    The relevant parameters for possible measurement errors are common mode voltage and common mode rejection ratio.


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    Re: Primary resistance of mains transformer

    Thanks, that ground floating to 115v could be a big problem if someone suddenly connects the PC USB cable to it........which would suddenly earth the 115V....what effect do you think this would have on the PC?...I mean its a high voltage, but there's a big impedance 'behind' it.


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    Re: Primary resistance of mains transformer

    Hi,

    With "the big impedance" you mean the isolation?
    If so: Yes, it will (should) reduce the 50Hz AC current to a safe value.

    The more problematic may be the capacitance (to earth ground or to mains).
    115V AC means about 160V peak as worst case.
    The initial discharge current may be high...

    Klaus
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    Re: Primary resistance of mains transformer

    Thanks, in post #10, in the schem there. i have breached the isolation with the 1 MEG resistors which are the Zin's of the INAMP...it is these resistances that i am speaking of.
    As you say the initial discharge current may be high, specially with the stray capacitance of the transformer taken into account....therefore , i believe we will have to connect the secondary gnd to earth gnd, would you agree?



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    Re: Primary resistance of mains transformer

    Hi,

    Yes, I'd connect bith GNDs, too, with a high ohmic resistor.

    Klaus
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    Re: Primary resistance of mains transformer

    Thanks, may i please ask why is that?...we were just going to connect the secondary ground directly to earth ground? (no high Ohm resistor)
    Last edited by treez; 14th March 2019 at 07:13.



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    Re: Primary resistance of mains transformer

    Hi,

    With high ohmic resistors you avoid Earth Ground loops.
    Additionally you reduce the ESD current.

    But if you don't expect these problems you are free to connect them directly.

    Klaus
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