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Mains voltage metering issues

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smuel

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Hi,

I'm working on a final year project with the aim of remotely metering mains voltages and currents in a 3-phase distrobution box.

here's our setup..

step down voltages with .35VA 6V transformers, apply 2.5V offset, take 64 samples per 20ms (for 50Hz) via the ADC in an AVR ATmega16 micro, then calculate the RMS value in software and shoot it off as serial data.

My problem, is that i have found out after building a prototype that the 6V transformers dont output a sine wave, it's more like a deformed triangle wave... So this is putting our results off. Is there any way of somehow making it more of a sine wave?

thanks

Sam
 

IanP

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Re: Mains Metering Issues

Try this:

Connect 100Ω/1W resistive load to the 6V winding and without anything else connected using an oscilloscope check what is the shape of the signal .. You should have morehteless nice sine wave - do you have it?

Regards,
IanP
 

smuel

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Re: Mains Metering Issues

thanks for the reply.

I tried it with the 100ohm resistor, and it is pretty much exactly the same shape. it helps a little bit, but it definetely doesn't make it a sine wave.

how does adding the resistive load change the wave shape?

any other suggestions would be greatly appreciated.

thanks

Sam
 

VSMVDD

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Re: Mains Metering Issues

try to use torroid transformers

it could just be how your scope is setup
maybe you use the probe on x1 mode or X10 type probe
you need to use a special probe to measue this type of AC

or use X1 mode and connect the ground to the other end of the 6v winding via a 1uf poly cap it will look more sine

i think to measure a transformers performance
use some audio signals and do a plot to see what resuslts you get at different frequencies

i use transformers in a bass amp and to get a clean signal i use a split tapped
transformer AND GROUNDED THE CENTER TAP via a capacitor

i did get distortion and had to change it from a single tap to the split type
 

smuel

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Re: Mains Metering Issues

My probe is a 10X. Why do you need a special probe for this kind of signal? I'm far from up to scratch on transformer theory...

The transformer i'm using has 2 secondary windings, 2x6V. I'll try sticking the cap in line with the probes earth lead.

thanks a lot,

Sam

Added after 8 minutes:

Nup, didn't work.. I also tried connecting the output of the tranny straight into the cro via eligator clips, and it still looks the same.

This, is driving me nuts.

Any other ideas?

thanks

Sam
 

IanP

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Re: Mains Metering Issues

Transformers will mortheless repeat the shape of the signal that is fed to them ..
Are you sure the input signal from the mains is different (pure sine wave) to what you have from the secondary?
That was the reason I asked you to connect pure resistve load and disconnect anything else ..
If the input to the primary winding is sinusoidal and the output from the secondary (with a resistive load) is triangular then this is not "normal" transformer ..

Regards,
IanP
 

smuel

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Re: Mains Metering Issues

The input on the primary is definetely sinusoidal (240V AUS mains). The waveform on the secondary isn't completely triangular either, it's something in between sin and trangle..

I'm pretty sure it's a normal transformer. 2x6V secondaries .35VA made by a german company called BLOCK. As far as i know they are very high quality transformers.

The interesting thing, is that i am also using a 2x15V transformer of the same type for a seperate power supply, and it's secondary voltage is almost a perfect sine wave. So I'm guessing this problem only occurs for the very low voltage transformers.

Thanks

Sam
 

nec3

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Re: Mains Metering Issues

where exactly do you inject the 2.5v offset?
 

shaz

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Mains Metering Issues

for measuring ac voltages rather than using transformers use resistor dividers and precision rectifier, this would serve you a better solution.
 

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