Logging mains voltage: Arduino or Raspberry Pi?

[tony sayer]

Care to expand of that statement Dennis?..

[jrwalliker]

Most small mains transformers are run very close to core saturation, so any large transients are likely to be clamped by the transformer. This could be overcome by putting a resistor in series with the transformer primary so that it is running at a small fraction of mains voltage. Have a load resistor on the secondary may improve the performance as well.

John

[The Natural Philosopher]

Oh dear. A pseudo authoritative statement from someone who doesnt know the difference between voltage and current.

This

Oh dear. Soemone who doesnt understand inductance either.

[jrwalliker]

So do you disagree with my claim that small transformers are usually run close to core saturation at their rated voltage?

Do you disagree with my expectation that grossly overdriving such a small transformer (with the transient to be measured) will cause its output to saturate and give an incorrect measurement result?

What exactly do you object to here? A resistively loaded transformer looks like a resistor (apart from the leakage inductance) so in conjunction with a series input resistor it will form a reasonable potential divider thereby moving the operation well away from saturation.

Please explain.

John

[Roger Hayter]

Saturation results from current flow. If the transformer has a high impedance across the secondary saturation will not happen. (The voltage output may not be precise for other reasons, but should be good enough for the sort of transients that are going to affect a heating element.)

The load resistor will not improve anything. The resistor in series with the primary will just make accurate voltage measurement impossible because of the poorly defined and frequency dependent primary impedance while providing no benefits.

[The Natural Philosopher]

Yes

Yes.

I am not here to teach you basic electronics when you think you know it all already

[The Natural Philosopher]

I admire your patience

[dennis]

What's to explain? they isolate the mains unlike autotransformers or some other ways to drop mains voltage. You aren't going to get a sever shock from a 6V isolating transformer like you could from a potential divider using resistors.

[newshound]

Modern Earlex, China I expect. 40 year old Earlex, UK I expect.

Today, it was 250. Sampling over a few days: THAT'S WHY I WANT A DATA LOGGER

and are

As I said earlier, it's rural on overhead lines

Sometimes but not always. They are failing open circuit, some but not all fail a megger test.

[Johnny B Good]

An effective way to avoid saturation issues when monitoring for excessively high voltage excursions is to wire up an identical pair with their 240v primaries in series and their low voltage (6 or 7 vac?) secondaries in parallel (in phase current aiding) so as to help maintain voltage balance between the primaries. Not only will this allow for 100% excursions beyond the nominal 240vac supply, it should also allow high frequency transients to be registered without molestation by saturation effects. However, unless the transformers incorporate a 'grounded' inter winding screen, high frequency transients can couple capacitively into the secondary circuit, effectively magnifying their prominence.

If this is just a temporary setup, you can use a PC or laptop to record the waveform using an audio recording application. Unless you need to detect extremely high frequency transients, you can choose an 8 or 16 bit mono sampling rate of 8 or 16 or 22.05 KHz to save disk space if planning on logging for more than 12 hours worth (Heads Up! 24 hours in 16 bit stereo at a 44.1KHz sample rate produces a 7GB file! DAMHIK, IJK).

If your main interest is checking for extreme voltage excursions rather than high voltage spikes. adjust the recording level on the 50Hz fundamental to -10dB FSD otherwise choose -20dB FSD if you're looking for high voltage spikes (sub millisecond transients). A -10dB setting allows you to see overvolting events just in excess of +200% and a -20dB setting will let you identify spikes as large as 2.4KV without clipping (assuming the transformer assembly can deal with such spikes).

A quick 'n' dirty way to check whether there were any 'events' worth zooming in on, is to ask the audio app to calculate a normalisation factor for the whole period. A normalisation factor of 9dB on a recording level of -10dB FSD represents a 12.2% voltage boost. OTOH, a normalisation boost value of 9.172dB represents a 10% overvolting event on a -10dB recorded level.

HTH & GL!

[charles]

a 6v transformer is just a transformer. Isolating transformers do just that _ they isolate - mains level voltage on the output. Used to be an essential workshop item when dealing withn live cahssis equipment.

[charles]

ask your supply co to install a voltage logger.

[dennis]

8<

Not all transformers are isolating! Why do you suppose I said an isolating transformer rather than an auto transformer or any other type with a single coil?

[jrwalliker]

The OP is convinced that slow voltage surges are the problem not fast high-voltage transients. However, this is just an assumption - there might be fast transients causing insulation breakdown. Nobody actually knows.

Therefore it would make sense to try and achieve a reasonable bandwidth if it can be done with little extra effort.

It might be something completely different such as corrosion of the elements.

Its an interesting problem. I will have a rummage around for a suitable transformer and make some measurements and report back in a few days.

John

[Roger Hayter]

Not all, but nearly all! Except for special applications for autotransformers, most applications require not only isolation but an insulation resistance tested to some insulation resistance standards, if only to avoid killing people who touch the secondary circuit and/or tripping RCDs when the secondary circuit is permanently or transiently connected to earth. Reference to an "isolating transformer" tends to imply one used *only* for isolation with a one to one ratio of turns.

[jrwalliker]

A very good approach if two identical transformers are available.

John

[Roger Hayter]

Just to generalise, and by no means to accuse the OP, the commonest cause of submerged element failure is probably allowing the water level to get too low, or possibly displaced sideways if the vessel is not kept horizontal.

[Roger Hayter]

FAOD, I should have said nearly all *mains* transformers. There are of course other kinds.

[Dave Liquorice]

Just report the "high" voltage, 250 is only 3 short of the permitted maximum. The DNO for here jumps pretty damn quickly at reports of wide voltage variation and/or approaching the limits.

If they do install a logger you don't get to see the data.

High voltage will shorten the life incandescant bulbs quite a bit. We used to get through a bulb every week or two. Bought a UPS plugged it in it went straight into voltage reduction mode. Measured voltage,

250 ish, rang DNO, engineer at door two hours later, transformer tapping adjusted next or the day after, bulb consumption fell noticabley...

I wouldn't expect a 1? 2? kW water heating element to be as sensitive to voltage as light bulbs unless it's really cheap and nasty. Or made such that 240 is the max rather than nominal voltage...

[The Natural Philosopher]

THERE ARE NO SATURATION ISSUES UNTIL CURRENT IS DRAWN

WE ARE NOT DRAWING POWER