DVM versus Analogue

[Speedy Builder5Participant @speedybuilder5]

I don’t understand,  I am making an interface to read data from our French Smart Meter marketed as Linky.  The interface provides 3 terminals viz –

A and I1 Low voltage 130mw alternating current supply

I1 and I2  7bit serial data link.

If I put my DVM set to a/c 200 volts (Lowest a/c range) on A and I1, I get a zero reading.

If I use a cheap analogue meter on the 20 volt a/c range, it reads about 10 volts (about what is expected).

When I pass this through a rectifier bridge, both meters read 10 – 10.4 volts.

Could someone explain what is going on please.

Bob

[Andrew JohnstonParticipant @andrewjohnston13878]

A little searching comes up with this link:

https://lucidar.me/en/home-automation/linky-customer-tele-information/

The ‘power’ pins do not put out a 50Hz sine wave, but instead a 50kHz carrier with the 7-bit data amplitude modulated onto the carrier.

A digital DVM will most likely measure using dual slope integration, which will be much to slow to react to 50kHz. So it sees the signal as zero. The analogue meter will just have a half wave rectifier, ie, a series diode, so will produce a measurable waveform from the input.

The rectifier bridge produces a DC voltage as expected, which both meters will detect on DC voltage ranges.

Andrew

[Robert Atkinson 2Participant @robertatkinson2]

Andrew beat me too it. If you want to read the voltage (not that it will tell you much) you need a DMM capable of responding to higher frequencies. My favorite is the Fluke 8060A. An old, non-autoranging model but 41/2 digit and true RMS measurement up to 100kHz.
An oscilloscope, preferably storage (digital) is a better tool for the job.

Robert.

[Speedy Builder5Participant @speedybuilder5]

Thank you gentlemen, I was beginning to think that I had popped the meter interface!  The next stage after the voltage regulator is to pass through a Super Capacitor. Prices vary wildly from 46 euros to about 10, China being the cheapest again buying one offs, postage is more than the component.

Apparently the super cap is advised as a power storage device coupled with sleep mode on the Arduino to keep the power consumption within limits of the power supply (if that makes sense).

Bob

[Martin ConnellyParticipant @martinconnelly55370]

Would a low pass filter let you read the AM signal and, if necessary, amplify the information of the AM part?

Martin C

[John HaineParticipant @johnhaine32865]

Er, which voltage regulator?  And what final voltage do you want, 5V or 3.3V?

0.68 Farad (that’s 680,000 microfarads) supercaps are £1.50 each in a pack of 5 from RS.  Working voltage is typically quite low, these are 3.6V.  Best to use a switched-mode buck converter down to 3.3V and if you need 5V a boost converter from 3.3V.  This is because the tolerance on the capacitors is -20 to +80% so if you put two in series to get a higher working voltage one of them may have a significantly higher voltage on it than the other depending on tolerance – of if you can measure the capacitance then try to get the closest matching pair from the batch.

If it isn’t obvious, don’t try connecting the supercap to the bridge-rectified 10V unless you buy one (actually an array) specified for say 16V – RS do sell these but they cost more like £150!  You can buy pre-built buck/boost converter boards for peanuts.  Use a conventional say 100uF electrolytic on the 10V then feed to the converter.

[Speedy Builder5Participant @speedybuilder5]

Hi John H,  I am no electronics engineer, just following what I have found available on the net.

Basically, my Linky meter is down at the gate about 200 meters away from the house. I have an Arduino and code cobbled from the internet using twisted pair to get the signal to the house – and this works OK, but who needs a cable trailing around the house when I can get the signal via LoRa radio wherever I want.

The problem is that (apparently) there is very little power available at the meter (no 240 volt socket to power the Arduino) and the meter is sealed so I can’t break in and get power there. Hence use of what is available (Ref Andrew’s post). Yes I could use the twisted pair to the house, and have my LoRa or WiFi there – but where is the fun in that !

Once the signal has passed the Arduino, it then goes to the LoRa radio module which has more range than WiFi but this all consumes the precious power available.  The transmitting Arduino Pro Mini is the lowest power consumer once various components have been stripped off of it, the LoRa radio will be put in sleep mode except when transmitting – probably every 10 seconds and with the super capacitor doing its stuff on the 3.3volt regulated supply, apparently it becomes a working solution.

I have to say, this is none of my own work, but using what has already been achieved by others.

[Andrew JohnstonParticipant @andrewjohnston13878]

Low power electronics design is not straightforward, as in all engineering the devil is in the detail. It requires proper system design, careful consideration of power consumption at each stage and a detailed knowledge of the modules/components.

If there is a working solution, using wires, I’d stick with it. One could always put a LoRa, or other interface such as Bluetooth, at the point where the wires enter the house, and mains power is available. That way the information would be available throughout the house without the need to run extra wires everywhere.

From a simplicity viewpoint a linear IC regulator at the meter end is good. But to maximise the power available at the regulated output then I agree with JohnH that a buck converter is a better solution.

Be careful with super capacitors, read (and understand) the datasheets. When supercapacitors first came out they were used to backup RTCs and could only supply microamps. There are now supercapacitors that can accept/supply much higher currents. But as always they also have a premium price. For instance looking at some 0.68F supercapacitors from RS they have a DC impedance of 50 ohms. So with a current draw of 20mA that’s a 1V drop; not good!

Andrew

 

[John HaineParticipant @johnhaine32865]

There is theoretically a duty cycle limit in Lora to minimise band occupancy as it operates in license free spectrum.  That may limit how often you can transmit. From memory it’s 1%.

[duncan webster 1Participant @duncanwebster1]

Solar panel and battery down by the gate? I can’t remember when I last looked at my smart meter display, when it first arrived we kept looking at it, but I know tumble dried s cost a lot, so we hardly use it, but if I want a cup of tea, the kettle us going on no matter what the meter says

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