Author Topic: Safest way to measure AC mains 220-250 voltage?  (Read 4432 times)

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Offline soldar

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Re: Safest way to measure AC mains 220-250 voltage?
« Reply #25 on: July 19, 2019, 03:21:04 pm »
I do not understand how those things work. It would seem to me that it would not distinguish between reactive and resistive loads.

I would check to see how precise they are because bad information is worse than no information.

Whatever trick they use I see they have a USB model which you can connect to a computer if that's what you want to do.

Personally I have a similar device but it is placed at the mains panel and is connected to the wires so it does have voltage information as well as using that to supply power so it does not need batteries. I find that is good enough for me as the panel is right at the front door and I feel no need to carry the receiver around with me around the house.

Most of the time I am only concerned with the current being used because in Spain we have current limiters and pay by the max current you can pull so I have a lower current limit but have to watch how much current I am pulling at any one time and disconnect something if I am over the limit.
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Offline paulcaTopic starter

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Re: Safest way to measure AC mains 220-250 voltage?
« Reply #26 on: July 19, 2019, 06:14:45 pm »
Peak demand based billing is only applied to large commercial and industrial supplies here.  Thankfully.

I think we have different aims.  If the energy monitor I have tells me I'm drawing 450W and that figure it out by 10% because one of the loads is inductive like the washing machine, I'm fine with that. 

I do not need this to verify or cross check the electric utility meter (magnetic rotating disc and clockwork digits type).  I really just want indicative, comparable measurements.   

If logged and graphed this can show me an estimate of the base load and see where and how hard the spikes hit when the "big hitters" are used, shower, washing machine, cooker, space heaters etc.

As to hacking into an off the shelf module, it's took fiddly.  As to the USB style monitors; they often save a CSV file to a memory card and so not realtime data, that or a prepriatory file which you have to pen with their "app".

I might just follow the open energy monitor spec and see how far I get.
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Offline richnormand

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Re: Safest way to measure AC mains 220-250 voltage?
« Reply #27 on: July 19, 2019, 09:19:22 pm »
Interesting timing for your post. 

A few days ago Ontario Hydro had a statement about a test where they would lower the mains voltage by 5% to test their various systems across the province.
So, using stuff in the misc and spares drawer, I put a test system to monitor the line voltage behaviour.
I was not comfortable going straight to the mains so I located a wall-wart that had a non-regulated AC output (ie: just a transformer).
I then put together a full bridge rectifier with very low filtering  and a resistive load on it so it would react fast to changes in voltage (just the opposite of what you do for a good supply). I then put a resistor/LED to signal the system is on.

For monitoring I used my Lascar voltage monitor. Could have used the computer monitoring on the DMM but the Lascar is simpler.

Photos:
Setup.
Results.
Used a Variac to change the line voltage and get a calibration reading rms voltage from DC generated.

Edit: just realised that the dip was in sample units. The 5% voltage drop was from 2:00 to 3:00 pm.
« Last Edit: July 19, 2019, 11:07:51 pm by richnormand »
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Offline james_s

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Re: Safest way to measure AC mains 220-250 voltage?
« Reply #28 on: July 19, 2019, 10:43:18 pm »
Is there a reason you put a rectifier and capacitor on it instead of just taking the raw AC out of the transformer? I suppose it shouldn't make much difference either way but I figure it's less stuff to potentially influence the reading.
 

Offline richnormand

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Re: Safest way to measure AC mains 220-250 voltage?
« Reply #29 on: July 19, 2019, 10:48:58 pm »
No not really, but in my case the Lascar only take 0 to 30VDC as an input.
All depends on what you have to monitor mains voltage.

Edit: just added in my previous post that the voltage dip was one hour long. Most people probably never noticed.

« Last Edit: July 19, 2019, 11:09:38 pm by richnormand »
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Offline soldar

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Re: Safest way to measure AC mains 220-250 voltage?
« Reply #30 on: July 20, 2019, 08:12:06 am »
If logged and graphed this can show me an estimate of the base load and see where and how hard the spikes hit when the "big hitters" are used, shower, washing machine, cooker, space heaters etc.

In this case you can just measure current, you do not need to measure voltage. If you are going to assume constant voltage then measuring only the current is a directly related magnitude.

But you need to measure RMS current because measuring peak current is going to be very misleading. A diode charging a capacitor is going to give you a big peak.

So you can put the 50 mA of the current transformer through a bridge and get DC pulses. A current transformer should never be left open so I put a Zener diode to prevent that. The Zener is just a protection and has no use in normal operation.

I guess you could follow with a low pass filter to get rid of current spikes.

ETA: I guess I am a bit confused because the title of the thread is how to measure voltage but then in the discussion the OP says he wants to measure power and without need for precision and power is correlated with current since we can assume voltage is stable. So I am not really sure what the purpose of all this is.
« Last Edit: July 20, 2019, 09:00:09 am by soldar »
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Offline paulcaTopic starter

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Re: Safest way to measure AC mains 220-250 voltage?
« Reply #31 on: July 20, 2019, 11:42:15 am »
ETA: I guess I am a bit confused because the title of the thread is how to measure voltage but then in the discussion the OP says he wants to measure power and without need for precision and power is correlated with current since we can assume voltage is stable. So I am not really sure what the purpose of all this is.

I wanted to find the easiest and safest way to measure the voltage, but then started to question if I needed to or not.

With the current clamp, through a burden resistor and into a voltage divider to size it for my MCU there is very little that can go wrong, there is no mains connection. 

When I come to measuring voltage, I need to actually connect something to the mains and I don't like mains.

I'm still not 100% decided on whether to try and measure the voltage or not and if I do, how.
Option 1: Chinese module: https://uk.banggood.com/Single-phase-AC-Active-Output-Voltage-Transformer-Voltage-Sensor-Module-p-1228141.html?gmcCountry=GB&currency=GBP&createTmp=1&utm_source=googleshopping&utm_medium=cpc_bgcs&utm_content=garman&utm_campaign=pla-gbg-ele-diy2-pc&ad_id=339489662626&gclid=CjwKCAjw98rpBRAuEiwALmo-yi4ig3-CtTFTSgPBmLTfxfxN_iy3w0Mn14YECcKBKPcyJBi3743RDhoCZcUQAvD_BwE&cur_warehouse=CN

Option 2: Ac-Ac power supply, but finding the right one that isn't regulated does not appear to be easy, although I could modify it.  Then I need to start considering the transformer phase shift, voltage dividers, offsetting the voltage etc. etc.
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Offline MrAl

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Re: Safest way to measure AC mains 220-250 voltage?
« Reply #32 on: July 20, 2019, 12:16:09 pm »
Hello,

When measuring AC voltage and current usually the AC voltage is converted into DC and then the DC level is calibrated against the measured AC level.  This way you get good readings.

For voltage, one way to do it is to rectify the AC into peak AC which is DC, then calibrate it using known AC voltages.
The drawback to this method is that if the peak is loaded down (often the case with softer AC lines) then you dont get an accurate reading unless of course you are just interested in the peak AC.
You could look into averaging methods for better results.

For AC current, it is usually to use a current transformer.  The current transformer output is rectified into DC.  The DC is then calibrated against a known AC current.

The reason for the calibration in these methods is because each method is somewhat non linear and thus the scale factor changes depending on actual signal level.  It's not too hard to do though just input some known levels and note the output then include that in your measurement algorithm.

Often a microcontroller is used to do the measuring.

Also note that playing with live mains is not always a good idea.  To measure voltage an AC transformer can be used that steps the voltage down to a more manageable level and also isolates your measurement equipment from the live mains line.
 

Offline richnormand

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Re: Safest way to measure AC mains 220-250 voltage?
« Reply #33 on: July 20, 2019, 03:49:31 pm »
Hello,

When measuring AC voltage and current usually the AC voltage is converted into DC and then the DC level is calibrated against the measured AC level.  This way you get good readings.




Indeed for voltage using a wall-wart with an AC transformer is most likely the safest way to have isolation and low voltages for hobby measurements. Calibration is easy like pix 3 of post 27 in this thread.......
Current measurement will require a loop/ring to keep a non-contact setup an can be calibrated using various loads on the circuit.

« Last Edit: July 21, 2019, 07:39:42 pm by richnormand »
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Offline paulcaTopic starter

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Re: Safest way to measure AC mains 220-250 voltage?
« Reply #34 on: July 20, 2019, 04:35:14 pm »
When measuring AC voltage and current usually the AC voltage is converted into DC and then the DC level is calibrated against the measured AC level.  This way you get good readings.

For voltage, one way to do it is to rectify the AC into peak AC which is DC, then calibrate it using known AC voltages.

Now I'm even more confused.  Why do I need to rectify them?  If I offset them so that I end up with, say, 0-3V with 1.5V being the mid point, when I do the RMS calculation it will (by it's squaring nature) end up positive.  Although I can see that a rectified waveform would give more accuracy as I could use the full 0-3V for both positive and negative giving twice the accuracy.

Also will rectifying the voltage not change it requiring I calibrate out the diode drops?  I realise rectifying the current transformer will have little to no effect as it's a current source.

So it's looking like I do need to measure the voltage and I might as well go the full RMS voltage RMS current calculation.

I already have the current transformer.  Gives me 50mA per 100A, a correctly calculated burden resistor and an offset divider.

For voltage I'm going to need a 12Vac adapter and a voltage divider to get it down to 3Vac offset by 1.5V.

What about this transformer phase issue?  Do I need to worry about it?  How much will it shift the phase by?  Is it determinable/measurable/negligible?
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Offline paulcaTopic starter

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Re: Safest way to measure AC mains 220-250 voltage?
« Reply #35 on: July 20, 2019, 04:48:25 pm »
I found this:
https://meters.co.uk/products/single-phase/

Would need the electrician to install it, but it has an RS485 MODBUS output and it would mean I don't need to feck about with the mains or blow myself up.

Thoughts?
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Offline soldar

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Re: Safest way to measure AC mains 220-250 voltage?
« Reply #36 on: July 20, 2019, 05:32:52 pm »
I found this:
https://meters.co.uk/products/single-phase/

Would need the electrician to install it, but it has an RS485 MODBUS output and it would mean I don't need to feck about with the mains or blow myself up.

Thoughts?
Looks nice. I think you can install it yourself but if you feel more comfortable having a pro do it then go for it. If you do it yourself you can get the split core which is easier to install. If you get a pro to do it then you can get the solid core which gives a bit better precision.

Basically it is the same that I have except mine has no communications capability. Mine is just the cheap Chinese eBay version. Just connect two wires to voltage and the current transformer around the main wire. You can do it with the general breaker cut off so there is no risk involved. Easy peasy.

This one costs the same and looks a bit better if you have the space for it.
« Last Edit: July 20, 2019, 06:15:46 pm by soldar »
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Online ledtester

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Re: Safest way to measure AC mains 220-250 voltage?
« Reply #37 on: July 24, 2019, 05:04:43 am »
Here's a design which utilizes HCPL-7520 linear optoisolators to provide isolation:

https://atmega32-avr.com/powerbox-the-safe-ac-power-meter-using-atmega32/

Perhaps others can comment on whether or not this part is appropriate for this kind of use.


Also, a couple of power meter projects that I've found useful to study:

- http://elm-chan.org/works/heco/report_e.html

- http://ww1.microchip.com/downloads/en/AppNotes/Atmel-2566-Single-Phase-Power-Energy-Meter-with-Tamper-Detection_Ap-Notes_AVR465.pdf

In particular, the way you measure the current has to be carefully considered since that has quite a large dynamic range -- from a few milliamps to several amps. The Elm Chan project solves that by using a 16-bit ADC. The Atmel/Microchip design uses op amps whose gains are dynamically controlled by the microcontroller.
« Last Edit: July 24, 2019, 05:08:52 am by ledtester »
 


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