TheBackShed.com

Forum Index : Microcontroller and PC projects : Measuring AC voltages with an MM170

Author

Message

Bizzie
Senior Member

Joined: 06/07/2014
Location: Australia
Posts: 192

Posted: 10:23pm 25 Oct 2018

Copy link to clipboard

Print this post

Hi All,

We have a problem with the pressure unit supplying water to our house and gardens. I would like to log when the pump is on so I purchased a Split core current transformer here is the Datasheet .

The current ratio is 30A/15mA AC (milli volts) of course.

What is the easiest way to convert the above to a usable input to the MM?

I am close to ignorant when it comes to the electronics to do the above.

Rob White

hitsware
Guru

Joined: 23/11/2012
Location: United States
Posts: 535

Posted: 11:05pm 25 Oct 2018

Copy link to clipboard

Print this post

Perhaps you can adapt this .....

Grogster

Admin Group

Joined: 31/12/2012
Location: New Zealand
Posts: 9610

Posted: 11:54pm 25 Oct 2018

Copy link to clipboard

Print this post

Are you wanting to measure voltage or current?
Or both?

Those CT's (current transformers) are for measuring AC current, not for measuring voltage.

How much current does your pump motor suck under load?
To get even 50mV output from those CT's, you'd need an AC load of 10-amps.....

Once you get down to 50mV or so, it is going to be too hard for the MM to reliably read, so you would normally connect the CT to some kind of amplifier stage before you connect it to the MCU input.

Voltage is much easier. You could just use another mains transformer on the pump wiring, and with a 230v(or 110v) primary and a 3v secondary, bridge rectifier and decoupling cap(and perhaps zener snubber diode), you can feed that directly into the MM input pin. If the pin is high, the pump voltage is on, if it is low, the pump voltage is off.

Can you clarify if you want to measure the voltage being switched to the pump, or the currrent sucked by the pump - or both. That will help us to help you.

Smoke makes things work. When the smoke gets out, it stops!

bigmik

Guru

Joined: 20/06/2011
Location: Australia
Posts: 2950

Posted: 12:08am 26 Oct 2018

Copy link to clipboard

Print this post

GDay Bizzie,

What sort of current does your Pump usually draw?

It seems to me that unless it was to draw 20A or more it would produce pretty small voltages. ie. if draws only 5A it would output only 25mV.. (using the 10Ohm load as shown in the datasheet). To detect this you would need your MM close to the sensor.. as you would drop voltage across any long run.

Also you need to put this clamp over only the ACTIVE (or Neutral) wire not just the entire lead (clamped over both the active/neutral) as this would counter act and read a ZERO reading as the current going down the active would put a + read and the current going the neutral would put a - read which would cancel each other.

Do you just want to DETECT that the pump is running or measure the current itself?

If you only wish to detect when it is powered there may be other options..

I would like to see some more details of how the pump is powered (fixed wiring or 240V mains lead) where is the detection control of how the pump knows when to turn on etc.

Kind Regards,

Mick

Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<<

bigmik

Guru

Joined: 20/06/2011
Location: Australia
Posts: 2950

Posted: 12:09am 26 Oct 2018

Copy link to clipboard

Print this post

Damn Grogster,

You always beat me to the punch whilst I am typing..

Remind me not to go into a boxing ring with you..

MikEdited by bigmik 2018-10-27

Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<<

Chopperp

Guru

Joined: 03/01/2018
Location: Australia
Posts: 1097

Posted: 12:16am 26 Oct 2018

Copy link to clipboard

Print this post

I use one of these on my grey water connected to a counter input on a Maximite. Works well.
You may need a larger diameter one for a pressure pump.

ChopperP

Bizzie
Senior Member

Joined: 06/07/2014
Location: Australia
Posts: 192

Posted: 12:23am 26 Oct 2018

Copy link to clipboard

Print this post

Thanks all for your quick replies!

@Hitsware I am aware of that doc.

@Grogster

Quote Are you wanting to measure voltage or current?
Or both?

I was going to assume voltage is 240v and wanted to measure current as there are other things I was thinking I could use this for. I am aware the clamp must only go over one wire (thanks Bigmik).

@Bigmik

Quote What sort of current does your Pump usually draw?

Not sure at all it is a variable speed motor (read almost constant pressure) I am guessing about 1 Hp. The pump is plugged into a power point I will go and see what my multi meter and the CT measures and report back.

Thanks all

Rob White

Bizzie
Senior Member

Joined: 06/07/2014
Location: Australia
Posts: 192

Posted: 01:31am 26 Oct 2018

Copy link to clipboard

Print this post

An update:-

Here is my test setup

Here I have it hooked up to my oil heater the label says 2000w so that looks about right (see page two of datasheet above) at say 8amps.

Now the pump reads 10.3 mV so that's about 2amps

The pump label says 1.48kW (say 6amps?) max of course.

Rob White

Bizzie
Senior Member

Joined: 06/07/2014
Location: Australia
Posts: 192

Posted: 01:40am 26 Oct 2018

Copy link to clipboard

Print this post

I have just watched this youtube video.

Is the dangers he talks about (at about 3:30mins) of real concern?
If correct these should not be sold with the exposed plug!

Rob White

Bizzie
Senior Member

Joined: 06/07/2014
Location: Australia
Posts: 192

Posted: 01:45am 26 Oct 2018

Copy link to clipboard

Print this post

@ChopperP

Hey that makes my mind spin a few of those around the system will tell me lots!
Must see if I can locate some larger ones I think TassieJim uses 3/4" ones.

Rob White

Grogster

Admin Group

Joined: 31/12/2012
Location: New Zealand
Posts: 9610

Posted: 01:46am 26 Oct 2018

Copy link to clipboard

Print this post

You could TRY to read that with a MM just for the purposes of experimentation, but they are TINY voltages, and would be rather prone to background noise etc. Especially if you plan to cable that tiny voltage to your MM - that probably won't work for the reasons stated by Mik.

You're talking MovingMagnet record player cartridge or guitar pickup levels here.

It's still do-able, but you will need some kind of amplifier for the output of the CT, before you cable to your MM. This could be as easy as an op-amp with a gain of 10 or so.

I suggest you use a module such as this for three bucks. It has the CT and the op-amp voltage booster on-board for you, and an adjustable output voltage. Set it up, and job done. Even via some cable to the MM.

Or there is this slightly more expensive module which can output a serial datastream to your MM's COM port, and you just read the data.

Smoke makes things work. When the smoke gets out, it stops!

Turbo46

Guru

Joined: 24/12/2017
Location: Australia
Posts: 1642

Posted: 02:20am 26 Oct 2018

Copy link to clipboard

Print this post

Bizzie, Please be aware of the dangers of open circuiting a current transformer (CT) see here under the safety heading. where I used to work we used several power diodes in series (two lots in opposite directions) connected AT THE CT TERMINALS to protect against an open circuit. The output is a constant current so you could experiment with larger current sense resistors.

Bill

Keep safe. Live long and prosper.

Bizzie
Senior Member

Joined: 06/07/2014
Location: Australia
Posts: 192

Posted: 02:39am 26 Oct 2018

Copy link to clipboard

Print this post

Thanks Turbo I gathered that from the YouTube video.

I think I will order some of the second units Grogster linked to. That is much more like what I am used to.

Thanks all who have contributed.

Rob White

Turbo46

Guru

Joined: 24/12/2017
Location: Australia
Posts: 1642

Posted: 03:18am 26 Oct 2018

Copy link to clipboard

Print this post

Bizzie, One more thing, put protection at the CT terminals (it could be two back to back 3.3v zener diodes) and the current sense resistor at the point where you will be measuring the voltage. It is a constant current output so there is no concern about voltage drop.

Bill

Keep safe. Live long and prosper.

Chopperp

Guru

Joined: 03/01/2018
Location: Australia
Posts: 1097

Posted: 03:24am 26 Oct 2018

Copy link to clipboard

Print this post

Bizzie said @ChopperP

Quote

I think TassieJim uses 3/4" ones.

Yeh, he does. He set me some links to some a while ago but I couldn't work out if the 3/4" or whatever referred to the connection thread size or the actual pipe size. Might ask him one day if I get around to modifying my system

ChopperP

TassyJim

Guru

Joined: 07/08/2011
Location: Australia
Posts: 6283

Posted: 04:08am 26 Oct 2018

Copy link to clipboard

Print this post

My flow sensors are 3/4inch BSP thread with internal diameter about 15.8mm
they are rated 1-60 L/M which is more than enough for me.

If you go for bigger ones, any low flow rate is likely to be inaccurate/unreliable.

Re CTs
Open circuit can hurt.
Some have resistors built in as a safety measure but most don't.
As well as having an output too small for the micromite, AC is not easy. Doable but not easy.
If all you want is an on/off indicator, amplify the CT output by enough to force it into a square wave. Then look for pulses.

I have a power board that turns the slave outlets on when it senses load on the control. It makes turning on my radio a one-switch instead of three exercise (and reduces standby power wastage).

I am sure that Silicon Chip did a project for one a few years ago. You would get some ideas from that circuit.

Jim

VK7JH
MMedit

Print this page

To reply to this topic, you need to log in.