# Can I use my doorbell transformer to measure voltage for real power measurement?

**URL:** <https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336>\
**Category:** Hardware\
**Created:** [14 January 2024 23:41 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336 "2024-01-14T23:41:43Z")\
**Posts on this page:** 20\
**Page:** 1

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**Author:** ![BrutusBoots](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/brutusboots/32/28352_2.png) [@BrutusBoots](https://community.openenergymonitor.org/u/BrutusBoots)\
**Post date:** [14 January 2024 23:41 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/1 "2024-01-14T23:41:44Z")

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I have a transformer at my panel for my doorbell (see below). It’s marked 10VA. I’m thinking I could use this as a source for measuring the AC voltage in my house? I’ve gotten a CT current clamp working with EmonLib + ESP32-S3 + ADS1115/1015 and it’s giving me suitable measurements but voltage is the missing piece. I’d like to utilize this transformer since it’s readily available in my panel. I was also thinking I may power my electronics from it? (or will that affect voltage measurement?)

 ![20240114_174002](https://community.openenergymonitor.org/uploads/default/original/3X/7/e/7e43ee0103aaa4863d75a3c23ecda20355a937f5.jpeg)

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<div class="post-metadata">

**Author:** ![BrutusBoots](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/brutusboots/32/28352_2.png) [@BrutusBoots](https://community.openenergymonitor.org/u/BrutusBoots)\
**Post date:** [15 January 2024 00:07 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/2 "2024-01-15T00:07:35Z")

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Scratch the powering my device part… read more on that.

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**Author:** ![Robert.Wall](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/robert.wall/32/38246_2.png) [@Robert.Wall](https://community.openenergymonitor.org/u/Robert.Wall)\
**Post date:** [15 January 2024 01:06 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/3 "2024-01-15T01:06:03Z")

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Welcome to the OEM forum.

In theory, yes you can.

In practice, does accuracy of the result of the calculations matter to you? Do you have the expertise to recognise and counter or compensate for any errors that your transformer introduces? My concern would be that it’s not designed to faithfully reproduce a scaled-down version of your mains wave, but simply to produce enough current to ring the doorbell. And we wouldn’t be able to advise you on how to correct/compensate for whatever errors it introduces, because it’s highly likely nobody here has measured the characteristics of it. (And I’m ignoring transient effects like what happens to the waveform when somebody presses the bell!)

If you do decide it’s worth a try, there should be enough information in the ‘Docs’ section to tell you how to calibrate it - whether emonLib (I take it you mean the old Discrete Sample version) is able to apply sufficient adjustment without introducing additional errors is a different question, and you might need to heavily modify it to be able to calibrate it to get an acceptable accuracy - especially for the real power calculation.

> [@BrutusBoots](#):
>
> I was also thinking I may power my electronics from it? (or will that affect voltage measurement?)

It looks as if you’ve already abandoned this, but in any case, the answer is yes to both (if there’s enough voltage to be able to get a stable 3.3 or 5 V). Again, if the error it introduces is acceptable, that’s OK. In the emonTx V3, we reduced this error by carefully limiting the current the power supply drew at the voltage peak.

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<div class="post-metadata">

**Author:** ![BrutusBoots](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/brutusboots/32/28352_2.png) [@BrutusBoots](https://community.openenergymonitor.org/u/BrutusBoots)\
**Post date:** [15 January 2024 01:19 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/4 "2024-01-15T01:19:37Z")

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Thanks for the reply… I must say that I’ve referenced a lot of your posts when figuring out how to get my EmonLib setup up and running. Much appreciated!

It would be very convenient to power from the transformer in my panel as it eliminates me needing any plugs for any of this equipment. That being said, it sounds like it might be a bit of work to sort out if I go that route. I have enjoyed learning about the CT side of things so maybe this could be a fun challenge? 🙂

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<div class="post-metadata">

**Author:** ![BrutusBoots](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/brutusboots/32/28352_2.png) [@BrutusBoots](https://community.openenergymonitor.org/u/BrutusBoots)\
**Post date:** [15 January 2024 23:38 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/5 "2024-01-15T23:38:13Z")

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ok I have a circuit built that when connected to my doorbell transformer outputs .750V centered around 1.65V at the maximum voltage the transformer will put out unloaded. I should be able to read this with my ESP32 and toss it at EmonLib to see what output I get.

(I’m going to power my electronics separately via USB wall wart. The only action that transformer will see aside from my monitoring device is the doorbell that gets rung once every five years lol

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<div class="post-metadata">

**Author:** ![Robert.Wall](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/robert.wall/32/38246_2.png) [@Robert.Wall](https://community.openenergymonitor.org/u/Robert.Wall)\
**Post date:** [17 January 2024 15:39 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/6 "2024-01-17T15:39:23Z")

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This sounds like a 3.3 V ADC? I normally recommend designing to 1.1 V rms at the input to a 3.3 V ADC, and 1.6 V rms to a 5 V ADC (the input range, not necessarily the power supply!)

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<div class="post-metadata">

**Author:** ![BrutusBoots](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/brutusboots/32/28352_2.png) [@BrutusBoots](https://community.openenergymonitor.org/u/BrutusBoots)\
**Post date:** [17 January 2024 18:41 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/7 "2024-01-17T18:41:12Z")

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I may adjust. I want to do some testing with the ESP32 ADC which as I’m sure you’ve heard is notoriously inaccurate… I tried to keep the voltage near the center of the ADC range because that’s where the ESP32 seems to be the most accurate. As my testing progresses, I’m leaning towards using the ADS1015 for all my ADC stuff though. It’s a great little ADC.

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**Author:** ![Bill.Thomson](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/bill.thomson/32/1834_2.png) [@Bill.Thomson](https://community.openenergymonitor.org/u/Bill.Thomson)\
**Post date:** [18 January 2024 14:03 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/8 "2024-01-18T14:03:23Z")

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You may want to have a look at this:

> [@DIY Current Monitor on Raspberry Pi. My power calculation isn't accurate. Help please?](https://community.openenergymonitor.org/t/diy-current-monitor-on-raspberry-pi-my-power-calculation-isnt-accurate-help-please/13266/174):
>
> The two 100 mA c.t’s should read 110 A max, the 50 mA one 104.75 A (both with a little bit to spare). Now you can see where the money goes…

There’s some good info ~~about using the “1015” ADC~~ in that thread.

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<div class="post-metadata">

**Author:** ![BrutusBoots](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/brutusboots/32/28352_2.png) [@BrutusBoots](https://community.openenergymonitor.org/u/BrutusBoots)\
**Post date:** [18 January 2024 15:15 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/9 "2024-01-18T15:15:43Z")

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> [@Bill.Thomson](#):
>
> You may want to have a look at this:
> 
> [DIY Current Monitor on Raspberry Pi. My power calculation isn't accurate. Help please? - #174 by Robert.Wall](https://community.openenergymonitor.org/t/diy-current-monitor-on-raspberry-pi-my-power-calculation-isnt-accurate-help-please/13266/174)
> 
> There’s some good info about using the “1015” ADC in that thread.

Excellent. I will give that a thorough read. I had a GenerLink installed yesterday on my meter and while the power was off I installed my clamp sensors. I’m eager to start using them instead of my bench test setup but still a lot of development to do before I’m ready. So far in my testing the 1015 has been giving me fairly accurate readings using the calcIrms method but I really need to do some testing with the voltage sensor in place as well.

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<div class="post-metadata">

**Author:** ![Bill.Thomson](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/bill.thomson/32/1834_2.png) [@Bill.Thomson](https://community.openenergymonitor.org/u/Bill.Thomson)\
**Post date:** [19 January 2024 06:47 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/10 "2024-01-19T06:47:50Z")

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The link above is to post number 174 in that thread.  
(my mistake. wasn’t paying enough attention. 😁)

The link I _should’ve_ posted is: (this one takes you to the top of the thread)

> [@DIY Current Monitor on Raspberry Pi. My power calculation isn't accurate. Help please?](https://community.openenergymonitor.org/t/diy-current-monitor-on-raspberry-pi-my-power-calculation-isnt-accurate-help-please/13266):
>
> Hi there, I’ll try to keep this post brief and to the point. I’m currently building a home power monitor using a Raspberry Pi Zero W and the circuit diagrammed here: [Learn | OpenEnergyMonitor](https://learn.openenergymonitor.org/electricity-monitoring/ct-sensors/interface-with-arduino). My circuit uses 20 ohms for the burden resistor between CT leads. I’m in USA and I’m using a 120V 60Hz AC circuit for testing. It is functional, but when I calculate the load of the conductor my CT sensor is monitoring, it doesn’t match the load that a power meter is showing (ie, Kill-A-Watt). I put a …

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<div class="post-metadata">

**Author:** ![BrutusBoots](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/brutusboots/32/28352_2.png) [@BrutusBoots](https://community.openenergymonitor.org/u/BrutusBoots)\
**Post date:** [19 January 2024 23:15 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/11 "2024-01-19T23:15:49Z")

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Looks like the fella in that post tried the 1115 but didn’t try out the 1015 he ordered.

> [@Bill.Thomson](#):
>
> The link above is to post number 174 in that thread.  
> (my mistake. wasn’t paying enough attention. 😁)
> 
> The link I _should’ve_ posted is: (this one takes you to the top of the thread)
> 
> [DIY Current Monitor on Raspberry Pi. My power calculation isn't accurate. Help please?](https://community.openenergymonitor.org/t/diy-current-monitor-on-raspberry-pi-my-power-calculation-isnt-accurate-help-please/13266)

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<div class="post-metadata">

**Author:** ![Bill.Thomson](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/bill.thomson/32/1834_2.png) [@Bill.Thomson](https://community.openenergymonitor.org/u/Bill.Thomson)\
**Post date:** [20 January 2024 06:32 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/12 "2024-01-20T06:32:54Z")

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> [@BrutusBoots](#):
>
> tried the 1115 but didn’t try out the 1015 he ordered.

Nuts!

I forgot that he ended up using an MCP3008.  
That was 4 years ago. I’m surprised I remebered it at all! 😁

Still, plenty of good info in the thread.

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<div class="post-metadata">

**Author:** ![Robert.Wall](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/robert.wall/32/38246_2.png) [@Robert.Wall](https://community.openenergymonitor.org/u/Robert.Wall)\
**Post date:** [20 January 2024 12:14 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/13 "2024-01-20T12:14:00Z")

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> [@Bill.Thomson](#):
>
> Still, plenty of good info in the thread.

I re-read it yesterday and looking back, he did a fantastic job. Do an exact repeat (if all the information is still out there) and you won’t go far wrong.

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<div class="post-metadata">

**Author:** ![BrutusBoots](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/brutusboots/32/28352_2.png) [@BrutusBoots](https://community.openenergymonitor.org/u/BrutusBoots)\
**Post date:** [20 January 2024 14:03 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/14 "2024-01-20T14:03:45Z")

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Yep, I still read through it all 😃

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**Author:** ![BrutusBoots](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/brutusboots/32/28352_2.png) [@BrutusBoots](https://community.openenergymonitor.org/u/BrutusBoots)\
**Post date:** [20 January 2024 14:05 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/15 "2024-01-20T14:05:54Z")

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Honestly, half the fun for me is figuring out everything. There’s all kinds of off the shelf solutions and examples of devices people have made and I try to check them all out but I do enjoy the learning aspect of these types of builds. Having you two experts dumping your brains all over this forum for reference is invaluable!

> [@Robert.Wall](#):
>
> > [@Bill.Thomson](#):
> >
> > Still, plenty of good info in the thread.
> 
> I re-read it yesterday and looking back, he did a fantastic job. Do an exact repeat (if all the information is still out there) and you won’t go far wrong.

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<div class="post-metadata">

**Author:** ![BrutusBoots](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/brutusboots/32/28352_2.png) [@BrutusBoots](https://community.openenergymonitor.org/u/BrutusBoots)\
**Post date:** [20 January 2024 19:27 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/16 "2024-01-20T19:27:28Z")

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@Robert.Wall @Bill.Thomson

I have a 7.2A test load on my bench and the doorbell transformer with my above described circuit feeding into two ADS1015 ADC’s being read by an ESP32. I feel like I’m REALLY close to having this working properly and I just need some slight tweaking of the calibration values.

My kill-a-watt shows 115.1V, 0.72A, 0.99 PF. My CT is on a 10X AC line splitter and it’s seeing 7.2A confirmed by my clamp meter.

This is the output from my code. The Irms is almost bang on but the other values are iffy. Am I right to think the answer is in my calibration values?

```auto
realPower: -806.450418, apparentPower: 802.525900, powerFactor: -1.004890, Vrms: 111.712001, Irms: 7.183883
realPower: -808.429352, apparentPower: 804.401080, powerFactor: -1.005008, Vrms: 111.732405, Irms: 7.199354
realPower: -809.406273, apparentPower: 805.347553, powerFactor: -1.005040, Vrms: 111.717377, Irms: 7.208794
realPower: -790.941580, apparentPower: 786.696327, powerFactor: -1.005396, Vrms: 110.699411, Irms: 7.106599
realPower: -793.567831, apparentPower: 789.400843, powerFactor: -1.005279, Vrms: 110.671800, Irms: 7.132809
realPower: -795.405009, apparentPower: 790.970227, powerFactor: -1.005607, Vrms: 110.908621, Irms: 7.131729
realPower: -794.114144, apparentPower: 789.991446, powerFactor: -1.005219, Vrms: 110.910364, Irms: 7.122792
realPower: -790.627114, apparentPower: 786.579755, powerFactor: -1.005146, Vrms: 110.716731, Irms: 7.104434
realPower: -806.045420, apparentPower: 801.995503, powerFactor: -1.005050, Vrms: 111.954315, Irms: 7.163596
realPower: -796.708321, apparentPower: 792.448003, powerFactor: -1.005376, Vrms: 110.716234, Irms: 7.157469
realPower: -793.847827, apparentPower: 789.626218, powerFactor: -1.005346, Vrms: 110.732209, Irms: 7.130953
realPower: -795.405121, apparentPower: 791.249223, powerFactor: -1.005252, Vrms: 110.878893, Irms: 7.136157
realPower: -794.032091, apparentPower: 789.908474, powerFactor: -1.005220, Vrms: 110.629336, Irms: 7.140136
realPower: -794.387068, apparentPower: 790.215660, powerFactor: -1.005279, Vrms: 110.829208, Irms: 7.130031
realPower: -796.982294, apparentPower: 792.788894, powerFactor: -1.005289, Vrms: 110.872116, Irms: 7.150480
realPower: -810.542431, apparentPower: 806.298440, powerFactor: -1.005264, Vrms: 111.870528, Irms: 7.207425
realPower: -794.233934, apparentPower: 790.057322, powerFactor: -1.005286, Vrms: 110.651326, Irms: 7.140062
realPower: -790.023375, apparentPower: 785.727074, powerFactor: -1.005468, Vrms: 110.542516, Irms: 7.107917
realPower: -803.265009, apparentPower: 798.571866, powerFactor: -1.005877, Vrms: 111.057805, Irms: 7.190597
realPower: -795.229142, apparentPower: 790.886224, powerFactor: -1.005491, Vrms: 110.769297, Irms: 7.139941
realPower: -797.786514, apparentPower: 793.690712, powerFactor: -1.005160, Vrms: 110.905961, Irms: 7.156430
realPower: -795.726083, apparentPower: 791.167977, powerFactor: -1.005761, Vrms: 110.720662, Irms: 7.145622
realPower: -809.444397, apparentPower: 805.581942, powerFactor: -1.004795, Vrms: 111.807714, Irms: 7.205066
realPower: -794.953964, apparentPower: 790.607379, powerFactor: -1.005498, Vrms: 110.844495, Irms: 7.132581

```

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<div class="post-metadata">

**Author:** ![BrutusBoots](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/brutusboots/32/28352_2.png) [@BrutusBoots](https://community.openenergymonitor.org/u/BrutusBoots)\
**Post date:** [20 January 2024 19:30 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/17 "2024-01-20T19:30:50Z")

</div>

unrelated, it’s neat to change the clamp orientation and see the realPower change sign

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<div class="post-metadata">

**Author:** ![Bill.Thomson](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/bill.thomson/32/1834_2.png) [@Bill.Thomson](https://community.openenergymonitor.org/u/Bill.Thomson)\
**Post date:** [20 January 2024 19:54 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/18 "2024-01-20T19:54:55Z")

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The first thing I notice is your voltage measurements are about 10 volts low.  
(unless of course, your line coltage _really is_ 110 Volts, but it should be closer to 120 V)

Have you measured your line voltage with a reasonably accurate meter?

The other thing that sticks out is your measured power factor.  
PF can never be greater than 1, so you have an issue there as well.

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<div class="post-metadata">

**Author:** ![BrutusBoots](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/brutusboots/32/28352_2.png) [@BrutusBoots](https://community.openenergymonitor.org/u/BrutusBoots)\
**Post date:** [20 January 2024 19:58 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/19 "2024-01-20T19:58:58Z")

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yes, I measure 115.x V with my multimeter and the kill-a-watt agrees. I’ll keep testing but I figured I’d get some input on what I’m seeing so far. I was thinking my calibration values were to blame but I’ve double checked and I think I have them calculated properly.

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<div class="post-metadata">

**Author:** ![Bill.Thomson](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/bill.thomson/32/1834_2.png) [@Bill.Thomson](https://community.openenergymonitor.org/u/Bill.Thomson)\
**Post date:** [20 January 2024 20:03 UTC](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336/20 "2024-01-20T20:03:52Z")

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> [@BrutusBoots](#):
>
> but I’ve double checked and I think I have them calculated properly.

If your line voltage measures ~115 VAC, and your device is reporting ~110 VAC, that says _to me,_  
at least _one_ cal factor is off.

Then there’s the Power Factor. Something’s off there too.

[Next page](https://community.openenergymonitor.org/t/can-i-use-my-doorbell-transformer-to-measure-voltage-for-real-power-measurement/25336.md?page=2)