# Home energy monitor project, help needed please! **URL:** \ **Category:** Getting Started\ **Tags:** ct, arduino\ **Created:** [24 August 2018 01:55 UTC](https://community.openenergymonitor.org/t/home-energy-monitor-project-help-needed-please/8330 "2018-08-24T01:55:25Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![BenJammin](https://community.openenergymonitor.org/letter_avatar_proxy/v4/letter/b/f04885/32.png) [@BenJammin](https://community.openenergymonitor.org/u/BenJammin)\ **Post date:** [24 August 2018 01:55 UTC](https://community.openenergymonitor.org/t/home-energy-monitor-project-help-needed-please/8330/1 "2018-08-24T01:55:25Z")
Hi, I just joined the forum as I am about to start my own home energy monitor project. I saw some great information about using a CT clamp SCT-013-000 interfaced with an Arduino. ([Learn | OpenEnergyMonitor](https://learn.openenergymonitor.org/electricity-monitoring/ctac/how-to-build-an-arduino-energy-monitor)) The schematic suggests using a 9V AC/AC power adapter (along with resistors and a cap) to measure the voltage. However the CT specs say, that using an ‘inbuilt resistor’ you can measure voltage and current directly from the CT clamp. (“Two form of output - current, voltage (voltage output built-in the sampling resistor)”) So, is the 9V supply actually necessary, and can the voltage just be measured somehow from the CT as suggested? How would it affect the schematic? Any help greatly appreciated. ---
**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:** [24 August 2018 02:15 UTC](https://community.openenergymonitor.org/t/home-energy-monitor-project-help-needed-please/8330/2 "2018-08-24T02:15:59Z")
Hi Ben, and welcome! > [@BenJammin](#): > > The schematic suggests using a 9V AC/AC power adapter (along with resistors and a cap) to measure the voltage. The “voltage” mentioned there is your _mains voltage_. > [@BenJammin](#): > > However the CT specs say, that using an ‘inbuilt resistor’ you can measure voltage and current directly from the CT clamp. (“Two form of output - current, voltage (voltage output built-in the sampling resistor)”) That refers to the voltage developed across the burden resistor. The voltage is applied to the ADC input of the 328 microcontroller and used to calculate the _current_ flowing in the conductor the CT is clipped around. The burden may be built into the CT, or external. CTs with an internal burden generate a _voltage_ output. An unburdened CT puts out _current,_ and can be connected to an _external_ burden that does the same thing an internal burden does, i.e generate a _voltage._ > [@BenJammin](#): > > So, is the 9V supply actually necessary Yes, if you want to measure _real_ power. Without the 9 V AC-AC transformer, the measurement is only an approximation of _apparent_ power. > [@BenJammin](#): > > and can the voltage just be measured somehow from the CT as suggested? No. As described above, it’s used to determine _current._ ---
**Author:** ![BenJammin](https://community.openenergymonitor.org/letter_avatar_proxy/v4/letter/b/f04885/32.png) [@BenJammin](https://community.openenergymonitor.org/u/BenJammin)\ **Post date:** [24 August 2018 02:30 UTC](https://community.openenergymonitor.org/t/home-energy-monitor-project-help-needed-please/8330/3 "2018-08-24T02:30:02Z")
Hi @Bill.Thomson! Many thanks for your speedy response! 😃 > [@Bill.Thomson](#): > > That refers to the voltage developed across the burden resistor. Is that referring to the burden resistor that I am adding into the Arduino breadboard circuit, or the ‘inbuilt resistor’ referred to in the CT spec? Sorry if this seems obvious, however I want to get my thoughts very clear before I embark on my project. 😉 ---
**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:** [24 August 2018 02:33 UTC](https://community.openenergymonitor.org/t/home-energy-monitor-project-help-needed-please/8330/4 "2018-08-24T02:33:48Z")
> [@BenJammin](#): > > Is that referring to the burden resistor that I am adding into the Arduino breadboard circuit, or the ‘inbuilt resistor’ referred to in the CT spec? The key is in the CT part number. The 000 portion of the SCT-013-000 CT part number denotes it’s an unburdened CT. i.e it has no internal burden, hence the build project is speaking about the resistor you’ll place on your breadboard. > [@BenJammin](#): > > Sorry if this seems obvious, however I want to get my thoughts very clear before I embark on my project. NOT a problem. We all had to start somewhere. 😉 ---
**Author:** ![BenJammin](https://community.openenergymonitor.org/letter_avatar_proxy/v4/letter/b/f04885/32.png) [@BenJammin](https://community.openenergymonitor.org/u/BenJammin)\ **Post date:** [24 August 2018 05:25 UTC](https://community.openenergymonitor.org/t/home-energy-monitor-project-help-needed-please/8330/5 "2018-08-24T05:25:49Z")
Thanks Bill. I think I’ll go and purchase all the bits on my shopping list with confidence now! I really appreciate your help. 👍