EmonPi aditional CT sensor

Submitted by CidiRome on Wed, 02/12/2015 - 18:28

Hi.

I've seen here (http://openenergymonitor.org/emon/buildingblocks/how-to-build-an-arduino-energy-monitor) and other places on this forum that it is fairly simple to build the circuitry to hook a CT sensor to the arduino.

There is one thing that is causing me doubts: I see almost everywhere references about the burden resistor of 22Ohm when 3.3V supply voltage is used, but when I analyzed the EmonPi circuitry I can see two 22Ohm in parallel (R17/R22 or R26/R28) that would result in 11Ohm burden.
Can this be some kind of option between two different types of resistors that in the build moment only one is applied?

In this circuit what are the necessary changes to modify the supply voltage from 3.3V to 5V and keep the same calibration values inside the EmonPi Firmware?

Cheers.

Re: EmonPi aditional CT sensor

Submitted by Robert Wall on Wed, 02/12/2015 - 19:05.

You are right. If you look at the PCB Layout, you will see one is a SMT component and the other is wire-ended. If you look inside your emonPi, you will find the wire-ended component is not fitted.

Calibration is not affected, provided that you use the correct preprocessor directive inside your sketch. Details are in emonLib in the comments.

If you are feeding the top of R27 with 5 V, then very obviously R25 and R27 can no longer be equal, and you must change the value of one so that the voltage at their common junction is as close to 1.65 V (i.e. the mid-point of the 3.3 V ADC input range) as possible. That does not of course affect calibration.

Re: EmonPi aditional CT sensor

Submitted by CidiRome on Wed, 02/12/2015 - 19:35.

Thank you for the clarification.

I don't have easy access to my EmonPi board side because it is installed under the Raspberry and fixed to the plastic box where I have the material.

Regarding to the changes need for 5V supply I read the information you already gave me

Regarding the bias supply for the outboard input, I would use 5 V and keep all the same components except for the resistor in the R27 position, which needs to be adjusted so that the voltage at the junction with (C4) and (R25) is 1.65 V, which gives a value of 954 kΩ. That, very inconveniently, is almost exactly mid-way between two E24 preferred values. Using 910 kΩ, the voltage is fractionally closer, but 1 MΩ is only fractionally worse and probably easier to obtain. You will lose about 1.5% of the range, which is nothing to worry about. Or you could use 910 kΩ + 43 kΩ in series. If you can get a 953 kΩ resistor, that would be ideal, but values in the E48 series can be hard to find and costly.

You advised me to change the value of R27, but the result is a hard to find resistor value.
I have read else where that we can change R25 and R27 to a good range of values that shell be equal for the 3.3V supply, but maybe we can change both (R25 and R27) to easy to find values as long we have a ratio of 1.65V and 3.35V to make the correct voltage divider...

Later I will do some math on this.

Cheers.

Re: EmonPi aditional CT sensor

Submitted by Robert Wall on Wed, 02/12/2015 - 19:47.

You can use resistors in series or in parallel. There are many ways to 'play the numbers' and it is possible that you will find a combination that comes close, and that is because the resistor values are rounded from their theoretical values, which are 12 or 24 equal steps by ratio per decade.

Look at the picture of the Pi or an emonTx, or an emonTx Shield, for the burden. All do it similarly.

Re: EmonPi aditional CT sensor

Submitted by CidiRome on Wed, 02/12/2015 - 23:17.

Hi.

I think I will change my approach to this situation, the values are not easy to match, the best pair I could find was 787 / 390 and even this not very good.

Then I started to think that if I connect this to the +5V rail (at the RJ45 port) I will certainly get power fluctuations because this output is a direct connection to the USB input while if I get the 3.3V from the EmonPi I can have the same reference voltage for all the CTs and don't have any trouble with the resistor values. I think a good source may be the ISP1 connector that I don't remember being connected to anything.

Cheers.

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