Advice requested on EmonTX analogue input voltage & impedance


I have a 24V DC solar system. I am building a DC current monitor to measure consumption. For this I need 2 analogue inputs.

I have an EmonTX V3 and a V2 available. I have more or less decided to use the V2 as the analogue inputs are easier to connect to.

I am allowing for a maximum system voltage of 34V even though the highest battery voltage ever logged on the PV controller is 29.5V

As a voltage divider I had planned on R1 as 15K and R2 as 2.7K with a 5.1 zener to ground giving a maximum of 5V on the input. Reading a thread on the forum I learned that the analogue input maximum should be 3.3volts on the EmonTX V2. Is the analogue input 5V tolerant?  Or should I change the resistor network and zener to give a maximum 3.3V. 

Also I have read the impedance on the analogue inputs should be low. I believe I have chosen suitable resistor values but any comments would be appreciated. I have also seen refernece to using an OP amp between the resisters and the analogue input. Would this be of any benefit in this case? 



alco's picture

Re: Advice requested on EmonTX analogue input voltage & impedance

Q:Is the analogue input 5V tolerant?
A:No it isn't so make a voltage divider to match the max of 3.3. Volts.


Robert Wall's picture

Re: Advice requested on EmonTX analogue input voltage & impedance

The source impedance feeding the analogue input should, according to the data sheet, be less than 10 kΩ. This is to give enough current for the sample-and-hold capacitor to charge adequately when the multiplexed switches between inputs (all done inside the processor of course). However, you should also make the impedance high enough to not overload the input protection diodes that clamp the input to the supply (the 3.3 V rail in your case) in case of overvoltage. Greater than 1 kΩ should be adequate. But it would be wise to check knowing the maximum voltage you think possible. The ATmel data sheet says the absolute maximum is 40 mA per pin but 200 mA total for all pins.

So I would design the divider for 3.3 V with 34 V to the divider and a source impedance of somewhere round about 2 kΩ, say 1 mA down the divider,  33 kΩ 'top' resistor and 3.3 kΩ 'bottom' resistor and forget the zener. Even if the bottom resistor goes open circuit, the current is limited to a safe value. That should give you a count of about 980 at absolute maximum voltage and worst case tolerances on the 3.3 V supply and the resistors; and nominally 667 at your 24 V.

Ian Eagland's picture

Re: Advice requested on EmonTX analogue input voltage & impedance

Help and advice much appreciated.

Design altered as suggested.



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