AD636 with 0-230 VAC Input

Hi,

I'm building a voltage measurement device for measuring 0-230 VAC input for a light dimmer. I just learnt about Analog Devices AD536 and AD636 ICs. I have the following doubts:

1. I want millivolt precision for detecting voltages; can you provide me with a application note or circuit diagram for converting 0-300VAC to 0-5VDC (I want true rms converter, as waveform is not pure sine).

2. How to 0-230VAC input range be extended to higher range; what modifications are required in the circuit?

Regards,

Ajay.

  • 0
    •  Analog Employees 
    on Aug 11, 2015 4:20 PM

    Hi Ajay,

    First you need to reduce the 230VAC input voltage to a value that won't destroy the AD636. For safety reasons, this is usually done with a transformer so as to isolate the ac mains from the user. To maintain accuracy, you can use a high-voltage series capacitor and resistor divider. The divider ration will be 300/5 or 60:1. In simplest terms, you could use a 59kohm resistor in series with a 1kohm.

    I have attached links to an rms-dc applications guide (see pages 43 and 44) along with an application note AN-1341 (see figure 27).

    http://www.analog.com/media/en/training-seminars/design-handbooks/RMStoDC_Cover-Section-I.pdf

    http://www.analog.com/media/en/technical-documentation/application-notes/AN-1341.pdf

    Best,

    j

  • Hi jstanley,

    Thanks for your reply. I've been reading the RMS to DC converison application guide. I see from the guide that AD637 is best for measurement devices that don't need to be battery powered (ie. save energy) and also that AD637 is the best out of AD536, AD636 and AD637 in terms of accuracy and bandwidth.

    1. What device would you suggest for me?

    Thanks for clarifying on how to reduce the 230VAC input voltage. The transformer based solution will not be accurate enough. I think I'll use a 47uF 300V capacitor followed by a resistor divider.

    2. Having a capacitor will hold the ac voltage at its peak right? I don't have a pure sine at input. My crest factor is not square-root(2). Its lower than that (speed regulator application; 300VAC = full speed). If I add a capacitor in parallel with the ac input, the output of the resistor divider will be wrong. So, I'm just going to have a resistor divider of ratio 60:1 and convert 0-300VAC input to 0-5VAC input. Am I right with these statements?

    3. I hope AD637 can accept 0-5VAC output from the resistor divider as input directly?

    4. Can you verify the correctness of the following circuit, including the 1 pole output filter capacitor values and buffer input values? I've considered AD637, but based on your inputs I will switch to other devices if needs be.

    5. What will be the output DC voltage from the AD637 for 0-300V rms input (stepped down to 0-5V rms)? I need to give this to the ADC of a microcontroller to display on a LCD screen.

    Thanks and regards,

    Ajay.

  • 0
    •  Analog Employees 
    on Aug 12, 2015 10:03 PM


    Answers to questions above:

    1. AD637 is the most accurate part we have, also the most linear over the widest frequency and input range.

    2. You add the capacitor in series with the resistor, not in parallel. The idea is to isolate the user or load from the HV source.

    3. The AD637 will accept 5vac so long as you have +/-10V or more on the sullies. The AD636 won't.

    4. You need a ground on the connection of the 1k resistor and the low side f the voltage source.

    5. For an ac input of 5V you get 5vdc at the output, for 1vac input, 1vdc out, etc. That is why it's called an rms-dc converter.

    6. Is this an application for a commercial product or are we working on an educational or hobby project?