# Please let me know the risk of Feeding AC directly into ADC

• posted

Dear All,

I am working on a school project in which the average power measurement is made by a PLC having ADC module.

My plan is to use a 220~24V step down X'mer and resistive divider circuit. The range of ADC module is from -10V to +10V.

I am thinking of feeding the AC signal from the resistive divider into the ADC. On the other hand, I am also worried that unforeseen things would happen and make the ADC spoil.

Any opinion? Can I use bidirectional TVS

? Can it protect the ADC from electrical hazards?

I have a basic understanding, but I have little hands on on this.

Thanks and Best Regards

• posted

I'd use a series resistor (or JFET limiter) to the ADC with diode clamps to the power supply rails.

Those TVS' tolerances are too loose.

-- Cheers, James Arthur

• posted

The resistive divider into the ADC will be fine. Arrange for the ADC to see, maybe, +-6 or 8 volts peak when the AC line peaks. Use an output resistor in the 1K sort of range.

John

• posted

Wasn't clear to me if "[...]worried that unforeseen things[...]" meant during normal operation, or for surge protection. That's why I recommended a diode clamp--for surges.

-- Cheers, James Arthur

• posted

Since the pure 220 V mains feed is 311 Vpeak scaling down this to 8 V would suggest that the largest peaks on the mains should be less than

389 V.

Even some inductive load switching on or off, would cause higher peaks. Some protection would definitely be required.

ADC modules used in PLC usually have some internal protection (check the specifications), so a series resistor between the voltage divider and the ADC input, helping the job of the ADC internal protection circuit.

If the experiment is intended to calculate true power (with an additional current transformer), you need to scale the nominal peak values well below 50 % of full scale in order to get a decent headroom, before the protection circuit kicks in. Especially on the current measurement side, the current peaks can be much larger then the RMS current would suggest, if there are any semiconductor loads (rectifiers etc.).

• posted

Thank you so much for all your kind suggestions.

I have just started testing the sensing circuit, and it worked!

I will add 1k ohms resistor between the sensing point and the ADC for further protections.

Regards

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