hi im measuring the main voltage and im using a micro internal adc for input signal ! i want to use this circuit to scale down the main voltage with transformer from 312(vmax) to 31.2 and then using the op-amp to scaling down and shifting the ac voltage to move the negative part of signal to positive side and make it suitable for adc. is this circuit ok? is there better options? or other circuits better than this one? i simulated it in mathlab and it is working properly. any suggestion would be appreciated...
Re: is this circuit suitable for scaling down the main voltage?
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I don't see a reason why that should not work. If you do not want to have an op.amp., you can use the arrangement for the voltage monitor in Building Blocks, and change the resistor values to suit your transformer output voltage.
On word of warning - is your transformer output voltage given at zero load or at full load? You need to find out before you decide the value of R2 & R5. Most small transformers are specified at full load, and the no-load voltage can be considerably higher. For example, the shop 9 V ac adapter gives 11.6 V out at no-load.
Re: is this circuit suitable for scaling down the main voltage?
You don't show the power arrangement for the op amp - it is operating with +/- supply rails? If not, it will have a hard time dealing with the negative half cycle of the input waveform.
Re: is this circuit suitable for scaling down the main voltage?
tnx for reply
i did not build this circuit yet.so if i give a 5v to one end and make the other one ground its not gonna work?
what should i do ??
i need to shift the negative side of the wave with a 2.5v dc .
is it necessery to give op-amp oth -/+ supplay???
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Re: is this circuit suitable for scaling down the main voltage?
"i need to shift the negative side of the wave with a 2.5v dc ."
That is because the ADC can only measure a positive voltage. That is nothing to do with how the op.amp. works.
"is it necessery to give op-amp oth -/+ supplay???"
If you have an op.amp. that will work off a single-ended supply, no, but in that case you'll need to apply the voltage bias to the input before the op-amp - why not take a look in Building Blocks where you might see something that will help you? Otherwise, yes.
Re: is this circuit suitable for scaling down the main voltage?
The voltage-sensing part of this circuit would appear to do everything that you need. The op-amp only requires a single-ended supply (i.e. Vcc and Ground), The mid-rail offset is set by a pair of resistors, just as in the Building Blocks circuit.
http://openenergymonitor.org/emon/sites/default/files/Mk2_CircuitSchematic.pdf
Re: is this circuit suitable for scaling down the main voltage?
Thank you everyone ! All your replies were useful !
now i changed my circuit and got rid of op-amp. another question , is there any thing else that i should take into consideration regarding the circuit?
Re: is this circuit suitable for scaling down the main voltage?
Is "main voltage 1" your half-supply voltage bias? If it is, then R3 is redundant and C is probably also redundant. If "main voltage 1" is the full-scale ADC voltage, then you are missing a resistor. Remember that with no mains voltage, the ADC input should be exactly half of the full-scale ADC voltage.
Re: is this circuit suitable for scaling down the main voltage?
yeah "main voltage 1" is the 2.5v dc to shift the 1.5 ac voltage so the voltage alternates between 1v and 4 v
Re: is this circuit suitable for scaling down the main voltage?
Isn't that just the standard voltage-sensing circuit in the Building Blocks page: