# AC to DC conversion using single supply opamp

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#### mayuriv

##### New Member
hello,

I have a single supply(3.3v) available in my circuit. I have an ac input current coming in the circuit ranging from 0 to 350 mA. I have a burden resistor of 10ohms,2 Watt connected across the ac input. Across burden resistor ,I get an equivalent vrms. I need an ac to dc conversion circuit to convert this vrms into dc voltage using single supply opamp. Please do not suggest a diode. I have designed a circuit using opamp and a bulk capacitor but it is taking time to smoothen the dc voltage. The ac input current coming to the circuit has to be detected within 100ms but this circuit needs the input to stay for more than 200 ms. can you suugest some alternative circuit solution?

#### spec

##### Well-Known Member
Hello Mayuriv,

Just a few questions:
(1) I can't understand the circuit you posted- can you explain its function including the variable voltage sources shown?
(2) Is the 0 to 350mA AC input current RMS, ie 989.8 mA peak to peak?
(3) Is the 0 to 350mA AC input ground referenced, high referenced, or completely floating
(4) What is the frequency of the AC input current
(5) Can the burden resistor be changed, possibly to a lower value?

If you answer these questions it will assist in designing a circuit for you.

spec

#### mayuriv

##### New Member
1) First stage does full wave rectification, second stage converts rectified voltage into dc voltage,capacitor smoothens the dc voltage. Please neglect fourth opamp stage,it is just a comparator.
voltage source is 3.3 v and not variable.
2)0 to 350 mA ac current is rms.
3) rms current is ground referenced.
4) 50 hz is the frequency
5) Yes, we can change the burden resistor value but it should accommodate the current range.Not to forget-the supply voltage is 3.3v only. This is a battery powered device...Power saving is needed.

#### spec

##### Well-Known Member
1) First stage does full wave rectification, second stage converts rectified voltage into dc voltage,capacitor smoothens the dc voltage. Please neglect fourth opamp stage,it is just a comparator.
voltage source is 3.3 v and not variable.
2)0 to 350 mA ac current is rms.
3) rms current is ground referenced.
4) 50 hz is the frequency
5) Yes, we can change the burden resistor value but it should accommodate the current range.Not to forget-the supply voltage is 3.3v only. This is a battery powered device...Power saving is needed.

You say that you want to do the rectification function with a single of opamp, but I assume that a single comparator will also be suitable- comparators, say an LM393, tend to give a better performance than opamps as precision rectifiers.

Also, I assume that half wave rectification will be sufficient.

spec

#### ericgibbs

##### Well-Known Member
hi may,
These two pdfs have some single supply precision rectifier circuits.
E

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#### alec_t

##### Well-Known Member
10Ω at 350mA rms would give 3.5V rms across the resistor. That exceeds your power supply voltage, so you would need either to reduce the burden resistor or to put a voltage divider across it.

#### mayuriv

##### New Member

You say that you want to do the rectification function with a single of opamp, but I assume that a single comparator will also be suitable- comparators, say an LM393, tend to give a better performance than opamps as precision rectifiers.

Also, I assume that half wave rectification will be sufficient.

spec
The quiescent current is 0.5 mA for LM393..I want to save current,the unit being battery powered...Right now the current of entire circuit is 450 uA...

#### AnalogKid

##### Well-Known Member
alec beat me to the critical problem in post #6. Separate from that, another approach is a true RMS to DC converter chip. Analog Devices makes several.

ak

#### mayuriv

##### New Member
hi may,
These two pdfs have some single supply precision rectifier circuits.
E
I don't want to use diode due to its disadvantages as mentioned in the article.
The other article has circuit which uses only opamp but its not helping me. I also need to take a decision based on current. If a current increases a particular threshold ,I need to indicate it with led.This circuit is not allowing me to take such decisions.

#### ericgibbs

##### Well-Known Member
hi may,
This is what LTSpice shows for your circuit, the comparator threshold is approx 0.18v

E

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#### spec

##### Well-Known Member
The quiescent current is 0.5 mA for LM393..I want to save current,the unit being battery powered...Right now the current of entire circuit is 450 uA...
OK.

spec

#### mayuriv

##### New Member
hi may,
This is what LTSpice shows for your circuit, the comparator threshold is approx 0.18v

E
No sir,please do not consider the comparator part. I have put it by mistake. I just want you to consider U1&U2. I want to convert rms current to smooth dc voltage.

#### audioguru

##### Well-Known Member
The OPA379 opamp has very low output current when it is powered from only 3.3V then the capacitor takes too long to charge. Reduce the value of the capacitor so it charges faster.

The output of the second opamp discharges the capacitor about half of the time. Add a diode.

#### crutschow

##### Well-Known Member
The ac input current coming to the circuit has to be detected within 100ms but this circuit needs the input to stay for more than 200 ms.
I want to convert rms current to smooth dc voltage.
Those two requirements are mutually exclusive.
You can't get a fast response if you want smooth DC voltage.

What is the frequency of the signal?

#### mayuriv

##### New Member
Those two requirements are mutually exclusive.
You can't get a fast response if you want smooth DC voltage.

What is the frequency of the signal?
frequency is 50hz

#### mayuriv

##### New Member
The OPA379 opamp has very low output current when it is powered from only 3.3V then the capacitor takes too long to charge. Reduce the value of the capacitor so it charges faster.

The output of the second opamp discharges the capacitor about half of the time. Add a diode.
I have added a diode after second opamp's output and have even reduced the capacitor's voltage to 470uF.This also seem not to be working.

#### audioguru

##### Well-Known Member
470μF will take all day to charge. Use a much lower value like this:

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#### schmitt trigger

##### Well-Known Member
AG:
Indeed, Multisim is a lousy schematic editor

#### Mikebits

##### Well-Known Member
AG:
Indeed, Multisim is a lousy schematic editor
Multisim is a perfectly fine editor. You just have to know how to use it properly along with following common guidelines in schematic design.