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How to convert 3vDC to 3vAC?

Athosworld

Member
I need to convert 3vDC to 3vAC (EXTREMELY IMPORTANT!: KEEP THE SAME VOLTAGE BUT AC) but, is there any way to convert it WITHOUT a transformer? (If it contains capacitors: can my 16 volt capacitors work on 3vAC?
 

Diver300

Well-Known Member
Most Helpful Member
Generally, electrolytic capacitors and tantalum capacitors won't work with AC. Other types will.

AC can be made by switching between +ve and -ve, but the resulting voltage will have an average of half the supply voltage, where pure AC has an average of 0 V. There are two common methods of dealing with that. One is to put a capacitor in series, but the peak voltage is only half the supply voltage. The other method is to have a second device switching between +ve and -ve, but out of phase and the second AC wire is connected to that. The second solution results in twice the supply voltage as a peak AC voltage, and neither AC wire is always connected to the either the input +ve or -ve.

Simply switching the supply between +ve and -ve will result in a square wave. AC mains is a sine wave.

Sine wave voltages are often quoted as RMS (Root Mean Square) voltage, which is about 70.7% of the peak voltage, so if you want the voltage to be a sine wave, the peak voltage will have to be around 4.24 V.

The questions for you are:-
What frequency?
What power?
Do you need the output voltage to be a sine wave?
Does the RMS output voltage have to be 3 V, or is it the peak voltage?
Does one of the output connections have to be connected to on of the input connections?
Does the output voltage have to vary if the input voltage varies?

It would also be good to know what this is being used for.
 

Athosworld

Member
Generally, electrolytic capacitors and tantalum capacitors won't work with AC. Other types will.

AC can be made by switching between +ve and -ve, but the resulting voltage will have an average of half the supply voltage, where pure AC has an average of 0 V. There are two common methods of dealing with that. One is to put a capacitor in series, but the peak voltage is only half the supply voltage. The other method is to have a second device switching between +ve and -ve, but out of phase and the second AC wire is connected to that. The second solution results in twice the supply voltage as a peak AC voltage, and neither AC wire is always connected to the either the input +ve or -ve.

Simply switching the supply between +ve and -ve will result in a square wave. AC mains is a sine wave.

Sine wave voltages are often quoted as RMS (Root Mean Square) voltage, which is about 70.7% of the peak voltage, so if you want the voltage to be a sine wave, the peak voltage will have to be around 4.24 V.

The questions for you are:-
What frequency?
What power?
Do you need the output voltage to be a sine wave?
Does the RMS output voltage have to be 3 V, or is it the peak voltage?
Does one of the output connections have to be connected to on of the input connections?
Does the output voltage have to vary if the input voltage varies?

It would also be good to know what this is being used for.
1.60Hz
2.500mA
3.Yes (or no) Both are fine.
4.Peak voltage
5.No
6.Both are fine.
Just used for a LED experiment.
 

Papabravo

Well-Known Member
1.6 Hz. is almost the same thing as DC
Power is not measured in milliamperes (ma)
It can be a sinewave or not is a pretty meaningless requirement
Does 3V peak mean that the waveform goes from 0V to 3v, or from -1.5V to +1.5V (3V P-P)
No
Both are fine

You sure know how to define a precise set of requirements. I'm going to say this project is unlikely, since very few power devices will work at at such low levels. I think you're SOL. Note, I've been wrong before.

You can easily make a PWM signal that varies between 0V and +3V which you can use for LED experiments. For a square wave you can integrate the voltage over one period to get the average voltage output.
 

Pommie

Well-Known Member
Most Helpful Member
If it's just for experimental purposes then use an Arduino.

A Nano will run at 5V. Use two pins to alternate between 0V and 5V. This will give you 10V P-P square wave. Feed it through an RC filter to get a rough sine wave (probably more like a sawtooth wave). Use an opamp (with sufficient output current) to get the required voltage.

Mike.
Edit, just saw the requirement to keep the same voltage - so ignore this suggestion.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
I need to convert 3vDC to 3vAC (EXTREMELY IMPORTANT!: KEEP THE SAME VOLTAGE BUT AC) but, is there any way to convert it WITHOUT a transformer? (If it contains capacitors: can my 16 volt capacitors work on 3vAC?
Use a simple CMOS gate oscillator using a 40106.
Using a 0.1uF cap for Ct and 390K for Rt should give you something in the ballpark of 60Hz.
Or use eg. a 220K resistor and a 470K or 500K preset in series, to allow the frequency to be tuned.

Add another 40106 inverter stage with its input to the osc output, to get a complementary signal.

Then feed the osc output to two other inverter inputs and connect both their outputs together as one signal output.

And feed the complement signal to the inputs of the last two inverters in the IC; connect their outputs together and they become the other signal output.

Connect the load between the two outputs; they will have the same voltage between them as the supply, but reversing continuously at the osc frequency, so the load will see an AC voltage.

The gate drawing tool I found does not allow anything but gates, I can't add the resistors and cap..

So, the oscillator part is as the first image below, and the overall arrangement as in the second image.
In that, the top left gate is the oscillator & the two connections to the right are the "AC" output;

40106 data with pinout:



waveforms-tim29.gif



osc_to_ac.jpg


Edit - as Crutschow points out, it needs a power stage of some sort adding for 500mA...
Me, speedreading, saw the current as 2.500 mA :facepalm:
 
Last edited:

danadak

Well-Known Member
Most Helpful Member
Here is a basic approach. Essentially chop the DC signal at 60 hz
rate, followed by a BPF or LPF to extract sinusoid.

Shown is a simple BPF, might get better sine by use=ing Twin T Notch
or use OpAmp, Sallen Key as filter.

The waveforms show were a sweep of the DC value from 1 to 5V,
so if you wanted output pk-pk to match DC then the filter could be designed
with appropriate G to yield that for OpAmp solution.

1657975650460.png


Of course there is a lot of latency associated with this as you can see. To eliminate
that a multiplier solution as discussed in post #7 fixes that issue.

So V2 chops the DC with the 2N7000, you need a 60 Hz source, V1 is DC source of interest, ignore V3, it
was used when I was using a CMOS mux part in lieu of 2N7000.

Regards, Dana.
 
Last edited:

danadak

Well-Known Member
Most Helpful Member
Using a Sallen Key filter (maybe should try MFB as well) -

1657978199015.png


So in short you could use a dual opamp, one for creating 60Hz oscillator, the other for the filter,
and have a solution.


Regards, Dana.
 
Last edited:

crutschow

Well-Known Member
Most Helpful Member
If a square-wave output is okay, then use a circuit similar to that in post #10.
For 500mA, you will need a high current buffer on each output.
A MOSFET gate driver IC would work for that.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Some power opamps for buffer use -
None that work at 3V.

Four 3V logic level FETs would do it, two P channel for high drivers and two N channel for low.

A fairly high value gate resistor each so slow turn on, with a diode across the resistor to give fast turn off, should allow them to be driven by the outputs of inverter oscillator circuit I posted earlier & prevent shoot through, I think?

Edit - example FETs:

 
Last edited:

danadak

Well-Known Member
Most Helpful Member
Maybe a question or two for Athosworld :

1) Do you want to buffer both 3 V DC and the 3V AC ?
2) What supply do you have to provide the 500 mA, just the 3V or
another supply ?
3) Confirm you do not want AC to be sine or you do ?
4) What frequency accuracy you want of 60 Hz AC output ?
5) Environmentals, eg. T range of operation ?
6) Latency concerns from change in DC to AC change out >

Regards, Dana.
 

Athosworld

Member
Maybe a question or two for Athosworld :

1) Do you want to buffer both 3 V DC and the 3V AC ?
2) What supply do you have to provide the 500 mA, just the 3V or
another supply ?
3) Confirm you do not want AC to be sine or you do ?
4) What frequency accuracy you want of 60 Hz AC output ?
5) Environmentals, eg. T range of operation ?
6) Latency concerns from change in DC to AC change out >

Regards, Dana
1.Yes
2.A breadboard power supply powered by a 9V battery
3.Sine
4.The closest?
5.3V-4V
6.I dont understand.
 

danadak

Well-Known Member
Most Helpful Member

Athosworld

Member
This is a consideration for the time the circuit takes to respond to a
step change in the DC input to the AC output changing. Do you care
if its 1 uS or 1 mS or 1 sec....?



That ques was asking what Temperature range

9V battery discharge life at 1A output (point of this is not very long) :


View attachment 137892

Regards, Dana.
I found an 120 vAC to 6 vAC (I shocked while grabbing the wires) adapter and a resistor to step it down to 3 vAC and I could power an LED (flickers with potentiomete). Interesting fact: you can actually hear the AC cycles if you connect a small speaker, you hear fast repeating buzzings
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I found an 120 vAC to 6 vAC (I shocked while grabbing the wires) adapter and a resistor to step it down to 3 vAC and I could power an LED (flickers with potentiomete). Interesting fact: you can actually hear the AC cycles if you connect a small speaker, you hear fast repeating buzzings
It's simply 50Hz (or 60Hz), well in the audio range, and has been used as a simple source of audio for as long as electronics has been around. When you touch the input of an audio amplifier and it buzzes, that's mains hum - picked up by your body from the surrounding mains wiring.
 

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