Sinusoidal reference needed...IC?

[cupoftea]

Hi,
We wish to have a train of half-sinusoids (ie a rectified sine thats always positive instead of bipolar). We need it to be 1V pk and 50Hz (but it will be 100Hz because its rectified) ....and preferably variable to say 0.9Vpk..0.8vpk...etc etc.
Are there any ICs which give this kind of waveform out.?...it doesnt need to supply any current.

Its just the reference for a switching power current source.

 

[scopeprobe]

Rectify the Mains then use a resistor divider to get your desired peak voltage.

 

[cupoftea]

Thanks, thats a good idea..we do have mains on this product...i could even divide it down then do the bottom res as a digipot...thanks!

But we would have to put it through a wee mains txfmr first for isolation,....and i believe that will distort the mains...wont give a nice sine out...due to interwinding c and the winding r....

Also, we sometimes want it not in phase with the mains..

Any other ways also appreciated.....

I am actually thinking micro PWM into RC filter...their must be some half sine code out there on the web?

 

[scopeprobe]

Thanks, thats a good idea..we do have mains on this product...i could even divide it down then do the bottom res as a digipot...thanks!

Any other ways also appreciated.....

Depends on what you need. If its for PFC etc.. then taking from mains would be the best solution as it would be phase matched. If your just looking for a stand alone oscillator try A Bubba Oscilator https://www.hscott.net/bubba.pdf using OpAmps could give you 50Hz.

 

[BigBoss]

A high quality transformer does not distort the waveform since the current remains under the nominal value.

 

[danadakk]

This is overkill for sure as there are many resources left unused on
chip that could be included in other parts of design. This is single
chip solution, freq can be sync or async, selectable under user control
if so desired.

See right hand window for other resources that can be used on chip,
A/D, DSP, PWM, Quaddec, LCD, LUT, logic, timer/cntrs.......

Above uses its internal DDS component to gen the clock for a DMA driven Wavedac component
using a table that holds sample set for half sine. Then an OpAmp and pot used to control pk-pk
and buffer that signal.

Could use onchip A/D and external digipot (using onchip OpAmp) in control loop to set precise pk-pk.....or quaddec onchip to dial in pk-pk.....




Regards, Dana.

 

[BradtheRad]

4017 IC (decade counter), with resistor network, produces custom waveforms.
Positive polarity.
To design a proper smoothing filter requires more effort.
It's possible to tap a desired phase position.

For better resolution, cascade two of this circuit. Set smaller resistance intervals.

 

[betwixt]

I'm thinking of an 8-pin PIC producing a sine or half sine using PWM (as many as 1024 levels so very pure) and also producing a pulse out at the start of each cycle that can be used as a zero crossing indicator. Easily switchable between 50Hz and 60Hz, very stable and accurate frequency. Cost about 2 UKP.

Brian.

 

[KlausST]

Hi,

their must be some half sine code out there on the web

Every full wave sine code contains two half sine....so I guess there are millions.

Klaus

 

[KlausST]

Hi,

A high quality transformer does not distort the waveform since the current remains under the nominal value

I doubt that overcurrent increases distortion.

I rather think overvoltage leads to saturation and thus leads to distortion. Is there any other "source of distortion" than the magnetic field?

Overcurrent rather leads to less magnetic field in the core.

Klaus

 

[Easy peasy]

simple analog solution is an XR2206 followed by a precision rectifier, you can combine with an XR2208 if you want to synchronize to the local mains ....

 

[tsan500]

Hi,

I doubt that overcurrent increases distortion.

I rather think overvoltage leads to saturation and thus leads to distortion. Is there any other "source of distortion" than the magnetic field?

Overcurrent rather leads to less magnetic field in the core.

Klaus

Even small transformer gives quite nice sinusoidal waveform, if supply voltage is not too high. Of course the waveform it's not like in signal generator. I have used 6VA 230/9 VAC transformer in mains voltage monitor hobby project. Transformer started to saturate above ~250 VAC.

Usually utility mains voltage has a "flat top" which creates more harmonics than a small transformer. I have example measurement data on my small hobby site, but links to it is prohibited in forum rules ("Do not spam the forums with links to your site").

 

[BigBoss]

Hi,

I doubt that overcurrent increases distortion.

I rather think overvoltage leads to saturation and thus leads to distortion. Is there any other "source of distortion" than the magnetic field?

Overcurrent rather leads to less magnetic field in the core.

Klaus

As you see, increasing current pushes the transformer into saturation region..This is what I understood..

 

[KlausST]

---

Hi

I never heard of saturation to limit current. Usually it's the other way round: when saturation happens the inductance drops and the current increases faster.

Welding transformers have a core with a magnetic shunt. It's a second path for flux when the scondary carries too high current.
The secondary current causes a magnetic field in opposite to the primary field.

Klaus

 

[tsan500]

I'm thinking of an 8-pin PIC producing a sine or half sine using PWM (as many as 1024 levels so very pure) and also producing a pulse out at the start of each cycle that can be used as a zero crossing indicator. Easily switchable between 50Hz and 60Hz, very stable and accurate frequency. Cost about 2 UKP.

Brian.

I made rectified half wave PWM with 8-bit Arduino Duemilanove which has ATMEGA328 microcontroller like Arduino UNO. Samples of positive half wave are repeated. Example of samples which are repeated in order to get rectified sine wave

I used 200 8-bit samples for half wave and 31 kHz PWM frequency. Operation is interrupt driven using ATMEGA328 timer 1 and timer 2. Timer 2 generates PWM and it's reference value is updated in Timer 1 interrupt which fires at 20 kHz.

RC filtered (3,9k + 100 nF) output is shown below. 100 Hz signal doesn't go to zero when measured with 10x oscilloscope probe.

 

[betwixt]

That's exactly what I had in mind for 'cupoftea' and your Arduino code is almost the same as what I considered using a PIC. A PIC would still be cheaper because the ones I had in mind have on-board precision clocks so all the circuit would use is one capacitor across VSS/VDD for decoupling and one RC network for PWM smoothing.

With almost no extra code, the waveform could be half positive sine, half negative sine or full sine and a sync output could also be produced for zero crossing circuits to use.

Brian.