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Author Topic: Varying the AC Voltage Amplitude  (Read 4158 times)
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Thiru09
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« on: March 12, 2010, 08:16:19 08:16 »

Dear friends,

Please guide me with the following requirement.

The AC voltage shall be varied from 0 to 120 V (750VA).
Incoming source is 220V AC

I guessed two methods:
        1. Using Servo + Auto transformer
        2. Generating the AC voltage (DC to AC)


Can anybody suggest me an easier method to achieve this ?

Is that possible to vary the AC voltage in second method ? If so, please guide me.

I am planning to use micro-controller, but I'm poor in managing AC voltages.

Thank you.

Regards,
Thiru
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DreamCat
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« Reply #1 on: March 12, 2010, 09:36:29 09:36 »

I think it seems there is no other method to do that.

the first method is good, it is strong, and can provide enough current, but the mechanical part is hardly for me.

the second method is is somewhat complex for same low frequency, and you need High curren transistor, also need transformer. but its output voltage is stable by your feedback control.
if you use this method, suggest to use SPWM.

Regards.
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mrpicing
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« Reply #2 on: March 12, 2010, 11:01:09 11:01 »

I agree with DreamCat. 2nd Methode is best for your application. All UPS apply same technique to achive their goal. You can convert 220VAC to DC then using uC you can generate controlled 0~120VAC. Try to see UPS/Invertor details on the web. I think UPS technique may help you.
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carbontracks
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« Reply #3 on: March 12, 2010, 03:32:33 15:32 »

If you can easily get a variac/autotransformer and an appropriate servo motor, then you might as well try the first option.

Do you require isolation?
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pabsironman
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« Reply #4 on: March 12, 2010, 08:52:44 20:52 »

I think the best option to get it done is making    AC-->DC--->AC b/c you use the uC in the control of the stage DC--AC activating mosfet o bjts. If you used the autotransformer it's going to make alot of armonics.
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solutions
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« Reply #5 on: March 12, 2010, 10:24:46 22:24 »

One key spec to come up with the right solution is how often you need to vary this voltage and the time between settings.

Another is can these be discrete levels or do they need to be continuously variable (and you have to ask if not quantizable, why not?)?  If it can be quantized, how many levels are minimally needed?

Are there any EMI requirements, harmonics, DC offset, specs?

Again, what is required versus what would be nice to have....

If this is a public spec you are trying to meet, or if it is a fairly common widget (like a benchtop power supply), feel free to post this info versus snippets so we can understand what you are trying to do.
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Thiru09
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« Reply #6 on: March 13, 2010, 05:18:48 05:18 »

Dear friends,

I am very much thankful for your valuable suggestions.

Dear solutions,

Time between settings is greater than 1 minute.
0 to 120V with 1V step.
No EMI requirements.

I am planning the AC -> DC -> AC design with micro.
My main problem is how to vary the voltage with enough current.

With precise info and circuit, I will again ask your help.

Thank you very much.

Regards,
Thiru
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jzaghal
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« Reply #7 on: March 13, 2010, 09:53:55 09:53 »

Hi,

You did not mention what your load is?? a motor??

Cheers.

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Thiru09
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« Reply #8 on: March 13, 2010, 03:13:02 15:13 »

Quote
You did not mention what your load is?? a motor??

Inductive load.

Thiru
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pickit2
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« Reply #9 on: March 13, 2010, 09:04:34 21:04 »

At work we used a dummy load ciruit modified, from I think elektor or EPE to test output from an audio amp, (a load of fets to ajust the voltage to a bit of kit under test) I did not work on this project, so no details as to mods made.  
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pabsironman
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« Reply #10 on: March 14, 2010, 06:25:16 06:25 »

Hey man, you have to control very well power electronics IC as MOSFET, BJT, IGBTs (with DC) or SCRs, TRIACs(with AC) if you want to manage high currents, be careful with the phases and degrees of de-phase when you used inductive load. You may want to put power Diodes in antiparallel for avoiding that.
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jzaghal
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« Reply #11 on: March 14, 2010, 08:24:46 08:24 »

Hi,

Inductive Load is same as motor.

Look at this document:

http://www.dartcontrols.com/manuals/125DManual.pdf

Best wishes.
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andig
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« Reply #12 on: May 01, 2010, 10:41:38 10:41 »

Try searching for posts made by Walkura. He posted a document that describes High Frequency AC chopping and re-construction. I have the technology but it is commercial so cannot share on the forum. It is built on PIC18F1330. Basic Concept is you can run a High Frequency PWM on the AC signal kind of amplitude modulating it. A LPF integrates the high frequency chops to the fundamental AC. Best of Luck
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Walkura
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« Reply #13 on: May 07, 2010, 09:10:11 21:10 »

You are looking for a ac-ac chopper or a ac-ac buck converter .
The first is published as datasheet by st.
http://www.st.com/stonline/books/pdf/docs/12105.pdf
Think a small filter to it and you're almost there .
You also have a socalled ac-ac buck (or boost converter) with which you apply pwm on a (coupled) inductor (either in phase or antiphase depending on buck or boost)
(there is no dc part inbetween ,converting it to dc only add's to the losses and is not needed)
This way you create a increase or decrease on a ac voltage (at a relative high efficiency)
http://rapidshare.com/files/236066661/ac-ac-buck.pdf

Have fun (and as always be carefull when working with netvoltage) Walkura .
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gianmagna
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« Reply #14 on: June 09, 2010, 12:42:22 00:42 »

Try this link

http://web.tiscali.it/i2viu/electronic/variac.htm
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