Is it possible to build a Variac without using Transformers? - Page 1
Forum section: electronics / beginners

author: MMDuino
author_rank: Regular Contributor
timestamp: April 22, 2015, 11:05:58 am
content: In my works with High voltage (220VAc-50Hz) always i need to (variable AC voltages), so my inquiry is; are there anyone made variac without variable transformers?, for example by using Thyristors, Triacs or even Transistors and get on adjustable 220 vAc with normal sine wave in the result?Thanks in advance

author: Simon
author_rank: Unknown
timestamp: April 22, 2015, 11:12:34 am
content: simply no, if you want to do it with electronics parts your looking at making your own variable output inverter or buying one, very complicated and expensive.The real variac is the simplest and cheapest solution.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 22, 2015, 12:57:00 pm
content: You can build a buck converter using synchronous switching and bidirectional switches, which implements the same idea as its DC counterpart, but works for AC as well. At a fixed PWM%, it will behave much the same as a variac (equivalent winding resistance and everything).It most certainly will not have the surge and fault capabilities of a real variac, though.Tim

author: Simon
author_rank: Unknown
timestamp: April 22, 2015, 05:00:12 pm
content: You would need a dual polarity buck converter with a varying output.

author: robrenz
author_rank: Super Contributor
timestamp: April 22, 2015, 05:15:21 pm
content: You can do it with a bipolar operational power supply as shown inthis thread. You can drive it with a signal generator to get the frequency and amplitude you want. You would need a 500V unit to do the 220VRMS. Very handy piece of equipment in general.

author: SeanB
author_rank: Super Contributor
timestamp: April 22, 2015, 05:30:35 pm
content: If you do not mind having discrete steps you can just use a transformer with multiple taps, and select them as needed, using either a switch or relays. You can have one wound with a series of taps giving you from 60VAC ( really a good place to start) to 250VAC, using taps every 10V on the transformer. 21 wires later and you can go from 60 to 250VAC using some switching.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 22, 2015, 05:34:00 pm
content: Yes, it can be done, but it is a very dangerous and complex circuit that many people with decades of experience (including me) would never attempt to make.If you want a real sine wave output, you are looking at an enormous linear power amplifier, not something that can be built with thyristors.Of course it would never give you the ISOLATION of a true transformer. But then many variacs don't provide isolation either because they are simply auto-transformers.

author: Simon
author_rank: Unknown
timestamp: April 22, 2015, 05:37:28 pm
content: Quote from: SeanB on April 22, 2015, 05:30:35 pmThe whole point apparently is to avoid the transformer.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 22, 2015, 05:48:11 pm
content: I've seen a variable supply, for automated testing purposes, using a normal transformer and a thyristor dimmer on the primary.Depending on the required quality and power, you can achieve the same using stepped approximation, such as an UPS. Or maybe even dirty pwm like a motor drive. You might need some filtering with the last one.

author: German_EE
author_rank: Super Contributor
timestamp: April 22, 2015, 09:12:15 pm
content: It should be possible but I would not like to attempt the task.1) Rectify and smooth the incoming mains supply to DC.2) Pulse width modulate the DC to build a 50/60 Hz waveform at the wanted voltage.3) Put your output through a low-pass filter which will hopefully remove as much as possible of the RF hash and other rubbish that you will see on the output and leave you with a reasonable sine wave.

author: SeanB
author_rank: Super Contributor
timestamp: April 22, 2015, 09:37:25 pm
content: Quote from: Simon on April 22, 2015, 05:37:28 pmNo, to avoid the variable transformer. I had a transformer I got as surplus once which had a mains primary and secondaries of 20VAC, it just had 20 of them. Thus you could make almost any output from 20VAC to 400VAC just by wiring up the secondaries.

author: albert22
author_rank: Regular Contributor
timestamp: April 22, 2015, 10:36:58 pm
content: Here is a solution for a small current "electronic variac"http://web.tiscali.it/i2viu/electronic/variac.htmIMO this is the simplest solution for low power applications.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 23, 2015, 12:21:19 am
content: Quote from: albert22 on April 22, 2015, 10:36:58 pmThe first one is as good as any old rheostat (protip: you can use one of those here, too, in place of the transistor stuffs hanging on the bridge), with the difference that you need big heatsinks on relatively cheap transistors, instead of some bigass ceramic and metal monstrosity.The second oneis not, however, because it applies a constant current -- the output voltage might still be in regulation, but it won't be sinusoidal. An OTA could be used to multiply the feedback signal by the bridge voltage, implementing an electrically variable resistor. Then both output regulation (active AC regulation, cool!) and sinusoidal response (well, for the resistor part anyway) will be had.Tim

author: Dave
author_rank: Super Contributor
timestamp: April 23, 2015, 12:44:35 am
content: Well, you could rectify mains directly and use PWM and some nice fat IGBTs to generate the output waveform (well filtered, of course). Kind of like a variable frequency drive.But that would be a very tricky and potentially dangerous design job. Not something a beginner should attempt.

author: Simon
author_rank: Unknown
timestamp: April 23, 2015, 06:48:50 am
content: Quote from: MMDuino on April 23, 2015, 06:43:29 amThat won't work, you will get a square wave and you need to use a H-Bridge output to get negative and positive voltages that make up AC, as we keep trying to tell you this is not simple. How will you vary your voltage ? you can't.The way it works if you do it electronically is you use a high frequency PWM much higher than 50Hz and you use different PWM duties to simulate the varying voltage of the AC since wave so that you can approximate a sine wave, you can then also filter it to make it better but this is all very complicated.With your level of knowledge you should not be playing with the mains unless your qualified in mains work like being an electrician.

author: MMDuino
author_rank: Regular Contributor
timestamp: April 23, 2015, 07:43:26 am
content: Oh, really i forgot that, the result will be square wave with PWM,,ok, i'll change my direction by using Sine wave generator and transistor 2SC3322 (Cause i have some pieces of it)with 10k variable resistor on the base to control the output of the transistor..do you agree with me?please see the next sin wave generators...Quote from: Simon on April 23, 2015, 06:48:50 am

author: Simon
author_rank: Unknown
timestamp: April 23, 2015, 10:05:32 am
content: It's not that simple, your transistors control current not voltage so you need to vary the average amplitude of the drive voltage from your oscillator and have feedback from the output and you need to work in both negative and positive voltages.Again this really is not a simple thing to do and not advisable for you to try for quite some time.

author: macboy
author_rank: Super Contributor
timestamp: April 23, 2015, 03:44:04 pm
content: Quote from: MMDuino on April 23, 2015, 07:43:26 amNo.

author: German_EE
author_rank: Super Contributor
timestamp: April 23, 2015, 06:04:39 pm
content: Don't forget that if you want to feed a sine wave into a power transistor to get a bigger sine wave out then the transistor will need to be biased into the Class A linear region. This means that the efficiency will be very bad, somewhere between 33% and 25%, so for 100W of power in you will generate over 60W of heat.

author: Dave
author_rank: Super Contributor
timestamp: April 23, 2015, 07:18:30 pm
content: Quote from: Dave on April 23, 2015, 12:44:35 amHave I not stressed this enough?I believe the problem here is that you are not even aware of how little you know about the subject. While this is often very good, as it encourages learning, this is not the case here. Mains voltage itself is very dangerous and you would be rectifying it and charging a capacitor with it, making it even more lethal. One small mistake and you aredead.Buy a proper variac and forget about this thing for now. Start off with something easier, something that can't kill you.

author: Dago
author_rank: Frequent Contributor
timestamp: April 24, 2015, 07:03:19 am
content: Quote from: MMDuino on April 23, 2015, 07:43:26 amNo it will not work. You need a sine-wave modulated square wave (with dead-time) and you need to filter the transformer output to get a sine-wave.Like others have said, this is not an easy thing to design even for an experienced designer, not a suitable project for a beginner.

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: April 24, 2015, 01:36:45 pm
content: Just do what my dad did and put a sine wave into an protected audio amp.

author: Simon
author_rank: Unknown
timestamp: April 24, 2015, 01:45:55 pm
content: judging by the lack of responses from the op he has already killed himself.

author: MMDuino
author_rank: Regular Contributor
timestamp: April 26, 2015, 01:45:56 pm
content: Thanks for your care, but i'm still here breathing....that's mean you encourage me to tear down my UPS and use it's transformer to get good results..ok. i will try that, then i will post my results...Quote from: Simon on April 24, 2015, 01:45:55 pm

author: Simon
author_rank: Unknown
timestamp: April 26, 2015, 06:06:43 pm
content: Quote from: MMDuino on April 26, 2015, 01:45:56 pmand what makes you think a UPS transformer will help ? it may be totally the wrong type as it could be a high frequncy step up followed by a high voltage AC generation circuit.

author: rickselectricalprojects
author_rank: Regular Contributor
timestamp: April 30, 2015, 11:56:35 am
content: I brought a "scr dimmer" of ebay and it could apparently vary ac voltage from 10-240 v ac. I don't remember how i wired it up but i Ballested it with two heaters and put a resistive load (a lightbulb). I did get a output voltage of 10-240v but it DID NOT have a normal sinewave. The only thing that was good for was blowing up speakers and it still was very bad at that. So the best bet would be to go and pick up a cheap and/or used variac of ebay

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 30, 2015, 01:11:39 pm
content: Quote from: rickselectricalprojects on April 30, 2015, 11:56:35 amNo, you didn't really. You got the 240 VAC RMS sine wave chopped up to vary the average power to the resistive load with a good amount of "averaging" in the thermal mass of the incandescent lamp filament.

author: Refrigerator
author_rank: Super Contributor
timestamp: April 30, 2015, 01:20:15 pm
content: Well, if you put 350 diodes across your 240VACyou can use the diode voltage drop to chop the voltage up...

author: madires
author_rank: Super Contributor
timestamp: April 30, 2015, 01:22:57 pm
content: Quote from: Richard Crowley on April 30, 2015, 01:11:39 pmSome references including nice pictures and graphs:-http://www.ilight.co.uk/downloads/iLIGHT%20Binder-HowDimmers.pdf-http://home.howstuffworks.com/dimmer-switch.htm-https://en.wikipedia.org/wiki/Dimmer