Can you use a variac when prototyping on a breadboard? - Page 1
Forum section: electronics / beginners

author: tec5c
author_rank: Frequent Contributor
timestamp: September 30, 2012, 06:20:17 am
content: This may be a simple question but I figured I could use a variac instead of a mains (step-down) transformer when building up circuits on a breadboard. However, when I connected up a rectifier circuit I immediately blew the fuse in the variac. So I am a little unsure now and only have a limited supply of fuses that I can waste, so I would like to be certain. Are you able to use a variac in the way I am trying to use one?Here are the specs of the variac I am using:-Specifications:• Rated power handling: 500 VA (fused)• Input voltage: 240 VAC @ 50Hz• Output Voltage: 0~260 VAC @ 50Hz• Dimensions: 165 x 120 x 160mm (D x W x H)Any input is welcome

author: Bored@Work
author_rank: Super Contributor
timestamp: September 30, 2012, 07:21:19 am
content: A good way to die.

author: IanB
author_rank: Super Contributor
timestamp: September 30, 2012, 07:32:59 am
content: Completely insane.

author: David_AVD
author_rank: Super Contributor
timestamp: September 30, 2012, 07:49:51 am
content: Variacs are autoformers and not isolated. There is no primary and secondary as such. Think of them like a potentiometer but mains rated.They are used to vary the mains voltage to something that is mains powered. Using one to power a low voltage circuit is dangerous and could have fatal consequences.

author: david77
author_rank: Frequent Contributor
timestamp: September 30, 2012, 08:01:49 am
content: The cheap variety of variacs is always NOT ISOLATED from the mains, so it would be quite insane to hook one of those up to a bread board - or any circuit.I strongly suggest you find out what kind of variac you have before killing yourself by accident!!!Get a clue before fiddling with stuff that is potentialy lethal:http://sound.westhost.com/articles/variac.htmAs long as you use anISOLATING VARIACat low voltage (below about 50V AC) settings there's not more danger than using a transformer.If you blew the fuse in your variac I advise to check the circuit, you probably hooked something up the wrong way creating a dead short.

author: Psi
author_rank: Super Contributor
timestamp: September 30, 2012, 08:06:55 am
content: If you connect the variac output up to the 230V primary of a random transformer with ~24V output you then have a nice isolated variable voltage AC supply between 0 and ~30V.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 30, 2012, 08:10:09 am
content: NO! NO! NO! NO!!!!!!!Don't do this for these reasons.A Variac is an adjustable Auto-transformer,so there is NO isolation from the mains.This may seem OK,as the neutral side of the mainsshouldbe at a very similar potential to the safety earth,BUT,from time to time,Electricians inadvertently transpose the Active & Neutral lines in a power outlet (GPO).In that case,what youthinkis the Neutral at the output of your Variac has the full mains voltage ,say 240V w.r.t earth .To make things worse,if you have set the variable tap for a nice convenient 12V ,you will have 240V-12V=228V onthattap,so no matter which lead you touch you will get zapped.But,wait,there's more:-If you connected to a power point where Active & Neutral are normal,except that you have lost your Neutral connection,allthe transformers connections except the earth lead connected to the body of the thing can hurt you!And just in parting:-If everything is set up OK,& you inadvertently bump the big knob on the top of the Variac,you can cook all your Electronics.P.S.david77refers to an isolated variety of Variac in his posting,so thereareapparently such things.I would,however, suggest you treat all Variacs as non-isolated,until proven otherwise

author: PSR B1257
author_rank: Frequent Contributor
timestamp: September 30, 2012, 08:37:30 am
content: QuoteWell, there is one important information missing: The type of fuse (rated current and speed) you are using.QuoteThis is here in Germany (and other country's of course) even more critical. Because we have symmetrical power outlets, and therefore it is random, where you get the Active or Neutral line at the variac (or whatever type of electric gear).

author: Astroplio
author_rank: Regular Contributor
timestamp: September 30, 2012, 08:52:03 am
content: tec5c, sounds like you need to get a power supply :-)How to tell:You know that you are desperate for it when you find yourself trying macgyverisms

author: krish2487
author_rank: Frequent Contributor
timestamp: September 30, 2012, 12:35:06 pm
content: @the OPYou could go the cheap way, replace the fuses and effectively at some point pay for it with your life..or you could invest some money in a proper programmable AC source.It sounds like you need a variable AC signal for your breadboarding experiments.costs quite a bit of money, but it is much safer and looks cool on your bench!!!(Oh and one of the features of buying a proper AC source is when post questions like this on a forum, other members wont criticize you for being unsafe..):-)

author: free_electron
author_rank: Super Contributor
timestamp: September 30, 2012, 01:32:43 pm
content: I smell a 'Darwin Award' in the making...

author: M0BSW
author_rank: Frequent Contributor
timestamp: September 30, 2012, 05:44:38 pm
content: Now I don't know much compared with some people here, and I have one of these, which I respect most carefully , I have a question , " Do you have a death wish"the only time I've use mine so far is for the reason the previous owner used it , that is to slowly bring up the power on old valve radio equipment that not been run for sometime, like my old receiver that came out of ,a WW2 Warbird, I used it instead of banging high voltage in and blowing the old waxed caps .

author: David_AVD
author_rank: Super Contributor
timestamp: September 30, 2012, 09:35:42 pm
content: At work, we use ours for bringing up the mains slowly (while watching bias current) on some amplifiers after repairs have been made.The other use is for observing what happens when something we designed is subjected to over and under voltage.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 01, 2012, 09:46:30 am
content: Using a variac to slowing bring up an unregulated power supply is perfectly fine . The point is that you should treat the output exactly like the mains wiring. Generally that means you connect the output to a fixed, mains rated isolating step-down transformer with mains rated power switch and fuse. The problem is with directly rectifying the output and using it to power a circuit. You wouldn't (or shouldn't!) directly rectify the full mains voltage and power your circuit with it, so you shouldn't with a variac either.I have heard of isolated variacs but never seen one. Make sure it really is isolated before you use it as such. When in doubt, put an isolation transformer after it.

author: M0BSW
author_rank: Frequent Contributor
timestamp: October 01, 2012, 05:56:16 pm
content: This is whats on mine a seperate box Isolation transformer, which has been bolted on to the side, I'm still more than wary with it, i've even been known to take a step back from it, can't be to carefull

author: david77
author_rank: Frequent Contributor
timestamp: October 01, 2012, 07:11:44 pm
content: It is interesting to see that the isolated variacs are so uncommon in most places. For me it's the other way round, I have hardly ever seen a non-isolated one. You would find one of those isolated beasties in nearly every self respecting electronics workshop in Germany, even our local Ebay is flooded with them. I've got a 500W unit in my home lab and we use a ~1kW unit at work.A really useful piece of equipment.Maybe "isolated variac" is the wrong term in English?

author: TerminalJack505
author_rank: Super Contributor
timestamp: October 01, 2012, 08:20:34 pm
content: Judging by the OP's non-participation in the thread he created he may not have taken everyone's advice and is now 6 feet under.

author: ElektroQuark
author_rank: Unknown
timestamp: October 01, 2012, 08:31:40 pm
content: david77, can you write down how can I search for them in German? I will look at ebay.de. Thank you.

author: david77
author_rank: Frequent Contributor
timestamp: October 02, 2012, 04:38:37 am
content: They're called Regel-Trenn-Transformator in German.The important bit is the "Trenn" part, which means it's isolated.If it only says "Stelltrafo" or "Regeltrafo" on it it is probably not isolated.The ebay sellers obviously are not in agreement how to spell it, so there are a few ways to find them:http://www.ebay.de/sch/i.html?_nkw=regel-trenn-transformator&_sacat=0&_odkw=regeltrenntrafo&_osacat=0&_from=R40http://www.ebay.de/sch/i.html?_sacat=0&_nkw=regeltrenntrafo&_frs=1http://www.ebay.de/sch/i.html?_nkw=trennstelltrafo&_sacat=0&_lncat=0&_arm=1&_armm=94&_ruu=http%3A%2F%2Fwww.ebay.de%2Fsch%2FTransformatoren-%2F9728%2Fi.html%3F_nkw%3Dtrennstelltrafo%26_arr%3D1&_armi=Transformatoren

author: ElektroQuark
author_rank: Unknown
timestamp: October 02, 2012, 06:19:43 am
content: Thank you. I know what to look for now.

author: tec5c
author_rank: Frequent Contributor
timestamp: October 03, 2012, 11:16:17 pm
content: Quote from: TerminalJack505 on October 01, 2012, 08:20:34 pmHaha sorry for my lack of input in this thread, I've been quite busy with work. Well after reading all of the replies I feel glad that I had a "DER!" moment. I need some advice on designing SMPS circuits, though I think that can be for a different thread.Thanks again.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 04, 2012, 05:18:20 am
content: I think you may have missed what you really need.A 0-20v dc current limiting power supply. This will be the basis for all your breadboarding.They don't cost that much.Or you can build your own, if you're desperately short of funds you could try the Atx PSU hacked method, which is to use the 12 volt output and add the voltage control and current limiting yourself. You can even reserve part of the breadboard for this.ps. I like variacs, very useful, but I use an isolation transformer too. Well I do use one at work but at home I have been relying on having an RCD on the mains. I should probably get an isolation transformer.Main thing to do is learn as much as you can as to where the dangers lie in each setup.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 04, 2012, 09:03:42 am
content: Quote from: tec5c on October 03, 2012, 11:16:17 pmNot sure what your plan here is, but before you get too invested in it, a line powered SMPS is not a good beginner project. Getting isolated feedback right can be tricky, and finding appropriate off-the-shelf SMPS transformers is nearly impossible. Stick to low voltage DC-DC converters which are still quite useful, use standard power inductors, and much less dangerous if you screw up.

author: Richardchris
author_rank: Newbie
timestamp: September 12, 2014, 05:56:03 am
content: Well if you linked with a power point, where Effective and Fairly neutral are normal, except that you have missing your neutral connection, all the transformers connectionsexcept our earthcause linked with the body of the thing can harm you.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 12, 2014, 06:22:40 am
content: The rules are to never have a direct conducting path to mains.Except for mains earth.Note that voltage measurements using resistors also require an analog or digital isolator.Remember you can only do it wrong once.Buy an (adjustable voltage) wall adapter of at least 7 volts, and one of those bread board power boards with two LDO's (5 and 3.3 volts) to power your breadboard.https://www.sparkfun.com/products/8376

author: jlmoon
author_rank: Unknown
timestamp: September 12, 2014, 03:23:59 pm
content: Quote from: TerminalJack505 on October 01, 2012, 08:20:34 pmOh No!!! Let's hope not!

author: corrado33
author_rank: Frequent Contributor
timestamp: September 12, 2014, 04:38:45 pm
content: Not to hijack the thread, but do the same warnings apply for DC variacs? We have both DC and AC varieties in our lab and I use the DC ones for relatively low power devices often.

author: IanB
author_rank: Super Contributor
timestamp: September 12, 2014, 04:46:50 pm
content: Quote from: corrado33 on September 12, 2014, 04:38:45 pmWell a variac is a variable transformer--usually an autotransformer. Variable because it has a continuously adjustable tap.Since a transformer is by definition an AC device, can you be more specific about what a "DC variac" is in your lab?

author: corrado33
author_rank: Frequent Contributor
timestamp: September 12, 2014, 04:55:55 pm
content: Our "DC" variac looks just like an AC variac but it only has two banana jacks for outputs. It outputs DC voltage, so I'm assuming it simply has a rectifier and some smoothing caps inside. I guess I could take it apart really quick if I wanted to.EDIT: Our "DC" variacs are simply AC variacs (obviously) with a single high power rectifier. (MDA2504) Now I really want to measure the ripple on them. I have a sneaky suspicion that these variacs are homebrew (lab-brew?) jobs.

author: Wim_L
author_rank: Regular Contributor
timestamp: September 13, 2014, 09:28:00 pm
content: Quote from: david77 on October 01, 2012, 07:11:44 pmYou may find most of the modern ones really are 1:1 isolated mains transformers, followed by a normal variac. Still, functionally equivalent to an isolated variac, though likely a bit heavier (two cores).