# Building a circuit [Variac]

Discussion in 'General Electronics Discussion' started by OrangeArav, Aug 13, 2018.

1. ### OrangeArav

38
0
Jul 14, 2015
Hello friends! I am trying to build the circuit attached below and need your advise on picking the right transformer. I am currently thinking of buying a variac so that I am able to not only use it for 12VAC but for any voltage I would like. However, looking at this particular variac on Amazon (link provided), the output current is 3A! How can I connect this high current to my electronics? What resistance should I place in series with the hot black wire? Also, if I understood the schematic correctly I would need to ground my white, neutral wire. I have also referred to this article to better understand variable transformers. From what I understood, majority of variacs have a single winding on a core (unlike a regular transformer with primary isolated from the secondary); therefore, at any voltage equal to or below the input voltage output current will be the same (excerpt from article attached below). Please let me know if my thinking is logical. Thank you for your help and clarification.

P.S. All low-current Radio Shack transformers, ~450mA, seem to be used/obsolete (or long lead time) plus not the correct voltage I need for the job.

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Last edited: Aug 13, 2018
2. ### kellys_eye

4,717
1,312
Jun 25, 2010
A variac does NOT give isolation from the live mains connections and could well prove fatal.

Suggest you obtain a proper isolated primary/secondary transformer as per the schematic. They are not difficult to source. Most hobbyist electronics sites will have them.

https://www.mouser.co.uk/ProductDet...GAEpiMZZMuKmRn7rpQYPeTveWqeXvVe1HnB%2bpk9EJ8=

The current being drawn will depend on the connected load. You could use a 1000A transformer and your LOAD will only take what it requires, nothing more (unless there's a short circuit or other fault!)

Last edited: Aug 13, 2018
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3. ### WHONOES

1,215
330
May 20, 2017
Totally agree with Kelly_eye. The proposed path would be a fatal mistake. Don't do it.

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4. ### OrangeArav

38
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Jul 14, 2015
WHONOES, Kellys_eye,
Now you have me interested. The safety concern with the variac is if I screw up and touch the wires coming out from the output. Please reply to my questions below:
• In the case I touch the wires from variac output I would be a very small LOAD and current would flow at about 3A through me?
• Effectively, the 3A is a no-load output current at voltages equal to or below input voltage to variac?
• Is the article I mentioned in my post accurate in its statement that Volt*Amp of a variac is of minimal use to calculations for output current?
• How do I calculate the current on the output including the load in my circuit?
• Where can I purchase a 'safer' variac, one where I can change output voltage?

5. ### WHONOES

1,215
330
May 20, 2017
If you touch the wires on the variac, the current flowing through your body will be substantial but is dependent on your body resistance at the time. 100mA is generally regarded as being a fatal current, sometimes much less, depending on your physical condition. You can easily exceed that value.
Teasing information from you last post, it seems you have a 3A variac. 3A is it's maximum working current. With no load, no current will flow through the variac apart from its' core magnetizing current which will be small.
The current on the output will be what ever you load demands. If your load demands 1A that that is what the variac output current will be. No more or no less.
In the wrong hands, there is no such thing as a safe variac. As mentioned by other contributors, you need an isolating transformer for safety.

Finally, what do you actually want to do with it?

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6. ### kellys_eye

4,717
1,312
Jun 25, 2010
You can feed the output of a variac into a 1:1 isolation transformer and connect to the output but that still doesn't stop you winding the supply up past the critical 50V level at which stage an electric shock could still cause significant injury/death.

Forget about variacs until you learn electrical safety. Just FORGET about them. End of.

As I linked to, the proper transformer is easily obtained and not expensive so why not just get one?

If you are in need for different voltages to use in experimental work then look for a suitable variable voltage, current limiting power supply. They can be purchased from \$50 upwards and will give you greater personal safety and also protection for any circuit you wish to power (due to their current limiting capability)

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7. ### hevans1944Hop - AC8NS

4,704
2,203
Jun 21, 2012
Clearly, @OrangeArav has fallen into the deep end of the pool without wearing a life-jacket and without a life guard in attendance. Two qualified posters, @kellys_eye and @WHONOES have repeatedly warned of the dangers of trying to use a Variac (variable AC autotransformer) to power the circuit schematic that was posted, said circuit clearly showing a transformer with a 12 VAC secondary and a 120 VAC primary. Such transformers are inexpensive and @kellys_eye posted a link to one. @OrangeArav should purchase such a transformer.

Or purchase a 1:1 isolation transformer. Connect it's primary to the mains supply and its isolated secondary to the Variac transformer input. The Variac will then provide a variable AC voltage from the galvanically isolated secondary winding of the isolation transformer and this can be used for whatever purpose, as long as the VA rating of Variac and isolation transformer are not exceeded. We cannot over-emphasize the DANGER to hobbyist experimenters attempting to connect anything to the mains source wiring, whether that be 120 VAC or 240 VAC, "protected" by a circuit breaker, fuse, or not. House wiring has the potential (literally!) to kill you. You cannot depend on circuit breakers or fuses to protect you: these devices protect only the wiring from starting a fire if too much current is drawn, said current being FAR in excess of the amount required to kill you.

Always use a transformer (NOT an autotransformer) to isolate circuits from the mains supply. You will find so-called transformer-less circuits on the Internet whose intent is to reduce mains voltage to a safe user level by using a capacitor/resistor voltage divider. Don't use this type of circuit! They are typical in cheap Asian "cell phone chargers" but they can fail in a manner that places the full faith and credit of your local electrical utility service right in your hand or ear. Believe this: you do not want that to happen!

There are similarly-sized small devices that plug into a wall convenience outlet and deliver safe levels of isolated voltage, at limited current capacity, without the bulk and weight of a 60/50 Hz power transformer. They are called switch-mode AC to DC converters. Inside, the 50 or 60 Hz mains line voltage is rectified to direct current and used to power a high-frequency oscillator running at tens of kilohertz. This oscillator excites the primary of a physically small transformer, providing electrical (galvanic) isolation between the mains wiring and subsequent user circuitry. At such high frequencies, transformers can be built much smaller and more efficient than their 50 or 60 Hz counterparts. The high frequency secondary winding output is rectified, filtered to a smooth DC level, and then applied to either a buck, boost, or combination buck/boost regulator from whence it is delivered to the end user at a "safe" voltage and current capacity.

"Safe" is a relative concept. I own firearms that employ "safety" mechanisms, but these are lethal weapons nevertheless. Almost any source of electricity can be fatal if applied in an ill-advised manner, ill-advised meaning "I don't know WTF I am doing!" That you may be ill-advised is evident from your post #4 quoted in its entirely below:

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8. ### hemigreg426

20
4
Aug 4, 2018
I agree totally about the 1:1 xformer, they are all over at the "bay" cheap, but i would also add a GFI outlet to feed the x former, it will trip at 4ma leakage and avoid any real hurt in event of other failures and Rf arcs.

9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,852
Jan 21, 2010
Whilst I wouldn't recommend a variac (mostly because of the additional exposed mains wiring), there are ways it can be kinda safe, but I wouldn't do it the way you've suggested.

My first option would be to find a plugpack that has a switched variable output voltage. This keeps you well away from all mains voltage.

Another option would be a regulated variable power supply. This is likely to be the most expensive option, but you get a stable and clean DC source (when compared to just rectifying and filtering an AC voltage).

The next option would be a transformer with multiple output voltages. Whilst you need to wire mains, if you can do it once, safely, you have a variety of possible voltages (dependent on the taps).

If I was going to pick an option using a variac, I would decide the maximum voltage required and get a transformer with this output voltage. I could then use the variac to adjust the input voltage to this transformer. This will give you more control, and the safety of isolation. Having said that, this option is probably the heaviest and has the most amount of mains wiring. I would probably pick any other option before this one.

10. ### hemigreg426

20
4
Aug 4, 2018
this thread got me thinking I need a bench top isolation xformer. got a huge one feeds my studio electronicsbut wanted a small one to feed my variac for tests. anyway found a brand new open box medicak grade 800Va unit for 30 buxx and shipped for another 14. so 44 total and its got better than 5UA leakage.
so go for it ASAP they have 4 more on the Bay.

11. ### OrangeArav

38
0
Jul 14, 2015
Thank you all for your support and responses. What I have decided to try is to go with an AC Power Supply (link provided) which has isolation transformer built into it. The receptacle (2A max) is the isolated output. Now, just to make sure you have my blessing (as I am doing this the first time), I am planning on connecting output of this power supply like so:
1. Connect power supply to 110VAC, 60HZ, receptacle; turn on the unit
2. Adjust the AC volts control knob to 12VAC on the scale; turn off the unit
3. Plug in 14AWG cable plug into the output receptacle of the power supply and strip opposite end (black, white, green wires stripped)
4. Join 14AWG black wire to a black 22AWG wire, neutral 14AWG white wire to a 22AWG white wire, and green 14AWG wire to a green 22AWG wire via electrical tape. Feed 22AWG wires into breadboard and join white neutral and green wires together to produce a common point; this is our circuit ground.
5. 12VAC to electrical circuit is established.
Let me know if this is an ok way of bringing power to the breadboard circuit.

12. ### hevans1944Hop - AC8NS

4,704
2,203
Jun 21, 2012
The electrical circuit sounds okay, but the proper way to join (splice) two wires together is either with solder or a crimped barrel connector. You can then cover the splice with shrinkable tubing or wrap it with electrical tape. DO NOT simply wrap the bared ends of the two wires together and cover with electrical tape! Yeah, I know that will usually work, but it is not good practice and it's a bad habit to get into.

Take the time to make a "Western Union" wrap of the ends of the two wires and apply 60/40 tin/lead rosin-core solder to the join. Then slip a sleeve of shrinkable tubing over the soldered joint and apply sufficient heat to shrink the tubing tightly around the soldered connection. The tubing should shrink to about half its original diameter with very little shrinkage along its length. If you choose tubing that is too small in diameter, it will split when you attempt to shrink it. For solderless breadboard work, the 22 AWG wire should be solid, not stranded. If you do choose to use stranded wire, the ends should be stripped and tinned so the tips do not unravel and splay when inserted in the breadboard.

Back in the day, I had to work with scientists (many of whom held PhD degrees) that knew virtually NOTHING about electricity. One of their "favorite" uses for Variacs was to "control" the speed of gear-reduced synchronous clock motors by "starving" the motors for power. Now this type of motor doesn't use much power to begin with, since it derives it's torque from eddy currents induced in a thin fast-spinning aluminum disk by stator windings, the disk serving as the motor rotor. The lightly loaded disk rotates at a synchronous speed determined by the power-line frequency... when the stator winding is properly energized with AC. These bozos discovered that by reducing the voltage to the gear-reduced synchronous clock motors, they could reduce the output shaft rotation rate. Never mind that they had no idea how fast the output shaft was actually turning since the motors were no longer operating as synchronous motors...

Typically, these motors were installed inside a vacuum chamber used for sputtering. The sputtered material would find its way in a line-of-sight-path to the target specimen and deposit as a thin film. The specimen was rotated with the gear-reduced synchronous clock motor to ensure all sides were exposed to the sputtered material. Problems arose during pump-down and later, when the chamber was brought up to atmospheric pressure. During these two intervals the gas pressure became "just right" to allow a highly conductive plasma to form as a result of the AC applied to the motor. Often the chamber was backfilled with argon after pumpdown to deliberately form a plasma suitable for sputtering. This plasma, intentional or not, caused all sorts of problems when a Variac was used to control the gear-reduced synchronous clock motor..

Since the chamber was made of stainless steel and well-grounded, the "hot" side of the AC line was shorted to ground through the plasma, blowing the fuse in the Variac and sometimes tripping the circuit breaker serving the convenience outlet that the Variac was using. The problem was solved by placing an isolation transformer in the primary of the Variac and NOT connecting the white-wire neutral OR the green-wire ground from the Variac to the deposition chamber frame, which remained grounded to the power-line ground. Thus both leads of the synchronous motor were isolated and very little current actually flowed through the plasma.

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13. ### OrangeArav

38
0
Jul 14, 2015
hevans1944,
Thank you for your reply. My materials have finally arrived and I am connecting components together. I have also purchased a heat gun and shrink wraps to take care of my splice.
As I go through my discoveries/unsats, I will write here. Thanks again for your feedback and valuable advice.

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