Small variac

For the last 10 years, I have needed to use a variac for various bench testing. To perform tests in this manner, it was awkward to say the least. Then, when any change was needed or performed, it was swapping leads back and forth again. Then, one evening when I inadvertently connected the wrong end of this test lead to the wrong end of that jumper lead, the whole project went up in smoke!

For starters, that pile of leads and even the DMM had to go.

Small 5 AMP Variac Voltage Regulator Transformer

Also, I wanted more power than my little two amp unit would put out. What I envisioned was the smallest enclosure with the largest practical variac that I could cram into it.

small variac

I also wanted simultaneous voltage and current metering. One NEMA standard three prong outlet, along with a power on switch, and fused both on the input and output would do the trick. They have remained virtually unchanged in all that time which gives testimony to their simplicity and ruggedness of design. The unit is shown schematically in Figure 1 and pictorially in Figure 2.

The coil will reach a potential of VAC at its upper end. This is because the coil is wound on a common core — a hollow iron cylinder with a flatted area of bare exposed wire on the top surface called the commutator. The slider is a spring-loaded carbon brush attached to a central shaft, and when rotated around this commutator, picks off a variable output voltage V2 of 0 to volts.

This action is almost identical to the mechanics of a standard potentiometer. The position of the slider now forms a transformer which — for all practical purposes — follows all the rules of an isolation transformer: volts x amps in equals volts x amps out. Today, Superior Electric and Staco, Inc. But that only touches the tip of the iceberg, and I could go on and on about its uses. Initially, when designing this unit, I decided that a 10 amp variac would be the best compromise between power and manageable size of the finished product.

This has proven to be a good choice over the last couple years since I built it, as I have used it for testing low milliamp devices all the way up to three horsepower woodworking routers speed control design. Even though the name plate rating is 10 amps, it can put out considerably more current for short time intervals minutes. It all comes down to duty cycle and starting with a cold core, then adequate cooling time between these high current runs. To start off this project, the first item to purchase is a bare bones variac unit no case or accessories.

Of course, eBay was the place to look for one. The unit I chose was a Staco They just seem to have a steady stream of these for sale. Figure 2 is typical of these 10 amp units. The type with it sitting on its front face with the dial plate removed is what you want to look for.

What's a good Variac to get?

Now that the most expensive part is out of the way, the next step is to locate an enclosure to house this unit, a couple of digital panel meters, circuit board, power supply, and various hardware.

Fortunately, I had a metal box out in the garage that — with the right shoehorn — I could cram all the parts into I have a habit of filling a 5 lb bag with 10 lbs of parts. I screwed on a handle and some bumper feet that I scrounged from my junk box and I was all set.

I gave it a fresh coat of paint then picked up the rest of the hardware and associated panel meters that I needed. Since I knew the internal circuit board and its power supply board would be quite small, I just allowed about cubic inches of space to fit those in later. Do not go any smaller than this if you use a 10 amp variac.

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In fact, bigger is better. Of course, if you use a smaller variac, just reduce the enclosure accordingly. Once you have all the major components set out in front of you on your workbench, you can then best determine your layout and enclosure size.Welcome, Guest.

Please login or register. Did you miss your activation email? This topic This board Entire forum Google Bing. Print Search. Author Topic: Broken Variac - Fixable?? Read times. I picked up this broken variac for the low price of free. It is a General Radio W2-S Any information on the specs would be welcome. There are many W2s on ebay, but this one seems to be a variant. It has some broken wires. Any ideas on the best way to repair it? Gregg Frequent Contributor Posts: Country:. You haven't even shown what abominations are hiding under the duct tape.

Maybe you are afraid to look, but to me it looks like somebody had a go at it and may have botched it so badly that repair is beyond consideration. I'm sure it is more work than it is worth.

But there is a slight possibility that you could peel back the broken wires far enough to slide some small heat shrink over the longer end and solder a short length of new wire between the two ends. Make absolutely sure that you don't cross any wires or short two turns together. I would look into some UV hardening glue to keep the repaired section as close to original as possible using smallish dots of glue. The open coils are part of the transformer cooling system.

Small variac transformers (03/09/12)

Remember the Variac is a mains powered device and safety is tantamount. I'm sure there will be lots of posts pertaining to safety. Thanks for the reply. I just removed the electrical tape and added a picture to the album.

It appears you were correct with your guess about a previous repair job. The previous repair was using the tape as insulation. Doesn't seem like a terrible idea. Solder and then tape each layer apart as you go instead of struggling with heat shrink. So do the wires need to lay down in their original configuration? How important is this? There appear to be a total of 4 cut strands and 1 repaired strand.

Tape has two problems. First it covers more surface area of the wires leading to potential hot spots; this is why I suggested spots of glue to keep the wires aligned and apart after repairing.Welcome, Guest. Please login or register. Did you miss your activation email?

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This topic This board Entire forum Google Bing. Print Search. I have spend a two evenings to build a case and switches for a small autotransformer. I thought I share. The autotransformer itself is 3 amp unit I bought as a new.

All other parts, except the 2. Contains dangerous voltages and practices. I did start by cutting a nice piece of 2. One might wonder why, well this box have seen 'some' PSU projects along the years. The autotransformer itself is a IP00 unit so it does need a box. Nice feature is the spare carbon, doesn't cost anything for the manufacturer, but certainly is nice bonus for the user. Once you have used something similar the sidecutter method makes you. The wiring is still not as It will be, even the circuit will be a bit different.

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I firts though I put the phase first through classic fuse and then to switch and there to automatic fuse, but I forgot to drill the hole for the fuse holder. I did want the unit to be usable next day today to test the indication light for the unit as it were old V and as a neon bulb I did want to know if the shunt resistor is on the right size for the specific bulb as the current mains voltage is now What I did test it day before quickly with volts it did seem so bright that I did wonder if it were already on the high edge in V setup.

For the fusing I think I switch the thermal fuse to output line and see if it works and leave the classic fuse for the input line. Meratester is on the background measuring a stray voltages as usual. Starting to look something. Next day. I made a test setup for the glimmer neon light.The term 'Variac' from "vary AC" has become generic, and commonly refers to any continuously variable autotransformer.

Variac is a trade name, and has changed ownership several times since early s when the device was first produced. Other trade names include Powerstat and Dimmerstat, and I'm sure there are many others. Some manufacturers simply call their versions 'variable voltage transformers'. I shall use the term Variac small 'v' in its generic form - this does not imply that variable transformers with the Variac branding are more desirable than others, only that I'm used to the term having used it all my working life.

The Variac is a special type of transformer, generally having a single winding and a single layer. The top section of the winding is flattened and machined to remove the insulation and provide a smooth surface for the sliding brush that's used to select the voltage needed.

small variac

Some variations use a roller instead of a brush, but there seems to be no specific advantage. I still have my first Variac branded as such that I bought sometime aroundand it's been dismantled for service once in all that time - see Figure 1a.

It is described as a "Deluxe Slide Regulator" sic. Other low voltage versions use a separate secondary, but still wound in such a way that a brush can make contact with the secondary windings. A photo of one that I have is shown below, in Figure 2a. Isolated secondary versions are rather uncommon, and I don't know of any manufacturer who still makes them. The one shown is very old, and as is obvious was made by Carl Zeiss usually associated with precision optical instruments.

I've had it for years, but it still gets used almost daily. Output is zero to 15V at up to 8. It's still potentially useful of course, once I find a use for it. The traditional Variac is an autotransformer, and provides no isolation between primary and secondary.

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There can be no isolation, because there is only one winding. Isolated versions do exist, but are uncommon.

It's generally easier and cheaper to use a separate isolation transformer if full isolation from the mains is needed. Figure 3 shows the secondary of the Carl Zeiss "transformator", with the roller brush. There are also a few additions that I included to make it more useful as a workshop tool hence the extra wires. Note the toggle switch. Not all of them get used regularly, but all are functional and get used when needed. I must admit that I don't often need the 6kVA unit, but I did get it very cheaply at the time.

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Variacs were probably the first transformer to use a true toroidal core, wound with one continuous strip of suitable steel. It seems probable that early units would have been hand-wound, since it is important that every turn is perfectly aligned with the previous turn. While machines can do this, toroidal winders are a relatively recent invention.An auto-transformer is a nifty device. Unlike a conventional transformer, it only has one winding.

This winding is constructed with taps at various points to allow accessing different numbers of turns in the coil…. Similar in construction to a rheostat a variable resistor but instead of resistance, it is wound of typical transformer magnet wire and works with AC current only. Transformers change the voltage by a change in the ratio of wire turns on the primary to that of the secondary.

In the auto-transformer, however, there is only one winding and the ratio is achieved by selection of the points of contact in the overall winding.

The diagram above was photographed off of the data plate on one of my variacs. This is the normal, buck-boost application and it allows the user to compensate for high or low line voltage as well as apply lower voltages to items. Just like the dataplate schematic says. Now, terminal 3 is the moveable contact.

It has a brush and is connected to the control knob. When you twist it, it glides across and makes contact with any point on the winding you choose. If you take power out by way of terminals 2 and 3, you will find 0 volts when the contact is by point 2…. At this point, in our example, you would be connected to turns of wire. The output would then be v. So there you have it.

Now, as you may have guessed—if you only wanted v output and had no desire to boost it above the input value, you could achieve this by moving the wire connection from 5 to 4….

The big advantage of this, is that unlike a resistance based device, the heat dissipation is low. Additionally, unlike electronic dimmer devices that use SCRs or Triacs, there is no distortion of the waveform. The type of AC wave going in is the type that comes out. Only the amplitude voltage is changed. While these same types of result can be obtained with electronic devices, they are complex and expensive and not nearly as robust and reliable as a basic coil of wire that makes up the heart of a variac.

Variacs typically have found applications in radio gear, motor control, stage lighting, reforming of electrolytic capacitors, neon sign transformer matching, etc…They are also often used to drive saturable reactor chokes for control of larger current devices. Keeping this in mind will insure your variac lives a long, cool, life. This means full current can be available on the output. In either case, make sure to include proper fuse protection in the circuit.

In this diagram, the unit is not even wired as a transformer. It is more akin to the way you wire a rheostat. What good is that? If you wire one in this fashion and apply v to the input, no matter where you set it, it will show v at the output—so long as there is NO load on it.

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In this configuration, it has essentially become a variable inductance device. Inductance is to AC current what resistance is to DC. It basically becomes a big variable choke coil. If you connect an inductive load to it, like a small fan or other transformer, you can effectively control its speed or output this way.

I use this set up on my neon processing bench. It works. Arguably, a saturable reactor would be smoother—and offer more in terms of flexibility of installation layout my BIG variac has to be where I can reach the knob but those are small issues and I already had the monster sized variac on hand. Now, this also works on a small scale too. When run, it would get hot—heat kills things. So, I wanted to dim it down a bit.QRZ Forums. I am working on a HV power supply for a linear amp and I need a variac that will deliver volts at 20 amps.

These are very hard to find, and when I do find one, the cost is prohibitive.

What is a Variac, and how does it work? - Simple Explanation

A fellow ham suggested running two of lower amperage in parallel. Anybody tried this?? If connected accross the AC line in parallel and then tie the wipers together, will this rig up work??? W8FAXFeb 23, May not be a good idea.

If the wipers are not putting out exactly the same voltage, the high one will feed current back into the low one. You could get a high circulating current that could damage the contacts and maybe the winding.

Could work if the Variacs are identical and the wipers moved exactly together. It would help if you could put a small choke in series with each output to reduce any circulating circuit. One way to check such a setup would be to measure the no-load input magnetizing current of each one, then tie them together with the outputs connected together, then rotate the assembly. Any current in excess of the sum of the individual currents would indicate that there are extra circulating currents.

You just might get lucky! Let us all know how it works out. GuestFeb 23, I wouldn't do it! It is, for all practical purposes, impossible to get the variacs to track. The whole purpose of a variac is to be able to vary the voltage depending on the circumstances and you will spend a whole lot of time trying to get both variacs set to exactly the same voltage and may possibly damage one, or both, variacs in the process.

You will get even less current capability because part of the current from one variac will be trying to counteract the other. Also, why do you need so much power? The maximum power output that you can run is watts. Even if your amplifier is only 50 percent efficient, that would mean watts of input power and you are looking at almost a 50 percent load factor over this.

At volts and 20 amps you are getting watts, way over what any linear amplifier should need. If you use a 15 amp variac at volts, then you will get watts which is still considerably more power input than is needed for watts output. Glen, K9STH. K9STHFeb 23, What you need is a filament or maybe 24 or 48 volt transformer with the LOW VOLTAGE winding rated for the amperage you need, and wire that winding in series either buck or boost, maybe even with a dpdt switch to change FROM buck to boost, and then wire the variac into the primary of that transformer.

This will not of course give you total control from max on the plate transformer, but will give you some. Also remember that you cannot just go around varying your plate voltage on a linear amp--what you do is quickly change the plate impedance conditions, and you quickly run out of Q and so on with the plate tuning components, unless you are using a roller coil.

It may be a lot better in the long run to just design the plate components for one plate voltage and be done with it. Also, the size of the variac wouldn't have to be that great at all. Remember that 20 amps at 12 volts is only watts which equates to only 2 amps at volts which is what would be required on the primary of the bucking transformer.

small variac

Of course, if the bucking voltage were higher, the input to the primary would be greater. But even at 24 volts this would only be 4 amps and that is still only a small variac. At 48 volts this would be 8 amps, again well within the specifications of the 15 amp variac.

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