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Understanding Transformers
For Hot Wire Foam Cutter Power Supplies


How Transformers Work


There are many sizes, shapes and configurations of transformers from tiny to gigantic like those used in power transmission.  Some come with stubbed out wires, others with screw or spade terminals, some made for mounting in PC boards, others for being screwed or bolted down.


Transformers are composed of a laminated iron core with one or more windings of wire.  They are called transformers because they transform voltage and current from one level to another.  An alternating current flowing through one coil of wire, the primary, induces a voltage in one or more other coils of wire, the secondary coils.  It is the changing voltage of AC current that induces voltage in the other coils through the changing magnetic field. DC voltage such as from a battery or DC power supply will not work in a transformer.  Only AC makes a transformer work.  The magnetic field flows through the iron core.  The faster the voltage changes, the higher the frequency. 


The lower the frequency, the more iron is required in the core for the efficient transfer of power.  In the USA, the line frequency is 60 Hertz with a nominal voltage of 110 volts.  Other countries use 50 Hertz, 220 volts.  Transformers made for 50 Hertz must be a little heavier than ones made for 60 Hertz because they must have more iron in the core.  Line voltage can vary a little and usually runs between 110 volts and 120 volts or between 220 and 240 volts depending on country or power connections.  A house in the USA has 220 volts coming in but is split to two legs of 110V by grounding the center tap (see configuration section below)


The ratio of input voltage to output voltage is equal to the ratio of turns of wire around the core on the input side to the output side.  A coil of wire on the input side is called the primary and on the output side is called the secondary.  There can be multiple primary and secondary coils.  The current ratio is opposite the voltage ratio.  When the output voltage is lower than the input voltage, the output current will be higher than the input current.  If there are 10 times the number of turns of wire on the primary than the secondary and you put 120 volts on the primary, you will get 12 volts out on the secondary.  If you pull 2 amps out from the secondary, you will only be using 0.2 amps or 200 milliamps going into the primary.


Transformers can be built so they have the same number of windings on primary and secondary or different numbers of windings on each.  If they are the same, the input and output voltage are the same and the transformer is just used for isolation so there is no direct electrical connection (they are only linked through the common magnetic field).  If there are more windings on the primary side than the secondary side, then it is a step down transformer.  If there are more windings on the seconday side, then it is a step up transformer.


A transformer can actually be used in reverse and will work fine.  For example, if you have a step up transformer built for transforming 120 volts to 240 volts, you can also use it for a step down transformer by putting 240 volts into the secondary side and you will get 120 volts on the primary side.  Effectively, the secondary becomes the primary and vice versa.


Transformer Power Ratings


Voltage is measured in volts, current is measured in amps, and the unit of measure for power is watts.  Watts is equal to the volts times the amps.  There is a little loss of power in a transformer due to the combination of resistance and reactance.  Reactance is similar to resistance except it is the resistance to an AC current or more technically, the resistance to change in a change in current due to the change in the field created.  This heat is what limits the amount of current or power a transformer can handle.  The higher the current, the more heat is produced.  When the wires get too hot, the insulation breaks down and shorts with adjacent wires which causes more heat which eventually melts wires and ruins the transformer.


A basic transformer has no additional components and so nothing to protect it from overloading.  If you were to connect the two output wires directly together, that will constitute a short circuit and cause far too much current to flow in both the primary and secondary and you will burn out the transformer.  In the same way, if you use the transformer to power a hot wire foam cutter and you are using a wire with too little resistance for your foam cutter, you will burn out your transformer if you don't have it protected by a proper value fuse or breaker.  You have to make sure that the wire resistance, in other words, the gage or diameter, and the length is correct to limit the amount of current to under the rating of the transformer.


The higher the current, the larger the wires need to be that carry that current.  When the wires are larger, there is less resistance and so less heat.  The power that is changed to heat and lost can be calculated as P=I2R.  That means that if you double the current, the power lost to heat increases by four times.  If the transformer is a step down transformer, then there will be more current on the output and so the wire in the secondary windings will be heavier than the primary.  The reverse is true for a step up transformer.


A transformer may be rated in Amps, Volt-Amps (VA), or Watts (W).  For small transformers, VA and Watts are the same thing for all practical purposes.  In large industrial transformers, power factors get involved and the two can be different.  If the transformer is rated in amps, it usually says X amps at X volts and is rated on the output or secondary side.  A 120V transformer with 24V out rated at 2 amps means that you can only safely pull 2 amps from the secondary side.  You can find the power rating of the transformer by multiplying the rated amps times the output voltage so 2 X 24 = 48 watts.


If the transformer is rated in VA or watts, you can calculate the maximum allowable output current by dividing the VA or watts by the output voltage.  So if the transformer is rated at 48 VA with 24 volts output, the allowable output current is 48 / 24 = 2 amps.


Transformer Configurations


A 120 volt transformer with two wires in and two wires out is very simple.  You hook up the two wires on the primary side, the 120V side, to a wall outlet and your output voltage is on the two wires coming from the secondary side.


When a transformer is shown in an electronic circuit, it is shown as a diagram like shown here.  The parallel lines represent the laminated iron core, the curved lines represent the primary and secondary windings, the circles represent the terminations whether terminals or short wires.


Center Tap


A common configuration is a center tap or CT.  The secondary side has three wires out.  The middle wire on the output side is attached to the secondary coil, usually at the middle.  If the winding ratio is 5 to 1, then with 120V input, you get 24 volts output on the two outside wires but if you connect  an outside wire and the center wire, you get 12 volts because you are using only half the secondary winding making the connection a 10 to 1 ratio.  If the transformer is rated at 2 amps, you still can only use 2 amps output whether you use 12 volts or 24 volts.  Often the center tap is grounded so you then have two 12 volt sources that can be used to make + and - 12V DC after running through a converter (rectifier and filter).



Dual Output


The dual output configuration is similar to the center tap except that instead of connecting a wire to the center of the coil, the coil is separated into two separate coils with wires with terminals or wires coming out from both ends of both coils so four wires come out of the secondary side instead of three. 


If the transformer is a 110V input with two 12V outputs, you can connect the two secondary coils in series to get 24 volts out, or you can connect them in parallel to get 12V out.  You have to be careful to connect the right ends of the two secondary coils in both the series and in the parallel connections.  If you reverse the connections, you will get 0 volts out because the two voltages will cancel each other out.


If the transformer is rated at 48VA, then you can use up to 2 amps out for the 24 volt connection which is no different than the center tap or single 24V output configuration.  However, when connected in parallel, you get 12 volts out but double the output current available so you can get 4 amps out.  You get the full 48VA output where with the center tap 12V output, you can only get half the rated output or 24VA.  This is an advantage in hot wire foam cutters because you have a wider range of wire diameters and lengths depending on whether you connect the outputs in parallel or series.  The series and parallel connections are shown below.




Dual Input


The dual input transformer is often used to make the transformer able to be used in both countries with 120V line voltage and 240V line voltage.  The primary is separated into two separate windings with terminals at each end of both windings so there are four wires or terminals on the primary side.


To use it with 110 volts input, the two primary windings are connected in parallel as in the left diagram below.  Care must be taken to connect the correct ends together.  If they are reversed, the fields cancel each other out because the fields generated by each section of the primary are opposite.  Normally, terminals are labeled with numbers or letters and a diagram is provided on the transformer or in an accompanying data sheet showing how the connections must be made for 110V and 220V.


If the transformer is to be connected to a 220V supply, then the two coils are connected in series and again, care must be taken to connect the correct terminations together.  Parallel connections for 110V and series connections for 220V is shown below.




Dual Input and Output


And of course, you can have both a dual input and a dual output so you have four wires in and four wires out which gives even more flexibility to the use of the transformer. 


Some specialized transformers may have several secondary taps or several secondary windings to provide different voltages and they need not be even numbers.  A transformer could have a 3V, 5V, 12V, and 24 volt output for example.



Autotransformers (Variac)


An autotransformer is often referred to as a Variac which is actually one company's trade name for their autotransformer.  It has a continuous output voltage from zero to a little over the input value.  It works similar to a potentiometer or rheostat except the change in the voltage is due to the field change rather than resistance.  Another difference is that a potentiometer or rheostat is very inefficient because it converts the current flowing through it to heat (Watts = Amps X Volts).  As in all transformers, the resistance is low so the amount of heat generated is much less and so much more efficient at transforming voltage



An autotransformer has only one winding which serves as both the primary and the secondary winding.  Because there is only one winding, there is no electrical isolation between the input and the output but if isolation is not required, then it provides an alternative to multiple winding transformers in some situations.


This transformer has the input wires connected to one end of the winding and the other a little ways from the other end.  The secondary is connected the same point as the input side that is on the end.  The other secondary connection is to a wiper that rides on the top of the windings where the insulation has been removed so the wiper can make contact with the windings at any point on one surface.  The wiper is connected to a knob on the top of the autotransformer so a person can turn the knob to get the voltage they want.  Because one primary wire is connected a ways from the end of the winding, the wiper can go past that point and so provide a voltage higher than the input, typically a 110V output can go up to around 130V on the secondary side.



Because the autotransformer has only one winding, there is only one wire size and so the maximum input current is also the maximum output current.  If a 110 Volt autotransformer is rated at 10 amps, then the maximum output current is 10 amps regardless of the voltage.  If it is rated in Watts or VA, then the amps is calculated by dividing the Watts or VA by the rated input voltage.


The autotransformer is a good alternative to a step down transformer when the range of desired voltages is on the high end or the whole range of voltages is needed but becomes more expensive if the range is on the low end because you have a lot of unused windings.  A step down transformer is more economical.


For hot wire foam cutting, an autotransformer is much more expensive than step down transformers in most applications.  If the voltage required is more than 24 volts, then an autotransformer might be considered.



Phases and connecting multiple windings


For simplicity's sake, I have not mentioned phase but when connecting two or more windings together, the phase becomes very important.  AC current is a sine wave and the voltage changes from positive to negative and back in a sinusoidal rhythm many times per second.  How often the voltage changes is called frequency and used to be called cycles per second but is now called Hertz (abbreviated Hz).  Household current in the USA and some other countries is 60 Hz, in other countries is 50 Hz.  When talking about two wave forms such as you have in two windings, the relationship between the two sine waves is the phase.  If the sine waves line up, they are in phase, if the positive peak of one wave lines up with the negative peak of the other wave, the two waves are 180 out of phase.  The phase between one end of a coil and the other are also 180 out of phase.  When one end is at the positive peak, the other end will be on the opposite peak.  Since there must be a difference in voltage between two points for current to flow, the two ends of the winding must be opposite voltage at any point in time.


The phase difference between two windings depends on the direction of windings and how they are connected so in electrical schematics a dot at one end of the winding indicates the beginning of that winding.  For simplicity's sake, I have left the dots off the schematics in this article.  However, when connecting two coils together, it is very important to connect them correctly. 


For a series connection you must connect the end of one winding to the start of the other winding (windings for multiple coils are always wound in the same direction).  If you connect the start of one winding to the end of the other winding in a series connection, the fields will cancel out and you will get zero output.  This will not hurt the transformer but you will get no output voltage.


When connecting two windings in parallel, you must connect the start of one winding to the start of the other winding and the two ends of the windings together.  In a parallel connection, connecting the wires in reverse will burn up your transformer if not properly protected (proper current rating) by a fuse or circuit breaker. Be very careful when connecting two coils together.


Further reading


This has basically just been an overview for a layperson.  Although physically a transformer is a fairly simple device with few parts, how it actually works is pretty complicated.  I recommend Rod Elliot's excellent articles if you are interested in understanding them better:

Transformers - The Basics (Section 1), (Section 2), (Section 3)

He also has many other articles on electronics including power supplies.



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