Full transcript
0:30Transformers are everywhere inside and
0:32outside of your home they're in TV sets
0:35sound systems computers and almost every
0:37electronic device this thing is a very
0:41familiar Transformer that is often used
0:43to provide lower voltages to electrical
0:45devices that cannot connect directly to
0:47the higher voltages of the
0:49wall also familiar are the Transformers
0:52that look like trash cans high up on
0:54telephone poles there's probably one
0:56near your school or neighborhood these
0:58Transformers are used to step down the
1:00voltage and correspondingly up the
1:02current to make it safer in the
1:06home understanding Transformers involves
1:08knowing that currents can produce
1:10magnetic fields and conversely that
1:13changing magnetic fields can also
1:15generate
1:16currents when I pass current through
1:18this coil the compass will
1:21deflect it's an
1:24electromagnet now we perform the reverse
1:27experiment
1:31when I move this magnet through the coil
1:33the galvanometer indicates that a
1:35current is
1:37Flowing notice that when I hold the
1:39magnet still no current is produced the
1:42magnetic field must change in order to
1:44produce an electric field that motivates
1:46the charges note also the direction I
1:48move the magnet makes a
1:52difference since Michael Faraday was the
1:54first person to publish this discovery
1:56it's usually known as Faraday's law now
1:59the Transformer puts both of these ideas
2:02together but instead of using a bar
2:03magnet it uses an
2:06electromagnet when I turn on the power
2:08supply it briefly produces a changing
2:11magnetic field in the first coil and
2:13that produces a brief electric current
2:15in the second
2:16coil also when I turn the power supply
2:19off again there is a brief current but
2:21in the other
2:23direction this is the fundamental design
2:25of a transformer two coils that are
2:28connected magnetically but electrically
2:30separated constitute a
2:33Transformer the magnetic flux between
2:35the coils is greatly increased when a
2:37soft iron bar is added this improves the
2:40efficiency of the Transformer by
2:42strengthening the field and preventing
2:43it from diverging out and being
2:45wasted whoa much
2:50better the Transformers found in home
2:52appliances usually look like this here
2:55are the coils which are wound on top of
2:57one another and this surrounding
2:59structure is soft iron shaped to guide
3:02the magnetic field lines back around and
3:04through the
3:05coils typically a Transformer is
3:08supplied with an alternating current
3:10since if the magnetic field does not
3:12change the Transformer would not work
3:15incorrectly using DC for the input of a
3:17transformer like say a battery might
3:20even damage
3:22it the number of turns on an Ideal
3:25Transformer is in proportion to the
3:27voltage across those turns
3:30here I have turns in an 8:1 ratio and
3:33the voltage of 3 volts from my signal
3:35generator is stepped up by that same
3:38ratio it should be 3 volts to 24 volts
3:42but this is not an ideal Transformer and
3:44there are some losses mostly due to flux
3:48leakage we need Transformers because
3:50it's more efficient to send electricity
3:52at higher voltages
4:01this is the symbol for a Transformer but
4:03here it's connected in an unusual way
4:05there's a light bulb in series with the
4:07primary coil which is connected to the
4:09power source but also another light bulb
4:12connected to the secondary coil which is
4:14powering it
4:16also here's the same setup in real
4:19life as you can see both bulbs
4:23glow now what do you think will happen
4:25if I remove the bulb in the primary
4:27circuit
4:32no real surprise there I cut the power
4:34to the whole thing like opening a switch
4:37the next question is what do you think
4:38will happen if I remove the bulb in the
4:40secondary circuit but leave the bulb
4:42connected in the
4:47primary aha without the second bulb
4:50there is an increase in impedance to the
4:52current
4:54flow impedance Z measured in ohms is the
4:57ratio of the voltage drop across the
4:59device to the current that passes
5:01through it this is very much like
5:03resistance but it's a more general term
5:05that refers to all the elements of the
5:07circuit inductor coils capacitors and
5:12resistors we usually discuss
5:14Transformers for stepping up and down
5:16voltages but they should also be thought
5:18of as impedance matching devices when
5:21the secondary circuit is open there is
5:23no current to flow to oppose the changes
5:25in the primary circuit and it
5:27experiences self imp
5:32if you want to do this demonstration
5:33yourself you must have an efficient
5:35Transformer and you should try to have
5:37the same wattage bulbs at their
5:38appropriate voltages in the primary and
5:40secondary circuit for this purpose we
5:43used an isolation Transformer that is a
5:46Transformer with a turn ratio of 1:
5:50one it is convenient to alter a duplex
5:53Outlet so that each outlet is in series
5:55with the other one this can easily be
5:57done with an inexpensive duplex out
5:59outlet that has had one of the copper
6:01connections between the two outlets
6:03cut using an extension cord with the
6:06insulation Stripped Away connect to
6:08either screw of the cut
6:10side and now we have our series Outlet
6:13Mount this in an outlet box for safety
6:16and clearly label it as a series Outlet
6:18it's not unsafe to mistake this for a
6:20regular outlet but most items will
6:22probably not work in
6:25it if you don't already own an isolation
6:28Transformer
6:30you can buy a small Triad transformer
6:33for about
6:38$20 its only shortcoming is it has a
6:40small power rating and should only be
6:42used with bulbs of less than 25
6:45watts if you have a highquality Step
6:47Down Transformer like this 10:1 toroidal
6:50then you can still do the demo but you
6:52should work to ensure that your light
6:54bulbs have the same wattage
7:00toroidal Transformers have excellent
7:02flux linkage and so they tend to be more
7:06efficient one more thing since
7:09Transformers have to match impedances in
7:11order to work it would make sense that
7:13you would have different Transformer
7:14constructions for different
7:16frequencies these are highfrequency
7:19Transformers for television
7:21application these are Balon Transformers
7:24that are used to match the impedance of
7:26the 300 ohm flat TV lead to the 7 5 ohm
7:30impedance of the coaxial cable lead if
7:32we look inside the Transformer we can
7:35actually see the windings the
7:37Transformer is toroidal and the wire is
7:39wrapped around this ferite
7:41core since it is used to match 300 ohms
7:44to 75 ohms the coils have a different
7:47number of
7:49turns Transformers are everywhere
7:51matching impedances in multiple ways a
7:54good lab might be to have your students
7:56try to figure out the turns ratio of a
7:58mystery Transformer by measuring the
8:00voltages remind your students that twice
8:03as many turns will have twice the amount
8:05of flux through them and therefore twice
8:07the voltage will be generated for the
8:09same change in magnetic field
8:36think