Notes on the Troubleshooting and Repair of Small Household Appliances and Power Tools


Chapter 9) AC Adapters and Transformers

  9.1) AC adapter basics

It seems that the world now revolves around AC Adapters or 'Wall Warts'
as they tend to be called.  There are several basic types.  Despite the
fact that the plugs to the equipment may be identical THESE CAN GENERALLY
NOT BE INTERCHANGED.  The type (AC or DC), voltage, current capacity, and
polarity are all critical to proper operation of the equipment. Use of an
improper adapter or even just reverse polarity can permanently damage or
destroy the device.  Most equipment is protected against stupidity to a
greater or lessor degree but don't count on it.

The most common problems are due to failure of the output cable due to
flexing at either the adapter or output plug end.  See section below on
repair procedure.

1. AC Transformer.  All wall warts are often called transformers.  However,
   only if the output is stated to be 'AC' is the device simply a transformer.
   These typically put out anywhere from 3 to 20 VAC or more at 50 mA to
   3 A or more.  The most common range from 6-15 VAC at less than an Amp.
   Typically, the regulation is very poor so that an adapter rated at 12 VAC
   will typically put out 14 VAC with no load and drop to less than 12 VAC
   at rated load.  To gain agency approval, these need to be protected 
   internally so that there is no fire hazard even if the output is shorted.
   There may be a fuse or thermal fuse internally located (and inaccessible).

   If the output tested inside the adapter (assuming that you can get it
   open without total destruction - it is secured with screws and is
   not glued or you are skilled with a hacksaw - measures 0 or very low with no
   load but plugged into a live outlet, either the transformer has failed or
   the internal fuse had blown.  In either case, it is probably easier to
   just buy a new adapter but sometimes these can be repaired.  Occasionally,
   it will be as simple as a bad connection inside the adapter.  Check the
   fine wires connected to the AC plug as well as the output connections.
   There may be a thermal fuse buried under the outer layers of the
   transformer which may have blown.  These can be replaced but locating
   one may prove quite a challenge.

2. DC Power Pack. In addition to a step down transformer, these include at
   the very least a rectifier and filter capacitor.  There may be additional
   regulation but most often there is none.  Thus, while the output is DC,
   the powered equipment will almost always include an electronic regulation.

   As above, you may find bad connections or a blown fuse or thermal fuse
   inside the adapter but the most common problems are with the cable.

3. Switching Power Supply.  These are complete low power AC-DC converters
   using a high frequency inverter.  Most common applications are laptop
   computers and camcorders.  The output(s) will be fairly well regulated
   and these will often accept universal power - 90-250 V AC or DC.

   Again, cable problems predominate but failures of the switching power
   supply components are also possible.  If the output is dead and you have
   eliminated the cable as a possible problem or the output is cycling on
   and off at approximately a 1 second rate, then some part of the switching
   power supply may be bad.  In the first case, it could be a blown fuse,
   bad startup resistor, shorted/open semiconductors, bad controller,
   or other components.  If the output is cycling, it could be a shorted
   diode or capacitor, or a bad controller.  See the "Notes on the
   Troubleshooting and Repair of Small Switchmode Power Supplies" for more
   info, especially on safety while servicing these units.

  9.2) AC adapter testing

AC adapters that are not the switching type (1 and 2 above) can easily
be tested with a VOM or DMM.  The voltage you measure (AC or DC) will
probably be 10-25% higher than the label specification.  If you get no
reading, wiggle, squeeze, squish, and otherwise abuse the cord both at
the wall wart end and at the device end.  You may be able to get it to
make momentary contact and confirm that the adapter itself is functioning.

The most common problem is one or both conductors breaking internally
at one of the ends due to continuous bending and stretching.

Make sure the outlet is live - try a lamp.

Make sure any voltage selector switch is set to the correct position.
Move it back and forth a couple of times to make sure the contacts are clean.

If the voltage readings check out for now, then wiggle the cord as above in
any case to make sure the internal wiring is intact - it may be intermittent.

Although it is possible for the adapter to fail in peculiar ways, a
satisfactory voltage test should indicate that the adapter is functioning

  9.3) Pocket wall adapter tester/polarity checker

This handy low cost device can be built into an old ball point pen case or
something similar to provide a convenient indication of wall adapter type,
operation, and polarity:

            Probe(+) o-----/\/\-----+----|>|----+---o Probe(-)
                         1K, 1/2 W  | Green LED |
                                       Red LED

* The green LED will light up if the polarity of an adapter with a DC output
  agrees with the probe markings.

* The red LED will light up if the polarity of an adapter with a DC output is
  opposite of the probe markings.

* Both LEDs will light up if your adapter puts out AC rather than DC.

* The LED brightness can provide a rough indication of the output voltage.

The operating range is about 3 to 20 V AC or DC.

  9.4) AC adapter repair

Although the cost of a new adapter is usually modest, repair is often
so easy that it makes sense in any case.

The most common problem (and the only one we will deal with here) is the
case of a broken wire internal to the cable at either the wall wart or
device end due to excessive flexing of the cable.

Usually, the point of the break is just at the end of the rubber cable
guard.  If you flex the cable, you will probably see that it bends more easily
here than elsewhere due to the broken inner conductor.  If you are reasonably
dextrous, you can cut the cable at this point, strip the wires back far
enough to get to the good copper, and solder the ends together.  Insulate
completely with several layers of electrical tape.  Make sure you do not
interchange the two wires for DC output adapters!  (They are usually marked
somehow either with a stripe on the insulator, a thread inside with one
of the conductors, or copper and silver colored conductors.  Before you
cut, make a note of the proper hookup just to be sure.  Verify polarity
after the repair with a voltmeter.

The same procedure can be followed if the break is at the device plug end
but you may be able to buy a replacement plug which has solder or screw
terminals rather than attempting to salvage the old one.

Once the repair is complete, test for correct voltage and polarity before
connecting the powered equipment.

This repair may not be pretty, but it will work fine, is safe, and will
last a long time if done carefully.

If the adapter can be opened - it is assembled with screws rather than
being glued together - then you can run the good part of the cable inside
and solder directly to the internal terminals.  Again, verify the polarity
before you plug in your expensive equipment.

Warning:  If this is a switching power supply type of adapter, there are
dangerous voltages present inside in addition to the actual line connections.
Do not touch any parts of the internal circuitry when plugged in and make
sure the large filter capacitor is discharged (test with a voltmeter)
before touching or doing any work on the circuit board.  For more info on
switching power supply repair, refer to the document: "Notes on the
Troubleshooting and Repair of Small Switchmode Power Supplies".

If it is a normal adapter, then the only danger when open are direct
connections to the AC plug.  Stay clear when it is plugged in.

  9.5) AC adapter substitution and equipment damage

Those voltage and current ratings are there for a reason.  You may get
away with a lower voltage or current adapter without permanent damage but
using a higher voltage adapter is playing Russian Roulette.  Even using
an adapter from a different device - even with similar ratings, may
be risky because there is no real standard.  A 12 V adapter from one
manufacturer may put out 12 V at all times whereas one from another
manufacturer may put out 20 V or more when unloaded.

A variety of types of protection are often incorporated into adapter
powered equipment.  Sometimes these actually will save the day.
Unfortunately, designers cannot anticipate all the creative techniques
people use to prove they really do not have a clue of what they are doing.

The worst seems to be where an attempt is made to operate portable devices
off of an automotive electrical system.  Fireworks are often the result,
see below and the section on: "Automotive power".

If you tried an incorrect adapter and the device now does not work there
are several possibilities (assuming the adapter survived and this is not
the problem):

1. An internal fuse blew.  This would be the easiest to repair.

2. A protection diode sacrificed itself.  This is usually reverse biased
   across the input and is supposed to short out the adapter if the
   polarity is reversed.  However, it may have failed shorted particularly
   if you used a high current adapter (or automotive power).

3. Some really expensive hard to obtain parts blew up.  Unfortunately,
   this outcome is all too common.

Some devices are designed in such a way that they will survive almost anything.
A series diode would protect against reverse polarity.  Alternatively, a large
parallel diode with upstream current limiting resistor or PTC thermistor, and
fuses, fusable resistors, or IC protectors would cut off current before the
parallel diode or circuit board traces have time to vaporize.  A crowbar
circuit (zener to trigger an SCR) could be used to protect against reasonable

I inherited a Sony Discman from a guy who thought he would save a few bucks and
make an adapter cord to use it in his car.  Not only was the 12-15 volts
from the car battery too high but he got it backwards!  Blew the DC-DC
converter transistor in two despite the built in reverse voltage protection
and fried the microcontroller.  Needless to say, the player was a loss but the
cigarette lighter fuse was happy as a clam!

Moral: those voltage, current, and polarity ratings marked on portable
equipment are there for a reason.  Voltage rating should not be exceeded,
though using a slightly lower voltage adapter will probably cause no harm
though performance may suffer.  The current rating of the adapter should
be at least equal to the printed rating.  The polarity, of course, must be
correct.  If connected backwards with a current limited adapter, there may be
no immediate damage depending on the design of the protective circuits.  But
don't take chances - double check that the polarities match - with a voltmeter
if necessary - before you plug it in!  Note that even some identically marked
adapters put out widely different open circuit voltages.  If the unloaded
voltage reading is more than 25-30% higher than the marked value, I would
be cautious about using the adapter without confirmation that it is acceptable
for your equipment.  Needless to say, if you experience any strange or
unexpected behavior with a new adapter, if any part gets unusually warm, or if
there is any unusual odor, unplug it immediately and attempt to identify the
cause of the problem.

  9.6) Replacing batteries with an AC adapter

While most appliances that run off of internal batteries also include a socket
for an wall adapter, this is not always the case.  Just because there is no
hole to plug one in doesn't necessarily mean that you cannot use one.

The type we are considering in this discussion are plug-in wall adapter that
output a DC voltage (not AC transformers).  This would be stated on the

The first major consideration is voltage.  This needs to be matched to the
needs of the equipment.  However, what you provide may also need to be well
regulated for several reasons as the manufacturer may have saved on the cost
of the circuitry by assuming the use of batteries:

* The maximum voltage supplied by a battery is well defined.  For example,
  4 AA cells provide just over 6 V when new.  The design of the device may
  assume that this voltage is never exceeded and include no internal regulator.
  Overheating or failure may result immediately or down the road with a wall
  adapter which supplies more voltage than its nameplate rating (as most do
  especially when lightly loaded).

* Most wall adapters do not include much filtering.  With audio equipment,
  this may mean that there will be unacceptable levels of hum if used direct.
  There are exceptions.  However, there is no way of telling without actually
  testing the adapter under load.

* The load on the power source (batteries or adapter) may vary quite a bit
  depending on what the device is doing.  Fresh batteries can provide quite
  a bit of current without their voltage drooping that much.  This is not
  always the case with wall adapters and the performance of the equipment
  may suffer.

Thus, the typical universal adapter found at Radio Shack and others may not
work satisfactorily.  No-load voltage can be much higher than the voltage at
full load - which in itself may be greater than the marked voltage.  Adding
an external regulator to a somewhat higher voltage wall adapter is best.  See
the section: "Adding an IC regulator to a wall adapter or battery".

The other major consideration is current.  The rating of the was adapter must
be at least equal to the *maximum* current - mA or A - drawn by the device
in any mode which lasts more than a fraction of a second.  The best way to
determine this is to measure it using fresh batteries and checking all modes.
Add a safety factor of 10 to 25 percent to your maximum reading and use this
when selecting an adapter.

For shock and fire safety, any wall adapter you use should be isolated and
have UL approval.

* Isolation means that there is a transformer in the adapter to protect you
  and your equipment from direct connection to the power line.  Most of the
  inexpensive type do have a transformer.  However, if what you have weighs
  almost nothing and is in a tiny case, it may be meant for a specific purpose
  and not be isolated.

* UL (Underwriters Lab) approval means that the adapter has been tested to
  destruction and it is unlikely that a fire would result from any reasonable
  external fault like a prolonged short circuit.

To wire it in, it is best to obtain a socket like those used on appliances
with external adapter inputs - from something that is lying in your junk-box
or a distributor like MCM Electronics.  Use one with an automatic disconnect
(3 terminals) if possible.  Then, you can retain the optional use of the
battery.  Cut the wire to the battery for the side that will be the outer
ring of the adapter plug and wire it in series with the disconnect (make
sure the disconnected terminal goes to the battery and the other terminal
goes to the equipment).  The common (center) terminal goes to other side of
the battery, adapter, and equipment as shown in the example below.  In this
wiring diagram, it is assumed that the ring is + and the center is -.  Your
adapter could be wired either way.  Don't get it backwards!

                       X V  | (Inserting plug breaks connection at X)
      Battery (+) o-------  |
      Adapter (+) o---------+------------------o Equipment (Ring, +)
      Battery/                            |
      Adapter (-) o-----------------------+----o Equipment (Center, -)

Warning: if you do not use an automatic disconnect socket, remove the battery
holder or otherwise disable it - accidentally using the wall adapter with
the batteries installed could result in leakage or even an explosion!

  9.7) Converting an AC output wall adapter to DC

Where a modest source of DC is required for an appliance or other device,
it may be possible to add a rectifier and filter capacitor (and possibly
a regulator as well) to a wall adapter with an AC output.  While many wall
adapter output DC, some - modems and some phone answering machines, for
example - are just transformers and output low voltage AC.

This is also the simplest and safest way to construct a small DC power supply
as you do not need to deal with the 110 VAC at all.

To convert such an adapter to DC requires the use of:

    * Bridge rectifier - turns AC into pulsating DC.
    * Filter capacitor - smoothes the output reducing its ripple.
    * Regulator - produces a nearly constant output voltage.

Depending on your needs, you may find a suitable wall adapter in your junk
box (maybe from that 2400 baud modem that was all the rage a couple of years

The basic circuit is shown below:

            Bridge Rectifier   Filter Capacitor

    AC o-----+----|>|-------+---------+-----o DC (+)
            ~|              |+        |
   In from   +----|<|----+  |       +_|_   Out to powered device
   AC wall               |  |      C ---   or voltage regulator
   Adapter   +----|>|----|--+       - |
             |           |            |
    AC o-----+----|<|----+------------+-----o DC (-)
            ~                -


* An AC input of Vin VRMS will result in a peak output of approximately
  1.4 Vin - 1.4 V.  The first factor of 1.4 results from the fact that the
  peak value of a sinusoid (the power line waveform) is 1.414 (sqrt(2))
  times the RMS value.  The second factor of 1.4 is due to the two diodes
  that are in series as part of the bridge rectifier.  The fact that they
  are both about 1.4 is a total coincidence.

  Therefore, you will need to find an AC wall adapter that produces an output
  voltage which will result in something close to what you need.  However,
  this may be a bit more difficult than it sounds since the nameplate rating
  of many wall adapters is not an accurate indication of what they actually
  produce especially when lightly loaded.  Measuring the output is best.

* Select the filter capacitor to be at least 10,000 uF per 1000 mA of output
  current with a voltage rating of at least 2 x Vin.  This rule of thumb will
  result in a ripple of less than 1 V p-p which will be acceptable for many
  devices or where a voltage regulator is used (but may be inadequate for
  some audio devices resulting in some 120 Hz hum.  Use a larger or additional
  capacitor or a regulator in such a case.

* Suitable components can be purchased at any electronics distributor as well
  as Radio Shack.  The bridge rectifier comes as a single unit or you can put
  one together from 1N400x diodes (the x can be anything from 1 to 7 for these
  low voltage applications).  Observe the polarity for the filter capacitor!

The following examples illustrate some of the possibilities.

* Example 1: A typical modem power pack is rated at 12 VAC but actually
  produces around 14 VAC at modest load (say half the nameplate current
  rating).  This will result in about 17 to 18 VDC at the output of the
  rectifier and filter capacitor.

* Example 2: A cordless VAC battery charger adapter might produce 6 VAC.
  This would result in 6 to 7 VDC at the output of the rectifier and filter

Adding an IC regulator to either of these would permit an output of up to
about 2.5 V less than the filtered DC voltage.

  9.8) Adding an IC regulator to a wall adapter or battery

For many applications, it is desirable to have a well regulated source of
DC power.  This may be the case when running equipment from batteries as
well as from a wall adapter that outputs a DC voltage or the enhanced adapter
described in the section: "Converting an AC output wall adapter to DC".

The following is a very basic introduction to the construction of a circuit
with appropriate modifications will work for outputs in the range of about
1.25 to 35 V and currents up 1 A.  This can also be used as the basis for a
small general purpose power supply for use with electronics experiments.

What you want is an IC called an 'adjustable voltage regulator'.  LM317 is one
example - Radio Shack should have it along with a schematic.  The LM317 looks
like a power transistor but is a complete regulator on a chip.

Where the output needs to be a common value like +5 V or -12 V, ICs called
'fixed voltage regulators' are available which are preprogrammed for these.
Typical ICs have designations of 78xx (positive output) and 79xx (negative

For example:

            Positive                   Negative
        Voltage Regulator          Voltage Regulator
     -----------------------    -----------------------
         7805      +5 V             7905      -5 V
         7809      +9 V             7909      -9 V
         7812     +12 V             7912     -12 V
         7815     +15 V             7915     -15 V

and so forth.  Where these will suffice, the circuit below can be simplified
by eliminating the resistors and tying the third terminal to ground.  Note:
pinouts differ between positive and negatve types - check the datasheet!

Here is a sample circuit using LM317:

                    I +-------+ O                       
    Vin (+) o-----+---| LM317 |---+--------------+-----o Vout (+)
                  |   +-------+   |              |
                  |       | A     /              |
                  |       |       \ R1 = 240     |
                  |       |       /              |           ___
                 _|_ C1   |       |            +_|_ C2      |_0_|  LM317
                 --- .01  +-------+             --- 1 uF    |   |  1 - Adjust
                  |  uF   |                    - |          |___|  2 - Output
                  |       \                      |           |||   3 - Input
                  |       / R2                   |           123
                  |       \                      |
                  |       |                      |
    Vin(-) o------+-------+----------------------+-----o Vout (-)

Note: Not all voltage regulator ICs use this pinout.  If you are not using an
LM317, double check its pinout - as well as all the other specifications.

For the LM317:

1. R2 = (192 x Vout) - 240, where R2 in ohms, Vout is in volts and must be at
   between 1.2 V and 35 V.

2. Vin should be at least 2.5V greater than Vout.  Select a wall adapter with
   a voltage at least 2.5 V greater than your regulated output at full load.

   However, note that a typical adapter's voltage may vary quite a bit
   depending on manufacturer and load.  You will have to select one that
   isn't too much greater than what you really want since this will add
   unnecessary wasted power in the device and additional heat dissipation.

3. Maximum output current is 1 A.  Your adapter must be capable of supplying
   the maximum current safely and without its voltage drooping below the
   requirement in (2) above.

4. Additional filter capacitance (across C1) on the adapter's output may help
   (or be required) to reduce its ripple and thus the swing of its input.
   This may allow you to use an adapter with a lower output voltage and reduce
   the power dissipation in the regulator as well.

   Using 10,000 uF per *amp* of output current will result in less than 1 V
   p-p ripple on the input to the regulator.  As long as the input is always
   greater than your desired output voltage plus 2.5 V, the regulator will
   totally remove this ripple resulting in a constant DC output independent
   of line voltage and load current fluctuations.  (For you purists, the
   regulator isn't quite perfect but is good enough for most applications.)

   Make sure you select a capacitor with a voltage rating at least 25% greater
   than the adapter's *unloaded* peak output voltage and observe the polarity!

   Note: wall adapters designed as battery chargers may not have any filter
   capacitors so this will definitely be needed with this type.  Quick check:
   If the voltage on the adapter's output drops to zero as soon as it is pulled
   from the wall - even with no load - it does not have a filter capacitor.

5. The tab of the LM317 is connected to the center pin - keep this in mind
   because the chip will have to be on a heat sink if it will be dissipating
   more than a watt or so.  P = (Vout - Vin) * Iout.  

6. There are other considerations - check the datasheet for the LM317
   particularly if you are running near the limits of 35 V and/or 1 A.

  9.9) Testing a power transformer

Here are some simple tests to perform where you want to determine if a used
(or new) power transformer with known specifications is actually good:

0. Look for obvious signs of distress.  Smell it to determine if there is any
   indication of previous overheating, burning, etc.

1. Plug it in and check for output voltages to be reasonably close (probably
   somewhat high) to what you expect.

2. Leave it on for awhile.  It may get anywhere from just detectable to
   moderately warm but not to hot to touch and it shouldn't melt down, smoke,
   or blow up.  Needless to say, if it does any of the latter, the tests are

3. Find a suitable load based on: R = V/I from the specifications and make
   sure it can supply the current without overheating.  The voltage should
   also not drop excessively between no and full load (but this depends on the
   design, quality of constructions, whether you got it at Radio Shack :-),

  9.10) Determining power (VA) ratings of unknown transformers

For a transformer with a single output winding, measuring temperature rise
isn't a bad way to go.  Since you don't know what an acceptable temperature
is for the transformer, a conservative approach is to load it - increase
the current gradually - until it runs warm to the touch after an extended
period (say an hour) of time.

Where multiple output windings are involved, this is more difficult since
the safe currents from each are unknown.

(From: Greg Szekeres (szekeres@pitt.edu)).
Generally, the VA rating of individual secondary taps can be measured.  While
measuring the no load voltage, start to load the winding until the voltage
drops 10%, stop measure the voltage and measure or compute the current. 10%
would be a very safe value. A cheap transformer may compute the VA rating with
a 20% drop. 15% is considered good. You will have to play around with it to
make sure everything is ok with no overheating, etc.

(From: James Meyer (jimbob@acpub.duke.edu)).

With the open circuit voltage of the individual windings, and their DC
resistance, you can make a very reasonable assumption as to the relative
amounts of power available at each winding.

Set up something like a spread-sheet model and adjust the output current to
make the losses equal in each secondary.  The major factor in any winding's
safe power capability is wire size since the volts per turn and therefore the
winding's length is fixed for any particular output voltage.  

  9.11) Determining the ratings of a fried power transformer

A power transformer can die in a number of ways.  The following are the most

* Primary open.  This usually is the result of a power surge but could also
  be a short on the output leading to overheating.

  Since the primary is open, the transformer is totally lifeless.

  First, confirm that the transformer is indeed beyond redemption.  Some have
  thermal or normal fuses under the outer layer of insulating tape or paper.

* Short in primary or secondary.  This may have been the result of overheating
  or just due to poor manufacturing but for whatever reason, two wires are
  touching.  One or more outputs may be dead and even those that provide some
  voltage may be low.

  The transformer may now blow the equipment fuse and even if it does not,
  probably overheats very quickly.

  First, make sure that it isn't a problem in the equipment being powered.
  Disconnect all outputs of the transformer and confirm that it still has
  nearly the same symptoms.

There are several approaches to analyzing the blown transformer and/or
identifying what is needed as a replacement:

* If you have the time and patience and the transformer is not totally sealed
  in Epoxy or varnish, disassembling it and counting the number of turns of
  wire for each of the windings may be the surest approach.  This isn't as bad
  as it sounds.  The total time required from start to dumping the remains in
  the trash will likely be less than 20 minutes for a small power transformer.

  Remove the case and frame (if any) and separate and discard the (iron) core.
  The insulating tape or paper can then be pealed off revealing each of the 
  windings.  The secondaries will be the outer ones.  The primary will be the
  last - closest to the center.  As you unwind the wires, count the number of
  full turns around the form or bobbin.

  By counting turns, you will know the precise (open circuit) voltages of each
  of the outputs.  Even if the primary is a melted charred mass, enough of the
  wire will likely be intact to permit a fairly accurate count.  Don't worry,
  an error of a few turns between friends won't matter.

  Measuring the wire size will help to determine the relative amount of
  current each of the outputs was able to supply.  The overall ratings of
  the transformer are probably more reliably found from the wattage listed
  on the equipment nameplate.

If you cannot do this for whatever reason, some educated guesswork will be
required.  Each of the outputs will likely drive either a half wave (one
diode), full wave (2 diodes if it has a centertap), or bridge (module or
4 diodes).  For the bridge, there might be a centertap as well to provide
both a positive and negative output.

* You can sometimes estimate the voltage needed by looking at the components
  in the power supply - filter cap voltage ratings and regulators.

* The capacitor voltage ratings will give you an upper bound - they are
  probably going to be at least 25 to 50 percent above the PEAK of the input

* Where there are regulators, their type and ratings and/or the circuit itself
  may reveal what the expected output will be and thus the required input
  voltage to the regulators.  For example, if there is a 7805 regulator chip,
  you will know that its input must be greater than about 7.5 V (valleys of
  the ripple) to produce a solid 5 V output.

* If there are no regulators, then the ICs, relays, motors, whatever, that
  are powered may have voltage and current ratings indicating what power
  supply is expected (min-max).

  9.12) Determining wiring of unknown power transformer

"I recently purchased at a local electronics surplus store at 35volt center
 tap 2A transformer for a model railroad throttle (power supply).  The
 secondary wires are red-red/yellow-red and I understand how to hook up the
 secondary in order to get two 17.5 volt sources.  My dilemma is the
 primary.  There are SIX black wires (black, black/red, black/blue,
 black/green, black/yellow, black/grey).  Two of the wires were already
 stripped and I hooked these up to 115 VAC but no voltage on the secondary
 side.  Does anyone have any ideas?  I don't know the manufacturer, the
 transformer is in an enclosed case (no open windings).  I also don't know
 if it has multiple primaries that must be connected or if it has five taps
 for different input voltages.  Any ideas????"

Of course, I assume you did measure on the AC scale on the secondary! :-)
Sorry, have to confirm the basics.  My natural assumption would also be
that the striped wires were the ones you needed.

Here is a suggestion:

1. Use an ohmmeter to determine which sets of primary wires are connected.
   The resistances will be very low but you should also be able to determine
   which are just taps as the resistance between them will be very low.

2. Since you already know what the secondary should be, power the secondary
   from a low voltage AC source like another transformer.  Then measure
   across each pair of primary wires.  You should be able to determine
   which are the main wires and which are the taps.

Using a combination of the above procedures should enable you to pretty
fully determine what is going on.  I suspect that you have a pair of
primary windings that can be connected either series (for 220) or parallel
(110) and a tap but who knows.  Do the tests.  If in doubt, don't just
connect it to 110 - you could end up with a melt-down.  Post your findings.

  9.13) Transformer repair

Some power transformers include a thermal fuse under the outer layers of
insulation.  In many cases, an overload will result in a thermal fuse opening
and if you can get at it, replacement will restore the transformer to health.

Where an open thermal fuse is not the problem, aside from bad solder or crimp
connections where the wire leads or terminals connect to the transformer
windings, anything else will require unwrapping one or more of the windings to
locate an open or short.  Where a total melt-down has occurred and the result
is a charred hunk of copper and iron, even more drastic measures would be

In principle, it would be possible to totally rebuild a faulty transformer.
All that is needed is to determine the number of turns, direction, layer
distribution and order for each winding.  Suitable magnet (sometimes called
motor wire) is readily available.

However, unless you really know what you are doing and obtain the proper
insulating material and varnish, long term reliability and safety are unknown.
Therefore, I would definitely recommend obtaining a proper commercial
replacement if at all possible.

However, DIY transformer construction is nothing new:

(From: Robert Blum (rfblum@worldnet.att.net)).

I have a book from the Government Printing Office . The title is: "Information
for the Amateur Designer of Transformers for 25 to 60 cycle circuits" by
Herbert B. Brooks. It was issued June 14, 1935 so I do not know if it is still
in print.  At the time I got it it cost $.10.

(From: Mark Zenier (mzenier@netcom.com)).

"Practical Transformer Design Handbook" by Eric Lowdon.  Trouble is, last I
checked it's out of print.  Published by both Sams and Tab Professional Books.

(From: Paul Giancaterino (PAULYGS@prodigy.net)).

I found a decent article on the subject in Radio Electronics,
May 1983. The article explains the basics, including how to figure what
amps your transformer can handle and how to size the wiring.

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Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server http://www.repairfaq.org/ [Copyright] [Disclaimer]