Winding house-keeping power-supply transformer
(manual procedure)
By
Kalpesh
Introduction:
This document illustrates how to wind the transformer used to obtain housekeeping power supply voltages on typical small circuits. Such transformers have
been used on PCBs .
This transformer is used in a blocking-oscillator configuration to generate the
relevant low voltages required to run chips on a typical PCB design. Power levels
up to a couple of watts can be supplied by this transformer. The input and output
sections/windings are completely isolated with respect to each other. In this
particular example the transformer output generates a +5V, +9V, -9V and
another isolated (with respect to the these three supplies) power supply of +5V.
Typical currents drawn from the supply: +5V@500mA, +9V@100mA, 9V@100mA, +5V@100mA. So the total power provided is approximately 5Watts.
Material required:
The following materials are required to wind this transformer:
1) EE 25x7 Ferrite core
2) Bobbin (horizontal : implying the core lies horizontally) for this core
3) Wire: 35 SWG (Standard Wire Gauge) enameled wire, or dual coated wire
4) Cello-tape (10-12mm width) or Taco Tape
5) PVC insulating tape for SMPS design (5mm wide roll)
6) Teflon tape – used by plumbers – available at any hardware store
7) Fevikwik or any equivalent cyano-acrilite adhesive
8) Glue stick pieces – for securing the winding in certain steps
9) Tools: Thin triangular file, knife, soldering iron/solder wire.
Ferrite Core and Bobbin for use in housekeeping supply
Transformer Making Procedure
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Material required: Tapes and Glue
Preliminaries:
The following diagram illustrates the winding details of the transformer. The
most important thing to follow is the dot-convention. The diagram along-side
shows the winding details for our house-keeping transformer. The view is
from the top. That is as
you see it from top of the
PCB. The pins are on the
other side of this view. All
the three primary side
windings (mains, 230V
side) are connected to
pins on one side of the
bobbin, and all the low
voltage secondary side
windings are on the other
side. This provides for an
extra degree of safety to
isolate the primary and
secondary side of the
transformer windings with respect to PCB connections and tracks.
Between each winding, electrical tapes of various kinds are applied, to
provide adequate insulation, and prevent shorting of turns. Summary of the
number of turns in each winding:
1) NP : 100T of 35 SWG wire on layer 1 - 2 or 35, between pins 8 (dot) and 9
2) NP2: 5T of 35 SWG wire on layer 3, between pins 9 and 10 (dot)
3) NP1: 5T of 35 SWG wire on layer 3, between pins 6 (dot) and 7
4) NS1: 8T of 35 SWG wire on layer 3, between pins 4(dot) and 5
5) NS2: 4T of 35 SWG wire on layer 3, between pins 2 and 3(dot)
6) NS3: 8T of 35 SWG wire on layer 3, between pins 1 and 2(dot)
Transformer Making Procedure
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Dot Convention:
The most important thing about these transformer designs is to strictly adhere
to the dot-convention. Dots indicate phase relationship (same or opposite
polarity) of different windings. Two windings that have dots on one of their
terminals will have the same voltage polarity with respect to the other
terminal, and any time.
Physically, this means that both windings, say starting from the ‘dot’ end, will
have the same sense of turning around the core (say clockwise for both or
anticlockwise for both). Both turn the same way, and this is the most
important thing to remember in dot conventions. If they turn the opposite way,
then the phase difference will be 180 degrees (i.e. of opposite polarity).
To avoid confusion during winding, it is best to mark small dots on the plastic
bobbin using the hot tip of your soldering iron. It is also good to mark pin 1
with a ‘1’ mark, so that all the other pins may be located unambiguously
(follow the same convention used in IC pin numbering). In the images below
the dots are marked. I did not get to mark the ‘1’ at this stage, but did it at the
end. It is better to mark at the start itself.
Construction:
The following procedure illustrates the step-by-step construction of this
transformer. Most of the steps are indicated with images, so that there is no
confusion about the steps.
Step1: Wind NP
Start NP, the primary side
winding.
We
start
by
removing insulation from the
end of the enameled wire
(35 SWG), say 15mm and
then tinning that end. Then
solder it to pin8 of the
bobbin.
This winding will terminate
on pin9 (dot). In the steps
we followed, we started the
winding sense going counter
clockwise (from our view in
this photograph).
Transformer Making Procedure
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The first few turns:
This shows how NP is
wound. The wire must
be
tightly
wound
(without stretching or
bending and kinking of
any kind) around the
bobbin.
Any
gap
between the turns must
be avoided. Try to keep
the gaps minimized by
pushing
the
turns
towards the side the
winding star
Dimensionally
the
bobbin can take up to
56-57 turns. Our target
is to put 55 turns on
this first layer.
Once the first layer is done, continue to wind on layer two, i.e. above layer 1,
until the winding reaches the starting end once again. Typically, if the winding
is done with care
(remember we are
doing this winding
manually),
one
should be able to
accommodate
55
turns on each of the
layers. The two layers
together will therefore
provide the full 110
turns
which
are
required for NP.
The photograph here
shows the two layers
of NP done. The wire
terminates on pin9
(dot), which is not yet
soldered.
This wire will continue to make the next winding to pin10. But first the surface of
this winding must be insulated with tapes. Notice a few small errors during the
winding led to a little anomaly close to the center of the winding.
Transformer Making Procedure
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Step2: Insulate NP
The Teflon tape being soft, makes a nice insulating layer, just above NP. The
surface becomes smooth when the softer and fragile Teflon tape insulation is
enhanced with cello-tape for mechanical strength. The other windings are may
be put on top of this insulating layer. Now that NP is secured well in the bobbin,
one can remove insulation from the enameled wire and solder it to pin9 (dot) of
the bobbin.
Step 3: Wind and insulate NP2
The
NP2
winding
consists of only 5 turns
of wire (35 SWG). For
winding NP2, pin10 is
the dot end. So, when
starting from pin9, one
winds the wire going in
the direction of pin8
(i.e. counter-clockwise,
as was done with NP).
The winding terminates
on pin10, which is the
dot end.
It is important to make
sure that the PVC tape
insulation
is
also
provided between the
return wire (going to
pin10 and the already
wound 5 turns of wires of NP2, starting from pin9. This process will be true for all
such windings.
Transformer Making Procedure
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Step 4: Wind and insulate NP1
Now all three primary side windings are done (pins 6-10). As the photograph
shows, these windings are all well insulated from any other winding that may be
created for the primary side (pins 1-5). In this application, the secondary (low
voltage) windings have very few turns, and they could all get accommodated on
the remaining half of the layer3 space. These secondary windings must also be
well insulated with respect to each other.
Step5: Wind NS2
Notice that pin3 is dot end
with respect to pin2 for
winding NS2.
The important thing about
this transformer is that the
dot convention may be
followed separately for all
the primary windings or all
the secondary windings.
Between all the secondary
windings, NS1, NS2 and
NS3, the dot convention
must be followed.
So here, we actually have
the winding’s direction
sense
reversed
as
compared to the primary. Here we have starting at dot end go counter-clockwise
(remember the bobbin is now seen from the other side!).
Transformer Making Procedure
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Step6: Wind and insulate NS3
NS3 was wound again making sure
that the cross-over of its end going
to pin1 is insulated, using PVC
tape, from its 8 turn winding below
it.
Cover this winding with PVC tape
so that the final winding NS1 can
be done on top of it.
Pin 2 is dot end, for this winding
between pin1 and pin2. 8 turns,
closely spaced, and wound closer
to the secondary side of the
bobbin, ensures decent functional
properties.
Step 7: Wind and insulate NS1
A final look at the dots on primary and secondary sides:
Transformer Making Procedure
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