6229 Magnetron Experimental Basic System Prototype II
Part 4
Pulse Transformer:
.
Before constructing the transformer using the Fair-Rite toroid type 5943003801; it is useful to know how long a PFN pulse can be used
at the primary before running out of volt-seconds. No intention of using a magnetization reset or bias is being considered; hence
the required VS is based upon 0 - to -Bsat.
Experimentally I believe I used a .1uF 600VDC metalized capacitor charged
to 300 volts and discharged it in the five turns wrapped around a single toroid core; the resulting waveform is shown below:
Since I am only interested in using the 0 - to -Bsat portion then the first positive peak is of interest. An expanded view of this
portion is shown below:
> have roughly about 300 volts for ~ 1uSec for 5 turns ==> 3E2 X 1E-6 = 3E-4 VS
> 3E-4/5 turns ==
6E-5 VS per turn 0 - Bmax
> using two cores stacked on top each other hence as stated previously in the Spice simulation
for 5 turns on the primary should have about 6E-4 VS
/__ 6E-4 VS / 325 V = ~ 1.8usec
should
be plenty adequate for 400nsec pulse width.
Since ~ 14:1 turns ratio is being considered then the secondary should have 70turns . For Prototype II I decided not to used
litz wire for the secondary due to the limited required space
Transformer Construction:
The first step I used in constructing the pulse transformer is stacking one ferrite core on top the other:
In order to wind the secondary the cores must be fixed relative to each other. Rather than gluing them together I decided to use a
technique that I saw someone else use that is held together by winding a single layer of blue painter’s tape:
Two separate secondary windings must be used for supplying the magnetron filament heater current.
After the two secondary windings are completed several coats of lacour are applied allowing each layer to dry.
Before winding
the primary over the secondary each must have sufficient insulation to withstand short duration 4KV pulse. One method that I have
been experimenting with consists of layering the inside and outside surfaces of the secondary winding with some sort of insulating
sheets ( ie mylar, or clear transparency sheets , etc). Below illustrates inside surface having been covered:
A word of caution should be mentioned; although the secondary is designed to operate at about 4KV; in situations where the secondary
is not properly loaded ( ie magnetron heater is not on ; ~ 5Kohm test load is not connected) then 8 or more KV pulses could arc to
the primary if not properly insulated from each other.
Next about 12” of the same sheet in wrapped around the outer portions
of the secondary; it can be fastened in place using super glue:
After the inner and outer transparent sheets are attached; the assembly is again coated with several layers of lacour. A top view
of the assembly is illustrated below:
The each primary leg consists of equal lengths of # 18 magnet wire wrapped in parallel; see below:
The next illustrations and photo show the respective primary leg of each connected to the PFN: