This project from the outset might seem so simple as to be ignored,
but it involves
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and so it encompasses all the items that makes it a valid project for the
Intermediate Course practical and you end up with a very useful test item.
Total time taken to make 3 1/2 hours from start to finish including time
to take photos.
Anticipated time for students 2 hours if the tin is already drilled
and the connector and solder tags fitted. |
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The tin shown is not the only one that would make a good dummy load enclosure
but it the one that was in the cupboards and emptied of treacle and washed
out dried and made ready for use in the project. |
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You can see that the lid has been drilled to take the aerial chassis socket.
Care is needed when drilling that the holes are made in the right place,
and are neither too small nor too big, but just right. A small round file
can be made to ease out the centre hole if you do not have a drill large
enough.
When drilling remember all the safety tips given in this regarding the use
of the drill, file and especially holding the piece that is being worked
upon. |
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Here you can see the 4 x 6ba screws three of which have their 6ba solder
tags in place the forth shows you that the solder tag in simply bent up at
an angle. |
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The chassis socket and the solder tags have now been attached to the under
side of the lid in preparation for soldering up the dummy load.
When you reach this part do make sure that the screws are nicely tight as
tightening then up later might be difficult!!! |
By this stage you have had to do "metal bashing", bending of
the solder tags and fitting the chassis socket to the lid ALL without injury
to yourself. If this is the first kind of construction that has included
"metal bashing" then well done - GOOD metal bashing is perhaps even more
difficult to achieve than good soldering !!! |
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Now form a piece of solid copper wire into a circle around the solder tags
and solder to the outside of the tags with good solder joints.
Then with another piece of solid copper wire solder it to the centre of the
chassis connector, then form the wire into a smaller circle than around the
solder tags and about 30mm from them and solder the end together. You should
now have the project as shown in the picture. |
Now make you should make your first continuity check that
from the copper wire attached to the solder tags to ensure that there is
a complete circuit to the outer part of the chassis socket. If not then find
out why not an correct.
Then make the second continuity check that from the smaller circle of wire
that there is NO CIRCUIT to the larger circle of wire. It there is then find
out why and resolve because this means that there is a sort between the centre
of the chassis connector and the outer part of the connector.
Now with both of those check satisfactory check that the smaller circle of
wire has a circuit to the centre of the chassis socket.
With all of the check completed to your satisfaction you can start to assemble
the resistors. |
Have you thought why you need 20 1k resistors of 2W rating
to make this 50 dummy
load ?
The resistors are being used in parallel thus the resultant resistance value
is the value of one of the resistors divided by the number of resistor. 1000
/ 20 = 50 and the rating of the unit is the number of resistor multiplied
by the rating of one of the resistor 20 x 2 = 40 so 40 watts. |
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Now those of you with eagle eyes will have noticed that the colour code of
the resistors used is Brown Green Red. Why has this value of resistor been
used - well because a wrong order was made and it was a case of use what
was available - so how many resistor will be needed to still make a
50 Dummy load?
Note the the resistors are crowded together. This is merely to ensure that
they will fit into the container. The minimum distance one to another should
be twice the diameter of the component - so even those on the outside will
get warm !!
So have you worked out the new number of resistors? |
The value of of one resistor -
1500 divided
by the (Number of resistors) =
50
So 1500 / 50 = number of resistors = 30
So what is the new power rating ?
30 x 2 = 60W but that is the maximum limit if the resistors were well spaced
and the Dummy Load was running for a long time. However fill the tin with
fresh cooking oil and leave a small gap at the top and put the assembly
into the tin and push down and you would from the original
calculation a Dummy load that for short periods of a few seconds would
comfortably handle 50W the power limit for the Intermediate Licence. If you
left it in use for a long time
the resistors would eventually make the oil hot
and that could be dangerous as there is no vent hole
to let out and internal pressure.
So be careful how you use the dummy load. No responsibility
can be accepted for accidents howsoever they are caused as you have control
of the situation.
The dummy load as shown above should be able to cope for an infinite period
of time at 10W and that is the power at which it will be tested.
The soldering on such a project needs to be "butch" and for certain no dry
joints else the health of the rig is at risk and with the 30 resistors used
there are 60 joint to the resistors, 4 more on the solder tags and at least
2 to make up the centre support link making a total of 66.
So let no one tell you that this is an easy project but it is a simple project.
The measured resistance of this dummy load with a digital meter was
50.4 which is well
within tolerance of the components. |
Component list |
Resistors |
Capacitors |
Semiconductors |
Value |
Required |
Value |
Required |
Type |
Required |
1k 2W carbon |
20 |
None |
None |
or 1k5 2W |
30 |
Other components |
If is essential that the resistors have no inductive component and thus must
be made of carbon |
Aerial connector chassis socket (female) preferable with screw fixing holes
and not a chassis socket which is secured by a single ring nut. |
4 6ba screws and 6 ba nuts
4 6ba solder tags |
Suitable container
two lengths of copper wire |
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