GL112 - Using a demountable (dissectible)
transformer
Introduction
This document gives information
on using the demountable (or
dissectible) transformer safely.
The demountable transformer
allows the teacher to do several
demonstrations, including the
step-up and step-down
transformer and the output
voltage dependence on the ratio
of the primary coil to secondary
coil turns.
A demountable transformer is
designed so that the iron core
can be separated to change the
primary and secondary coils (or
windings). The iron core is normally a U core (a U shaped laminated section) with a laminated
yoke (or I core) that is clamped on top, closing the U section. The demountable transformer kit
has a selection of coils that can be fitted to the core. The kit may also have accessories for
additional demonstrations of melting an iron nail, melting solder or wax, and the
jumping ring (Thomson’s ring).
Remember that transformers will only work on ac and as such any voltmeters or
ammeters used to measure the input or output must be designed for use with ac
(suitable multi-meters can be used where separate ac shunts or meters are not
available).
The demountable transformer can be demonstrated safely if set up properly. However, if it is set
up inappropriately or used carelessly, it would be possible to produce high voltages that could
cause a severe electric shock.
General guidance on safe use
Follow the supplier’s instructions, including on the maximum voltage rating of the coils. It is
advisable to keep the input and output voltages to below 28 V ac 1 (except for the mains input to
a 230 V mains primary coil). The primary coils are designed to be used when mounted on the
closed iron core. If the coils are connected when iron core is not closed, or the coil is used
without a core in place, large currents can flow through the coil and it could overheat quite
quickly leading to permanent damage. For the same reason, avoid switching on the 230 V mains
primary coil without an iron core in place. Most designs allow for connecting the 230 V coil to the
mains for short periods (e.g. no more than a few seconds) with the coil mounted on just the U
core with the yoke removed or partially removed. This allows you to demonstrate the jumping
ring and the effect of an air gap in the iron core.
1
CLEAPSS advises limits of 28 V ac / 40 V smooth dc for general practical work , in dry conditions, where electrical
conductors are exposed.
GL112 – RHW Jan 2013
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Step-up transformer or step-down transformer
Use a low voltage input from an ac power pack on the primary coil and avoid coil combinations
that would give a very high output voltage. Remember you can use the ratio of the number of
turns on the primary coil to the number of turns on the secondary coils to calculate the output
voltage and ensure that this is always less than 28 V ac. Do not use the mains coil as the
secondary coil when demonstrating a step-up transformer. The large number of turns on the
mains coil could produce a voltage significantly above 28 V ac – this would be extremely
dangerous because the output current is not limited to 5 mA. Note that a very high voltage across
the secondary coil is also likely to damage the coil insulation.
Melting a nail
When demonstrating the melting nail, use un-plated nails because the plating tends to flake off
when heated and small blobs of hot metal can fly across the bench. If you squeeze the
secondary coil ends together slightly before tightening the screws holding the nail, the nail will be
under a small amount of tension, and will give a better demonstration because the nail separates
cleanly when melting. The demonstrator and observers should wear eye protection because hot
sparks can fly off. Put a heatproof mat below the nail to protect the bench.
Jumping ring (Thomson’s ring)
The yoke is mounted vertically above the primary mains coil and a solid copper or aluminium ring
is placed over the core. When the alternating voltage is applied to the primary coil, the magnetic
field from the induced current in the ring causes it to jump up. It can jump quite a height. Ensure
that the current is switched on for a few moments only, just long enough to produce the effect as
the current through the mains coil will be higher than normal because the core is open, with
consequent increase in coil heating. Take care to avoid burns from the ring because it becomes
very hot.
Induction heating ring
Use eye protection when melting
solder or wax using the induction
ring.
Old or obsolete designs
The mains coils supplied with older
versions of this apparatus may not
be manufactured to current safety
standards. For example they have
open windings or unsafe
connectors for mains voltages, such
as 4 mm sockets for ‘banana’ plugs,
or spade connectors. The picture
right shows an example of a
transformer with unacceptable
mains connectors. The mains coil
should have a direct cord to the mains plug, or an IEC connector for
power supply cords for mains voltages. The windings should be
encapsulated or housed in such a way that you can’t touch them. If
you have an old style mains coil which doesn’t meet this standard of
safety, you should replace it with a modern design if available. If you
are in any doubt about the safety of the apparatus in your school
please contact CLEAPSS for advice.
GL112 – RHW Jan 2013
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