DESYNN SYSTEM

There are a variety of different types of Desynn systems available:

The Basic Desynn is generally operated by a rotary motion, however linear

versions are also found. The conversion from linear to rotary motion being achieved by a push rod and gear wheel.

The Micro Desynn was designed to magnify the small movement obtained by such items as pressure measuring devices. They are operated by linear motion.

The Slab Desynn was designed to overcome signally errors inherent in the basic Desynn system. In the vast majority of instances the errors in the basic Desynn could be considered insignificant.

4.1.1 THE BASIC DESYNN 4.1.1.1 Construction

In the basic Desynn system the transmitter comprises an endless resistance wound on a circular former, this arrangement being referred to as a 'Toroidal Resistance'.

Equally spaced at 120° intervals around the resistor are 3 tappings, it is to these that the signal wires are connected. Running on the resistor are two wiper arm type contacts that are spaced apart by 180° and insulated from one another, it is to these that system power is applied.

JAR 66 CATEGORY B1 MODULE 4

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JAR 66 CATEGORY B1 MODULE 4

ELECTRONIC FUNDAMENTALS

JAR 66 CATEGORY B1 MODULE 4

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FUNDAMENTALS

The indicator comprises a two pole permanent magnet rotor, pivoted to rotate inside a soft iron stator, the pointer being attached to the spindle. The stator carries three star connected windings that are connected to the three wires coming from the tappings of the transmitter.

4.1.1.2 Operation

When dc power, is applied to the wiper arms of the transmitter, current will enter the positive wiper arm and divide to flow in both directions, left and right, around the torroidal resistor. Both halves of the resistor have the same resistance, therefore the current in each path will be equal.

The resistance of the resister varies linearly. That is, the change in resistance for every degree of movement around it will be the same, therefore when 28 volts is applied to the system as shown in the diagram, the voltage at tapping 2 will be approx. 9.3 volts, as will the voltage at tapping 3. The voltage at tapping 1 will be 28 volts.

JAR 66 CATEGORY B1 MODULE 4

ELECTRONIC FUNDAMENTALS

JAR 66 CATEGORY B1 MODULE 4

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JAR 66 CATEGORY B1 MODULE 4

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FUNDAMENTALS

The differences in potential at the three tappings cause currents to flow in the wires that connect to the receiver. The flow of current creates magnetic fields around the 3 stator windings in the receiver, which combine to produce a resultant field across the stator. The permanent magnet aligns with the resultant stator field, in turn aligning with the wiper arms of the transmitter.

If, for ease of explanation, the transmitter wiper arms are rotated by 120° clockwise, the potential at tapping 2 will increase to 28 volts, the potential at tapping 3 will remain the same at 9.3 volts and that at tapping 1 will decrease to 9.3 volts. Current will now flow out of the transmitter at tapping 2 into the indicator at terminal 2,

through the first winding where it will divide equally, half returning to the transmitter via terminals 1, the other half via terminals 3. The resultant field now produced across the stator will be in line with stator coil 2, this will cause the permanent magnet rotor, band pointer, to swing around 120° clockwise to once again align with the wiper arms of the transmitter.

Irrespective of the position of the wiper arms in the transmitter, the current flow between transmitter and receiver will always create a field across the stator that aligns with their position.

4.1.1.3 Fail Safe Devices

A problem with the Desynn as shown, is that should the d.c. power to the system fail, the pointer will remain in its last position. This is not a satisfactory situation, the instrument should 'fail safe', that is it should respond in such a way that the fault will be identified. This is achieved by fitting a small permanent magnet in the indicator.

Under normal operation, the field of the permanent magnet is weak in comparison to the fields produced by the coils and therefore has no effect. When power is

removed, the small permanent magnet attracts the permanent magnet rotor, moving the pointer off scale.

JAR 66 CATEGORY B1 MODULE 4

ELECTRONIC FUNDAMENTALS

JAR 66 CATEGORY B1 MODULE 4

ELECTRONIC FUNDAMENTALS

JAR 66 CATEGORY B1 MODULE 4

ELECTRONIC

engineering

uk

FUNDAMENTALS

4.1.2 SLAB DESYNN

If the voltage at the 3 tappings of the transmitter of a basic Desynn are measured as the wiper arms are rotated 360°, it will be seen that they produce a sawtooth

waveform as opposed to a sinewave. This results in the pointer of the indicator not following the transmitter exactly. In most instances the difference is insignificant, however their may be certain circumstances where it cannot be overlooked.

The solution is to use a modified Desynn transmitter called a 'slab Desynn'. In a slab Desynn, the resistor is wound on a slab former and has the power supply connected to it, whilst the wiper arms now provide the output to the receiver, there being 3 wiper arms each displaced from the next by 120°. The output from this device is a sinewave. It can be connected to the same type of indicator and operates in the same way as the basic Desynn.