OBSERVATIONS ON TELEGRAPH INSTRUMENT USE IN CIRCUIT

by L.E. Trump AL7N

Mainline telegraph instruments with 100 to 150 ohm coil windings obviously introduce a high amount of inductance into any circuit they operate in. For this reason, circuits using these instruments (particularly if several are in the circuit) should use a high value of voltage. Upwards of one hundred volts is not too much.

Resistance is then added to the circuit to regulate the operating current to the desired value — nominally 45 to 50 mA, with limits of 30 to 60 mA or so. Commercial practice for landlines was to use a 130 to 160 volt terminal battery at each end of a single wire ground return circuit.

Snappy Operation

The reason for the use of these high voltages is that the high ‘open circuit’ voltage somewhat overcomes the inductive characteristic of the circuit and speeds the rise-time of current flow when the circuit transitions from ‘open’ to ‘closed’.

This results in clean ‘snappy’ relay and sounder operation. An additional advantage is that each telegraph instrument comprises such a small percentage of the total circuit resistance that several instruments in the circuit can be cut in or out without materially affecting the value of the operating current — or the operating adjustment of the remaining instruments in the circuit.

Voltage the Main Factor

Telegraph circuits can be set up using lower values of voltage, but when more than one or two instruments are in circuit the relay or sounder action tends to get sluggish and the instruments become very difficult to maintain in proper adjustment, especially if other instruments are cut in or cut out.

This is true regardless of the operating current used. In other words, a circuit with three or four 100 to 150 ohm mainline instruments in it and a supply voltage of 24 volts will work much poorer than the same circuit with 100 volts supply and additional ‘ballast’ resistance to obtain the same operating current. The instruments in the circuit with the higher supply voltage will have a wider range of adjustment with varying operating current values.

Rule of Thumb

This is something to keep in mind when setting up telegraph circuits for such purposes as demonstrations, museum exhibits or a home telegraph office. The rule of thumb is to use as high a voltage as possible for the supply and in any case not less than 100 volts or so for circuits with more than two instruments, and then add appropriate series resistance to regulate the operating current to 45—50 mA as desired.

Circuits connecting low resistance practice sets of 4 to 20 ohm should be operated at high current values of around 200 to 250mA. The supply voltage can be lower than that used with the high resistance instruments because there is less inductance in such a circuit to cause problems with current rise time as the circuit is keyed.

In general, all instruments on a given circuit, regardless of its length or the number of instruments used, should be of like resistance value whatever other differences in instruments may exist.

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