COLD CATHODE DIODE 



Another version of the circuit is in Figure 7.9b, for switching pulses of the 

 same polarity. The glow is switched on by making the anode temporarily 

 extra positive, increasing the anode-cathode potential difference as before, 

 but is switched off by making the cathode temporarily extra positive, thus 

 reducing the anode-cathode potential. Alternatively, of course, the circuit 

 may be switched 'on' by negative pulses at the cathode and 'off' by negative 

 pulses at the anode. 



Relaxation oscillator — Relaxation oscillators are those whose output wave- 

 form is non-sinusoidal. An extremely simple relaxation oscillator can be 

 made with a difference diode, a capacitance, a resistance and a direct voltage 

 supply, {Figure 7.10), which exceeds the striking potential of the diode. 



« Output 1 



Output 2o 



o + 1/ 



Output 3 



Output 1 



Output 2 



Output 3 



K_3^^ 



^^^^ 



Time 



Figure 7.10 



On connecting V, the capacitance charges via resistances R and i?j, and the 

 potential at A — Output 1 — rises exponentially towards +K. The tube is 

 non-conducting, so no current flows through R^ and there is no voltage drop 

 across it, so the P.D. between A and earth also appears across the diode. 

 When it reaches the striking voltage of the diode the latter becomes suddenly 

 a conductor of low resistance, and the capacitance discharges quickly, passing 

 current anti-clockwise round the circuit comprising the tube, 7?„ and R^,. As 

 a result of this, the potential at A falls rapidly toward the running voltage of 

 the tube, and eventually falls below it. The discharge in the tube is then 

 extinguished and recharging of the capacitance begins and the cycle is 

 repeated. An output in the form of positive going 'pips' may be taken off 

 across i?„, or negative going pips from across R^. The sum of the peak 

 amplitudes of the two sets of pips is given by the voltage differential of the 

 tube, and the sizes of the two sets are in the same ratio as i?„ to R^. If only 

 negative going pips are required, R^ may be omitted, and vice versa, but in 

 general R„ and R^ must be chosen so that 



Striking voltage — Running voltage 

 " ^ Maximum permissible tube current 



otherwise when the capacitance discharges the tube current will be excessive 

 and the cathode hable to destruction. 



The output at A has the form of a saw-tooth of peak-to-peak amplitude 

 also equal to the tube voltage differential. If C is small the discharge is 



125 



