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(1908) Wt. 58994—4723 15M 3/44 G&St, Gp. 690
JUN 13 1956
. \ t
°)ji 56AA 25420 i
SC.iELIFIED ; JODULATOR FOR COMPONENT TESTING
It is frequently necessary to oonstruot a modulator for the purpose
of life-testing or factory-testing major items suoh as valves'and networks. A
straightforward system for use in these oases was dovisod. It oontains no
valves other than H.T. roctifiors and a Trigatron or open gap. The small
number of components involved renders the unit readily serviceable, reliable and
The basio circuit is shown in Drawing No. ATR.70/775 Figure 1. A
high voltage transformer is grounded at ono end and the other end is attaohod
to the network via a rectifier, arranged to charge the oapacity negatively.
The same transformer winding is likewise connected to a second rectifier in
opposition with the first. This rectifier charges a small condenser C
(100 to 500 uuF) connected bet to on the trigger eleotrode of a spark gap S
and earth. ''
The spark gap S, which may be CV85, CV125 or CV100 according to voltage
requirements, is used to discharge the network N into the load R. The sequence
of operation is as follows. N is charged nogotivoly during one half of the
supply cycle and tho negative charge is maintained. On the positive half-cyole
of the supply the condenser C charges positively through a resistance R1 (about
1 m-ohm in value) to a value sufficient to break down tho trigger-anode gap.
Tho main charge in N thereafter discharges into the load R. The limiting
resistance R1 is used to reduce the current through the trigger-anodo-ground
circuit to a reasonably low value. At the same time R1 is such that tho time-
constant CR1 does not prevent the rise of voltage of C from being nearly equal
to that of the supply waveform. The resistance R2 across C is generally
10/20 times R1 in value and the time-constant CR2 is roughly equal to a quarter
of the time of one cycle of the supply frequency. None of the values is
critical, however, and a fairly extensive range of frequency can be tolerated
on a given system arrangod with the above principles in viow.
The network N is charged to nearly the peak nogative value of the
secondary waveform. Likewise the condenser C may be charged to approximately
the peak positive value (provided CR1 and R2 are properly chosen). It may be
found desirable to have this positive value higher than is obtained from a
singlo cannon winding though this depends on the characteristics of tho gap.
In such cases it is not difficult to extend the winding to give the extra
voltage at a low current rating for triggering purposes.
Operation as a Full-Wave Circuit
The mode of operation on full-wave output can be deduoed from Figure 1
and Figure 2 of Drawing GTR70/81A, The output pulso is always nogative in
this arrangement. The circuit constants were chosen for a supply of 50 c/a
frequency (output repetition rate 100 pps). The positively charged network
requires inversion of tho gap V6 and the anode and trigger are oonnocted by
20 M-ohms, Tho positive charging of tho triggering condenser is offectod
through the blocking condenser of capacity ,002/uF. Thus 0.8 of tho peak
voltage E is available for breakdown of the trigger electrode. This fraction
could bo increased if dosirod by, roduoing the trigger capacity to less
This is only offooted by altering tho supply frequency. However
it is usually desirable to maintain constant repetition rate and this is
automatically ensured. A 50 c/s source con be used at 50 or 100 pps.
Generators of 400 and 500 c/s are available and aircraft generators may be
usod to cover ranges from 800 c/s to 2500 c/ 3 . by changing gear-ratios.
Thus frequencies separated by discrete stops may be made available and
ropotition rates from 50 to 5000 pps realised.
The waveforms observed at various parts of the circuit are shown
in Drawing ATR70/775, Fig.l, and Drg. GTR70/814, Fig.2, for half-wave and ■
full-wavo operation respectively. The last refers to operation at 100 pps.
with a oircuit operating on a 50 c/s input supply frequency. It will
bo noticed that there arc slight differences observed in the pulse shapes
obtained on the altemato cyolcs. It seems that those differences aro due
to the effects of stray capacities and inductances which aro not exactly
alike duo to the asymmetry of components.
It should bo notod that the circuit is of the hold-off typo and
deionisation difficulties arc less marked. Higher repetition ratos may
bo cmployod than with the constant current charging systems.
Authors; S.C. Curran
Approved for Circulation: A. Ward.
19th June, 1944
Numbered Pages: 2.
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Date of Search: 4 November 2009
Record Summary: AVIA 26/694
Title: Simplified Modulator for Component Testing
Availability Open Document, Open Description, Normal Closure before FOI Act: 30 years
Former reference (Department): T-1692
Held by: The National Archives, Kew
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