Frans_M_Everaerts_Isotachophoresis_378342.pdf

W ABSORPTION METER
r
+15V -15V Mod
Ti : 2N3553 C1,C2’: 4.7pF
D~ - D : ~ i ~414a c3 : 120 pF
R1: 33n,1/4w C4- Ge: 4.7 nF
u2: 10kh,1/aW c7- c9: ca. 5 n ~
Lsm : radio -frequency choke
L: 3 + 12.5 windings, @ 2mm
Fig.6.24. Electronic circuit suitable for continuous excitation of the mercury electrodeless lamps. L
is a coil made of copper wire (2 mm diameter) of 3 + 12.5 turns; the outside diameter is 15 mm.
off, the modulation frequency must not be lower than 100 Hz (safe limit). Sometimes, if
no modulation is required, low-pressure lamps are difficult to ignite. This can often be
remedied simply by rubbing with a wool cloth. The modulation input ‘Mod’ is connected
with the point ‘to Mod’ shown in Fig.6.29. The supply current has an average value of
80 mA, whether or not modulation is chosen. This means that in both instances the
average amounts of W light are comparable (Table 6.4).
It is very important not to decrease the frequency at which the lamp is excited too
much. Tests on mercury lamps operating at a variety of frequencies showed that these
lamps become discernibly darkened in a few days when operated at a frequency below
50 MHz. Lamps operated continuously at 80 MHz for at least 1000 h showed only slight
discoloration, which could easily be corrected with the circuitry used.
The transistor oscillator circuit needs careful construction. The coil must be fixed
stably to the printed circuit plate because vibration may alter the capacitance between the
various parts and influence the coupling of the coil and the lamp. The light produced by
the lamp would change in intensity and noise would result. The solder points must be
made as short as possible and the wires that have to be used must be made as flat as
possible, because both solder points and wires may influence substantially the coupling
and hence the brightness of the lamp. In order to make a UV source that operates well, it
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