uniphy Phyaction 740 Series, Информация о сервисе

Страница: 19 / 68

Service Information Phyaction 740 & 790-series Page 17
level for the logic ICs.
2
.3 Electrotherapy unit
Figure 2 shows the functional blocks for the electrotherapy unit and the associated safety circuit. These
circuits are identical to the ones in the Phyaction 780-series. Just a minor simplification has been made
in the setting of the maximum current guarding because for the near future only generation of MF
currents and TENS is envisaged. Also as a consequence thereof the current-mode facility and the allied
switch-over options in the patient output circuit are not needed for the time being.
2.3.1 Waveform synthesis
The current waveform being selected, the microcontroller accordingly controls the waveform synthesizer
to yield the required waveform. The sample data of the waveform are stored in an EPROM and from
there transferred to the D/A-converters in the course of the phase. The magnitude of the resulting wave
is always proportional to the value of V
pot being set by the intensity control-knob because it is
established by modulating this control-voltage with multiplying D/A-converters.
2.3.2 Chopper
The main purpose of the chopper is to apply the waveform to one of the two inputs of the V/C-converter
and to establish in this way the polarity. With TENS the pulses are formed directly by activating the
chopper to pass its input voltage to one half of the V/C-converter.
In case of rectified current generation the chopper is operated on a substantially higher switch-rate and
the output current is rectified before being applied to the patient. The used switching rate yields an
unnoticeable current in case of a defective rectifier.
Figure 2, Functional blocks of the electro-therapy unit