 2002 by ELTRA GmbH Germany – January 2002 – Service Manual OH 900 / ON 900 / ONH 2000
2.1-3
In the regulated, stable condition the voltage on TP8 should preferably be 5.5 V.
If this voltage was 5 V, then the flow would reach the adjusted value immediately after
setting the main power switch to pos. 2
When 5.5 V are adjusted, see 2. 1 , the flow starts from a low value and reaches
the set value within a few seconds.
The following stage is a Pulse Width Modulator ( PWM ).
The average output voltage of the PWM can be measured on TP 10 with a multimeter, in
the range of 20 VDC.
The usual average voltage when the flow is regulated is about 4 VDC.
This PWM signal controls the board NR 32 through an optical insulator.
CAUTION ! High voltage !
The circuit of the NR32 is powered directly from the main.
It is not insulated with any transformer. The DC output voltage of the NR32 is controlled by
the FC21.It can be measured on TP1 of the NR32. The ground is TP2.
Caution:
The ground of the NR32 is not the same as the ground of the FC 21 and as the ground of
all the other boards inside the analyser.
The output DC voltage of the NR 32 ( TP1 ) is, with nominal flow, between 90 VDC and
100 VDC.
The NR32 circuit can supply an output voltage, ranging from zero to maximum about
200 VDC.
When this voltage reaches about 200 VDC, the analyser displays a warning, because the
pump has almost reached max. power.
One end of the coil pump is connected to the DC output of the NR 32.
The other end of the coil is connected to a switching transistor of the NR 32.
This transistor is switched ON and OFF by an oscillator.
This way the pump is supplied with a square shaped AC voltage.
The frequency is about 50 Hz, it is the operating frequency of the pump.
The frequency is adjustable with P1 of the NR 32, in order to set exactly the best
frequency for each individual pump, see 2. 1.
C A U T I O N ! High voltage !
The voltage at the end of the pump coil reaches 600 V peak, because of the inductively of
the coil. It can be measured on TP9 with a scope, however, this is not advisable for
safety reasons.
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