 2002 by ELTRA GmbH Germany – January 2002 – Service Manual OH 900 / ON 900 / ONH 2000
2.16-3
The furnace power is controlled by the software of the electronic rack.
The electronics send a digital command for starting the power and an analogue voltage
for setting the power level.
These commands are controlling the board PW 13, which is the furnace control board.
The digital command for starting the power is a digital "Low".
It can be measured on TP 11 of the PW 13.
TP 11 is connected to pin 6 of the 25-pin plug of the PW 13. This signal comes from
pin 26 of the 96-pin plug of the electronic rack.
The analogue signal for setting the power level can be measured on TP 14 of the PW 13.
TP 14 is connected to pin 20 of the 25-pin plug.
The signal comes from pin 12 of the 37-pin plug on the electronic rack.
One volt DC corresponds to one KW.
The usual range of the furnace power varies between 2 KW and 5 KW, depending on the
application, which means the usual range of the control voltage is between 2 V DC and
5 V DC.
The above mentioned commands, however, are not sufficient to have power. An important
precondition to power the furnace is the water flow. Without water cooling the furnace will
overheat. The message for the presence of water flow is given by the board ON 21,
transferred to pins 11 to 15 of the PW 13 and can be measured on TP 13 of the PW 13.
15 V DC on TP 13 means water flow, zero volts means no water flow.
When this voltage is zero, the thyristor control circuit of the PW 13 will be inhibited
and the furnace power will be off.
The water flow message of the ON 21 is also transferred to the electronic rack
( Pin 8 of the ON 21 to pin 7c of the 96-pin plug of the electronic rack ).
If this voltage is zero, the analysis cycle will not start and the message of the water flow
will appear on the display of the electronic rack. The digital and analogue commands for
the furnace power will not be released by the electronic rack.
This way there is a dual protection for the furnace: Firstly, there are no power enabling
signals from the electronic rack to the PW 13; and secondly, the thyristor control circuit is
directly disabled by the ON 21.
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