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ELETTRODOMESTICI
VIA CHECCHI 98
I 21013 GALLARATE
ITALY
SOFTWARE TRACKING FEATURE TO DETECT A SHORT CIRCUIT BETWEEN THE TEMPERATURE INPUTS
ON THE POWER BOARD MICROCONTROLLER. (F145 FAULT, double oven only)
Given that the temperature sensor input channels on the micro controller are close, UL requires an additional software feature to
detect if these inputs are shorted together and temperature measurement is not working correctly. In fact, if the inputs are shorted, the
control measures the same temperature in both cavities, which is probably lower than the actual one at least in one of them. As a
consequence, the control will try to warm up the cavity over and over, causing a runaway condition.
When the control measures the same temperature in both cavities, it realizes that there is the possibility of a short between the
sensors but also that the temperatures can be actually the same. For this reason, it starts a special software procedure to distinguish
these two situations:
1. The heaters are disabled in the cavity with the lower set point. Now the temperature in this cavity drops down, while in the
opposite the heaters continue to work correctly.
2. Every second the control checks if the short conditions are still present and increments a counter. When this counter overtakes
its threshold value, a F145 fault is generated and both cavities are switched off. As soon as the short conditions aren't detected
anymore, the counter is reset and the heaters formerly disabled are now restored. The short conditions are:
a. At least one cavity must be active, both measuring at least a 200°F temperature.
b. Measured temperatures are within a 20°F range. It can be 40/50°F also, due to different inputs calibration. Depending
on the micro controller internal construction, there is also a physical difference between the two inputs channel, adding a
20°F offset to the upper sensor.
c. Temperature slopes must be within a certain range. For example, if temperature is increasing in one cavity and
decreasing in the opposite, after a few seconds the counter is reset and no F145 occurs.
d. In the active cavity, the heaters must turn on within 6 minutes in any case. To prevent false alarm, there's no short
detection if both ovens are cooling down. The counter is reset.
Tuesday, June 18, 2002
BOSCH 700 SERIES:
SERVICE INSTRUCTIONS FOR OVEN TESTING AND BASIC TROUBLESHOOTING
Page 25 of 28
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