mohamed.elsabagh
Full Member level 2
I have a problem with max6675, I designed a board containing 5 max6675 IC with interface stated in datasheet. the serial interface of those are routed to a pin header which is connected through a flat cable to the main micro-controller board. As for the thermocouple interface the negative terminal is grounded as stated in the datasheet, and the connector is in two levels. The First connector is female to male connector, from the male connector the couple of wires are connecting it to the thermocouple female connector (where the thermocouple is connected to). The reason why I didn't use thermocouple connector directly on the PCB, is a design issue in the chasis of the device.
The problem: the Software on the micro-controller is supposed to read the 5 max's every 0.25 seconds, and when the thermocouple is disconnected the micro-controller should sense that (because the negative terminal of the thermocouple is grounded). For some reason when the thermocouple is disconnected the max doesn't send the reading that indicates that the thermocouple is disconnected but instead send random reading which by trials I found it ranges from around 258 to 273 according to the ambient temperature change, but this problem doesn't appear on all the 5 max's but it appears in one or two of them. When I assemble another board it may work perfectly or the problem appears on one or more IC. So this error is not fixed but nonconsecutive error.
i.e. if board#1 has a problem in max1 and rest of max's work fine if I replace max1 with max2 the problem will be on max2 and max1 will work fine.
board#2 may have problem on max 3 and rest of the max's will work fine.
Even for the board where the max's work perfectly in room temperature if the ambient temperature raises to 40 degree max's with disconnected thermocouple might give this error, although it's stated that the ambient temperature of the max IC is up to 80 degree.
I can't figure out where the problem is it a design problem or software or simply the problem is with the IC?
The problem: the Software on the micro-controller is supposed to read the 5 max's every 0.25 seconds, and when the thermocouple is disconnected the micro-controller should sense that (because the negative terminal of the thermocouple is grounded). For some reason when the thermocouple is disconnected the max doesn't send the reading that indicates that the thermocouple is disconnected but instead send random reading which by trials I found it ranges from around 258 to 273 according to the ambient temperature change, but this problem doesn't appear on all the 5 max's but it appears in one or two of them. When I assemble another board it may work perfectly or the problem appears on one or more IC. So this error is not fixed but nonconsecutive error.
i.e. if board#1 has a problem in max1 and rest of max's work fine if I replace max1 with max2 the problem will be on max2 and max1 will work fine.
board#2 may have problem on max 3 and rest of the max's will work fine.
Even for the board where the max's work perfectly in room temperature if the ambient temperature raises to 40 degree max's with disconnected thermocouple might give this error, although it's stated that the ambient temperature of the max IC is up to 80 degree.
I can't figure out where the problem is it a design problem or software or simply the problem is with the IC?
