Hi,
I am working on the vehicle air conditioning system.
There are 3 blower fans(24Vdc) and 4 temperature sensors(MCP9700) in my system.
The blower fans have 3 speed stages.
When the speed increases ,the sensor output voltage increases too.(Temperature is constant)
They have common ground(Vehicle body).
İs there anybody who knows that why sensor output voltage increases?

 

The Mcp9700 gives out 10mV per deg C,and has an offset of 500mV @zeroC, on a 5v supply, if your output is changing without temperature there must be a supply problem or the sensor is faulty.

 

The Mcp9700 gives out 10mV per deg C,and has an offset of 500mV @zeroC, on a 5v supply, if your output is changing without temperature there must be a supply problem or the sensor is faulty.

That's it. If the sensor VDD remains a constant less a temperature change the sensor output should remain constant. I would monitor VDD and see what it does with changes in blower speed. You may also want to look at VDD with a scope for any noise changes with blower speed change. Beyond that there really isn't much to look at.

Ron

 

I smell a crusty ground connection.

 

How is it all wired?
The sensors would be best to not share any wiring with the fans, but connect via dedicated wires back to the controller. Then and fan motor current induced voltage drops will not impact on the sensor reading. Like #12 says, suspect ground connections can cause problems. Some times really odd ones.

 

How is it all wired?
The sensors would be best to not share any wiring with the fans, but connect via dedicated wires back to the controller. Then and fan motor current induced voltage drops will not impact on the sensor reading. Like #12 says, suspect ground connections can cause problems. Some times really odd ones.

MCP9700 is on a pcb it takes 24Vdc voltage from main board via a cable.Main board controls blower fans.
and the sensor pcb ground connected to main board ground via a cable too.
I measured ground by an oscilloscope.I observed that when the fans start to work ,ground signal rises 930mV.

 

That's it. If the sensor VDD remains a constant less a temperature change the sensor output should remain constant. I would monitor VDD and see what it does with changes in blower speed. You may also want to look at VDD with a scope for any noise changes with blower speed change. Beyond that there really isn't much to look at.

Ron

I measured VDD,VSS and sensor output.
VDD remains constant, VSS(ground) rises 930mV and then remains constant 930mV and the sensor output rises by blower fans stage.
And sensor works well another system.

 

Hello,

Can you post a schematic?
What is the supply voltage of the sensor itself?
The datasheet states it must be between 2.3 and 5.5 Volts.

Bertus

 

I measured VDD,VSS and sensor output.
VDD remains constant, VSS(ground) rises 930mV and then remains constant 930mV and the sensor output rises by blower fans stage.
And sensor works well another system.

@930mV the sensor is sensing 43C, if it's not at that temperature you must have a bad supply, probably the ground.

 

VSS(ground) rises 930mV and then remains constant 930mV and the sensor output rises by blower fans stage.

I think you've found the problem!
Nearly a whole volt wrong on the ground is significant compared to a 5 volt supply.

 

Hello,

Can you post a schematic?
What is the supply voltage of the sensor itself?
The datasheet states it must be between 2.3 and 5.5 Volts.

Bertus

Hello,
Unfortunately I can post schematics because it is forbidden
The supply voltage of sensor is 5V.

 

Hello,

You CAN post schematics using the "Upload a File" button, below the reply window.
The pop-up will show you wich extentions for the file are allowed.

Bertus

 

@930mV the sensor is sensing 43C, if it's not at that temperature you must have a bad supply, probably the ground.

Yes you are right ,I figured out that the problem was ground conne

Hello,

You CAN post schematics using the "Upload a File" button, below the reply window.
The pop-up will show you wich extentions for the file are allowed.

Bertus

I mean the company I work for prohibits to share schematics.
Thanks anyway

 

Hello,
Unfortunately I can not post schematics because it is forbidden
The supply voltage of sensor is 5V.