hi J,
'TP' stands for Test Point, the Node or Position in the circuit where the signal is measured.

This is LTSpice with the FET's added. using equivalents for the simulation.
The FET analog selector will not work as you expect, consider an alternative method, eg: CMOS Analogue select IC or Relay.

Voltage source V4, inside the bridge wiring is used ONLY to generate a test signal for the circuit [ represents a change in the Thermistor value]

Check the LTS circuit for accuracy.

E

 

hi J,
'TP' stands for Test Point, the Node or Position in the circuit where the signal is measured.

This is LTSpice with the FET's added. using equivalents for the simulation.
The FET analog selector will not work as you expect, consider an alternative method, eg: CMOS Analogue select IC or Relay.

Voltage source V4, inside the bridge wiring is used ONLY to generate a test signal for the circuit [ represents a change in the Thermistor value]

Check the LTS circuit for accuracy.

E

Hi Eric, thanks for the reply. Is there any chance to use a simpler circuit? Both those fets aren't acquirable on my college so I'd have to buy them and wait for weeks to get it. Also, I can't seem to find the FDS4435A component in orcad so I can't sim it.

CMOS Analogue select IC

I suppose the fets solution you presented is this.

or Relay.

Is it something like I was doing with @wayneh ? What do I need to change given that I was also struggling to find a way for it to work? (Nothing against @wayneh, I'm guilty of not understanding much and not beeing able to actually find what's wrong ...)

 

hi j,
CMOS Analog IC, Integrated Circuit, not individual FET's.
Can you confirm what voltage change you have at the IA [ Bridge] inputs.?
Also the voltage Voutput from the IA amplifier.
E

 

hi j,
CMOS Analog IC, Integrated Circuit, not individual FET's.
Can you confirm what voltage change you have at the IA [ Bridge] inputs.?
Also the voltage Voutput from the IA amplifier.
E

According to temperature changes? I get a [0 - 2.2] voltage at the end of my bridge, while having [0 ... 5]V at the end of my IA.

If it's according that specific voltage point you were talking about, my bridge differential output is 0V and my IA output is -116,2 uV

 

hi,
OK,
I can confirm the 2.2V signal.
I have added a Reed relay in place of the FET's to show you as an example ONLY.
Check the TP4, the ADC input to the Arduino, the signal levels are not what you are expecting.
I cannot give you the answer as this is homework.
E

 

hi,
OK,
I can confirm the 2.2V signal.
I have added a Reed relay in place of the FET's to show you as an example ONLY.
Check the TP4, the ADC input to the Arduino, the signal levels are not what you are expecting.
I cannot give you the answer as this is homework.
E

Couldn't actually sim as I dont have that transistor on orcad either :\. Simulated with another one, voltage is 0V at the resistance no matter what.

Nontheless, I notice that voltage values aren't even near what they're meant to be. Perhaps it is something wrong on the output of TP3?

 

hi,
The Comparator switching temperature is incorrect.
Added a 'tmp' for the X axis, to show the problem.
E

 

hi,
The Comparator switching temperature is incorrect.
Added a 'tmp' for the X axis, to show the problem.
E

From what I can see, it simply sits at -12V for the entire 55 - 150 ºC range? Weird, it's correct for me. I've rechecked both circuits twice, they look exactly the same. Worth mentioning that I'm connecting my comparator straight to the IA output instead of using an AC voltage source, so it actually compares the output with my reference voltage.

 

Hi,
I have changed over the input pins to the relay, it now shows that the relay is operating at 95C,
Note the TP1 signal is only 0V thru +3.45v and TP2 signal is -3.2V thru +5V at the changeover point. [on TP4]
E

NOTE:
R21 the Comp input is from TP1

 

Hi,
I have changed over the input pins to the relay, it now shows that the relay is operating at 95C,
Note the TP1 signal is only 0V thru +3.45v and TP2 signal is -3.2V thru +5V at the changeover point. [on TP4]
E

You connected the 47 ohm resistance to TP1 instead of the IA output, that might be the / a issue? I think it's not supposed to connect directly to the signal conditioning circuit

 

I was messing around and actually found a plausible start to the issue. I was just connecting things around and simulating the best I could, turned out almost equal to what I pretend. Probably missing resistors on the branches???

 

hi,
This is what I see when Vo is on R21, it is looking promising.
E

 

hi,
This is what I see when Vo is on R21, it is looking promising.
E

So now the problem is that it goes past 5v which can be corrected by adding a resistor on the empty branch connecting TP1 to the relay right?

 

hi,
For TP1 going higher than +5V, have you considered reducing the Gain of OPA [U4] by changing the value of R11. [ Component Idents are for post #32 image]
E

Update:
Also note that TP2 does not start at 0v, needs correcting.

 

For TP1 going higher than +5V, have you considered reducing the Gain of OPA [U4] by changing the value of R11

Thing is, it was supposedly right. In calculations it stops right on 5V and so it does if I sim it alone.

Noticing that TP2 needs correction, I'm not sure what to do. Here the problem isn't "lowering" the voltage past 5V, it's lowering the voltage at 0,25V and adding a resistor to the empty branch doesn't do it. It simply reduces the end of the graph (therefore not reaching 5v instead).
Perhaps it's the OPA U5 fault? Een though, like I said above, they were supposed to work perfectly with this design.

Also, just noticed. Your signal conditioning circuits aren't commutating at the same point I chose (95ºC) but instead at 118ºC. Is there something wrong?

 

hi,
Look at the U5 resistor changes, ie: R14 and R17

Note: it often necessary to have some variable resistors in this type of circuit in order to correct for component tolerances.

E

EDIT:
Please post your latest circuit diagram.

 

hi,
Look at the U5 resistor changes, ie: R14 and R17

Note: it often necessary to have some variable resistors in this type of circuit in order to correct for component tolerances.

E

EDIT:
Please post your latest circuit diagram.

Got it.

I am afraid my circuit is pretty much incomplete since I can't find the 2N2905 transistor nor the 1N914.

(Idk if you read) Worth mentioning that atm your circuit isn't working in my temperature ranges. I was supposed to divide it into 55º-95º and 95º-150º, and atm your'se is 55º-118º and 118º-150º

 

hi,
The 2N2905 and 1N914 are just general purpose components, use equivalents.
I will look at tempr ranges.
E

 

hi,
If you check the Vo from the IA section, you will see that the Comp U6 TP3 is switching at 3.25v as required.

You have to change the circuit so that Comp TP3 changes at 95C.

E

 

hi,
If you check the Vo from the IA section, you will see that the Comp U6 TP3 is switching at 3.25v as required.

You have to change the circuit so that Comp TP3 changes at 95C.

E

In mine it changes at 3.25V which is the same thing as 95C. You're probably missing the sensor's expression to replace it instead of the resistance probably no? (Also, I'm trying to design the circuit with the components' equivalents like you said before)