Wheatstone Bridge Balancing PRT100 University Question

[Cmcada]

Good evening,

Fresh blood here!

I am busy revising for a 2nd year exam in Analogue electronics and I have hit a bit of a wall.

The question is simple enough:

Question 12

An instrument is needed to monitor the temperature of a swimming pool and should give a zero reading at 28°C; the desired poll temperature. The pool temperature is allowed to swing by one degree above and below this, where the instrument should give +/-1V respectively, which triggers on and off the heating system.
Assuming a 12V supply and a bridge ratio of one determine the values of R2. And the gain needed to achieve this.

Correct Answer:
R2 = 110.73Ohms
Gain=10.0

Response Feedback:
Yes you have correctly balance the bridge at 28°C and designed the gain to trigger the system



The resistor R2 is found using the characteristic equation for the PRT in question:

100*(1+((3.85e-003)*u)-((5.87e-007)*(u^2)))

Which gives 110.7304 as required.

Using the standard Wheatstone bridge equations, Vout is 0.0103v when the sensor reads 29c - To give the +-1v swing required the gain is therefore 100. Which is not the right answer!

Any ideas where I am going wrong?

I have even modelled this question in Simulink and came out with the same gain value of 100.

Can anyone put me out of my misery?

[Kleinstein]

The voltage at the bridge depends on the voltage driving the bridge. So at enough power to the bridge a gain of 10 might be enough. But here way more than 12 V would be needed.  So driven with 12 V a gian of 100 is about right.


In reallity,
however usually PT100 don't like to much driving voltage, but only a few 100 mV. So the bridge output voltage is much smaller, more like 0.2 mV / K. Thus much more gain (e.g. 5000) is needed to reach 1 V /K. One the other side - 1 V / K ist a pretty high amplification. Konparators can work with much lower levels and this would enable reading non regulated temperature as well.

[Cmcada]

Its an exam question that was set that I have spent the better part of three hours on (its a 10 minute question), at least I have seeded my knowledge that bit further.

It seems like the results I have produced are correct and the mark scheme is incorrect. It is a regular occurrence at my university mind so I would not be surprised!

I am waiting for a response from my lecturer and I will be waiting with baited breath

Thankyou for the response.