PWM Temperature-based Fan Controller

Hi all,

I have built a 3 stage Temperature-based Fan Controller to run off a single-rail DC supply of around 12-15V:
1) Wheatstone Bridge (with thermistor as one leg - all 10K resistors)
2) Instrumentation amplifier Instrumentation Amplifier (3 Op-Amp)
3) PWM Generator Basis of PWM Generator
The triangle wave outputs of circuit (2) and circuit (3) are passed into an Op-amp comparator. These output from this will go into digital buffer (4050) -> MosFET driver buffer (IR4288) -> MosFET (connected in ground path of the Fan(s)).
I have modded the PWM Generator a bit as I am using a single supply and I have added in digital buffers (from a 4050 chip) to get a higher trangular wave amplitude.
The PWM frequency is around 20KHz.

Anyway I want to determine the maximum of a number of seperate temperature points... I need someone to reality check this solution I have come up with!!

Basically I propose to have multiple sensors each with duplicate builds of the first 2 stages of the circuit:
1) Wheatstone Bridge+sensor 1 1) Wheatstone Bridge+sensor 2 ...
2) Instrumentation amplifier 2) Instrumentation amplifier ...


*********************************** THE QUESTION IS HERE.... !!! ********
Then I combine all the outputs from 3 instrumentation amplifiers, which will be a DC voltage of 1.2-10.8V above ground (i.e. Op-amp hitting supply rails). I do this by passing each output first through a Schotky diode (to minimise forward drop). The $1,000,000 question is: ' Will I end up with combined output which is the maximum DC voltage of 3 output voltages??'

********************************************************************

I am not quite clear what happens if I link 2+ diodes at the cathode when the anodes are driven by different DC voltages?? Anyone able to clarify this? I ran a simple 2 diode circuit through Spice (last year I think) which seemed to imply that I would get the maximum of the input voltages at the output... But will I also get any evil short circuits or other such nasties??

Thanks for any help you can give in this matter!!

Bob Wya

 

The short answer is... you will get the highest voltage instead of the sum. No bad effect as far as i know, that is what the diodes are for, to block the reverse voltage from higher voltage source to the lower voltage source.

 

Hi

Cheers for the quick reply!!

Cool I'll try that technique out.

Bob Wya