This is the circuit of a PWM temperature sensor, I will give you some details about the circuit:

  1. The Electronic control Module which supply power (+8V) at terminal (A)
  2. The Electronic control Module provide ground circuit at terminal (B)
  3. The output signal from the sensor at terminal (C)
  4. The oscillator, which provides the carrier frequency signal, depending on the application, the internal oscillator will provide a carrier frequency that can have the following approximate values 5kHz for temperature sensor
  5. A "Thermistor" element which varies its resistance with the changes of temperature, this variation is received by the amplifier and converted to a digital signal PWM.
  6. An output of the amplifier, which controls the base of a transistor and generates an output of "the duty cycle", measured in percentage of time that the transistor (this also provides protection against preserve circuits between C and B, and C and A terminals) "on" against time that it has been off.

Well I don't have enough experience in electronics so I want to know the sequence of how this circuit works:

  • For example, with the oscillator, what does it mean to provide carrier frequency and what's the function of it in the circuit?
  • How does the operational amplifier work in this circuit?
  • How is the signal converted to PWM?
  • How about the transistor; how does it work as a switch or what is the output signal at terminal (C)?

enter image description here

  • \$\begingroup\$ The device marked as "amplifier" is not just an op-amp. What it actually is you'll have to work out from whatever material you got this drawing from. \$\endgroup\$
    – The Photon
    Oct 31 '16 at 18:24
  • \$\begingroup\$ Hello Photon , thanks for your reply .. The device in training material called PWM amplifier so i don't know if u have info. about it or not .. \$\endgroup\$
    – M.R
    Oct 31 '16 at 19:29
  • \$\begingroup\$ Is the oscillator sinusoidal? Or something else? \$\endgroup\$
    – Reinderien
    Aug 30 '17 at 1:55

I suspect that some things are missing from the schematic you were given.

Consider this similar circuit.


simulate this circuit – Schematic created using CircuitLab

R1 is the thermistor. R2 is a fixed resistor, whose value might be adjusted to tune the circuit. V1 is an oscillator with triangle-wave output. CMP1 is not an op-amp, but a comparator.

Now, the output will be high for the portion of the oscillator cycle that the triangle wave output is greater than the voltage at VS, and low for the remainder of the cycle. Since the oscillator output shape is a triangle, this means the PWM output duty cycle varies linearly with VS.

Modifying the circuit or changing the oscillator waveform to make the output duty cycle vary linearly with temperature applied to the thermistor is left as an exercise for the reader.

  • \$\begingroup\$ Thanks photon for your concern , well i hope another people can help me or give me another ideas. \$\endgroup\$
    – M.R
    Nov 9 '16 at 10:56
  • 1
    \$\begingroup\$ @The Photon's description tells how the circuit must work. The diagram provided by the training material, at the very least, is incomplete. The amplifier, whatever it is, is being used as a comparator. \$\endgroup\$
    – user128351
    Jun 29 '17 at 14:54

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