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Thread: i have few doubts in the temperatre monitor circuit

  1. 26th February 2007 05:09 PM #1
    srimannarayanakarthik
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    Default i have few doubts in the temperatre monitor circuit

    One is he wrote that RL1 mentioned in figure is the Thermistor. Are there any specifications for it? And what is all the construction he made on the diode. how to assemble it on the board. what is it exactly. and the i want to change that circuit to 9V. so i am using lmv 358 instead of ua741. Is it ok to ue or will i face any problem. And also in the circuit provided the output is shown by glowing of led. i want to change that. i output can be only 1.2V. So
    i have made a small correction in circuit. That is the second photo i have attached. will these two work..
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    Last edited by srimannarayanakarthik; 26th February 2007 at 05:12 PM.
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  2. 26th February 2007 05:12 PM #2
    srimannarayanakarthik
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    Default 2nd circuit

    i was not able to add it in that msg. so i doing it here. This is the second one
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  3. 26th February 2007 06:29 PM #3
    j.p.bill
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    Let's try this stepwise:

    The thermistor is undefined. The op amp acts as a comparator. When the voltage across the thermistor is above the setpoint voltage on the 47K pot (has to be wrong, potentiometer values are 50K) the output of the op amp goes high, turns on the transistor, and that puts current through the relay coil.

    The diode is a suppressor to prevent reverse voltage from the relay coil damaging the teansistor at turn off.

    Provided the relay gets enough current, changing to a 9 volt supply will not affect circuit operation - unless it's a battery. Then battery life will limit operation.

    A 358 is as good as the 741 for this circuit.

    I am at a loss to understand the last part. If you are going to use 9 volts, why is the voltage on the transistor collector only 1.2? What kind of relay is going to operate on that voltage?
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  4. 26th February 2007 11:03 PM #4
    audioguru
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    You didn't see the datasheet for the LMV358? Its max supply voltage is only 5.5V. It comes only in surface-mount cases. Maybe you want to use an LM358 which is a fairly normal dual opamp in an 8 pins case.

    The extra transistor needs a resistor in series with its base.
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  5. 27th February 2007 12:46 AM #5
    bananasiong
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    what's the purpose of the 2.2khm: resistor from the base to ground on the 1st picture?
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  6. 27th February 2007 01:06 AM #6
    audioguru
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    Quote Originally Posted by bananasiong
    what's the purpose of the 2.2khm: resistor from the base to ground on the 1st picture?
    It forms a voltage divider with the other 2.2k resistor and hopefully will turn off the transistor when the output of the 741 opamp goes as low as it can to about +1.2V.

    The output transistors of opamps are usually darlingtons that have a 1.2V loss.
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  7. 27th February 2007 02:02 AM #7
    Russlk
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    The output of the LM358 goes nearly to ground, so you probably don't need the transistor. What are you trying to accomplish?
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  8. 27th February 2007 02:06 AM #8
    srimannarayanakarthik
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    Quote Originally Posted by audioguru
    You didn't see the datasheet for the LMV358? Its max supply voltage is only 5.5V. It comes only in surface-mount cases. Maybe you want to use an LM358 which is a fairly normal dual opamp in an 8 pins case.

    The extra transistor needs a resistor in series with its base.
    Sorry. Its LM358 only. Typing mistake
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  9. 27th February 2007 02:10 AM #9
    srimannarayanakarthik
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    Quote Originally Posted by Russlk
    The output of the LM358 goes nearly to ground, so you probably don't need the transistor. What are you trying to accomplish?
    The value above the tuned value of current by 47k resistor will make the led glow. that might be working at 9V. i Want that output to be 1.2V. so i that second circuit. will it serve my need
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  10. 27th February 2007 11:41 AM #10
    Russlk
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    You could connect the LED to the LM358 output and have the same 1.2 volt output without the transistor.
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  11. 10th March 2007 04:07 PM #11
    srimannarayanakarthik
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    sorry for the late delay of the post. i want to give that 1.2v supply to a VLSI board. This the datasheet of the board(http://www.altera.com/education/univ...de2-board.html) . Its input voltage is 7.5 and working voltage is 1.2v.
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