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i would like to create simple circuit to drive an computer fan (12V, 0.5A) is the temperature is above 42 celsius and stop when the temperature decreased below 36 celsius. I am simulating in LTSpice.

The schmitt trigger does trigger when I want. The problem is that the output does not go completely to zero, therefore the pnp transistor never goes fully on and then there is not enough voltage across the "load".

I have read something that this might be due the transistor at the output stage of the opamp. But i really do not now how to fix this problem.

Could someone explain me how to properly drive the load?

greetings,

David

enter image description here

n002 = output of opamp n003 = emitter transistor n005 = voltage acros R5

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  • \$\begingroup\$ Send that 741 back to the museum you got it from & get a real opamp from this century instead. \$\endgroup\$
    – brhans
    Commented May 22, 2016 at 3:00

2 Answers 2

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Reduce the value of R4, to allow more base current in Q1, and therefore more collector current.

If you want a comparator, it is much better to use a chip designed for that function, than to use an ancient op-amp. If you look at the LM741 datasheet, you will see that the 741 can't get its output closer than a couple of volts to either power supply pin.

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  • \$\begingroup\$ Do you have any suggestions for a dedicated comparator chip? \$\endgroup\$
    – d2key
    Commented May 21, 2016 at 20:03
  • \$\begingroup\$ LM339 or LM311 - search on Digikey, Mouser, or your favourite distributor for "analog comparator". \$\endgroup\$
    – Peter Bennett
    Commented May 21, 2016 at 20:25
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I agree with Peter but if you still want to stick with the 741 you will need more gain at the output stage. Something like this:

enter image description here

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  • \$\begingroup\$ I'd recommend reducing R4 to 1k. Plus, the limited 741 swing will not reliably turn the load off. \$\endgroup\$
    – WhatRoughBeast
    Commented May 21, 2016 at 20:41
  • \$\begingroup\$ I will replace the 741 with another chip. And I would also like to add more gain like you did. But just to be sure, you used the 2n3904 as an example right? This transistor will not be able to supply the 6W to r7, or am I missing something? \$\endgroup\$
    – d2key
    Commented May 22, 2016 at 14:51
  • \$\begingroup\$ You are right, a more powerful transistor is needed. \$\endgroup\$
    – Claudio Avi Chami
    Commented May 22, 2016 at 15:37

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