I can't understand why Multisim tells me that R2 and R1 will burn out after a while.

What are the correct resistor values that I should use. Or am I doing something wrong?

When a 3.3 V signal is received, the output of the PNP needs to be 24 V. Otherwise, it is to be 0 V.

Simulation works OK when I toggle the switches. However, when I leave the switch on for about 3 secs, R1 and R2 burn out and the PNP output does not work.

enter image description here

  • \$\begingroup\$ Anyway, what do you want to know? You say they will "burnout" - what do you mean by that? \$\endgroup\$ – awjlogan Feb 27 '18 at 14:37
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    \$\begingroup\$ Please can you edit your post and greatly improve the quality of your question, showing your conclusions so far. More details and good spelling will help people understand your problem. The clearer and better your question is, the better the quality of the answers it will attract. Thanks. \$\endgroup\$ – TonyM Feb 27 '18 at 14:37
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    \$\begingroup\$ I have edited the question \$\endgroup\$ – JOHNDOE Feb 27 '18 at 14:42
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    \$\begingroup\$ R2 is gobbling over half a watt, R1 over a quarter. What wattage of resistors are you using? \$\endgroup\$ – Trevor_G Feb 27 '18 at 14:43
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    \$\begingroup\$ Make them bigger... \$\endgroup\$ – Trevor_G Feb 27 '18 at 14:45


Here is the power draw for the two, presumably broken, resistors:

  • 1 kΩ resistor \$\rightarrow\frac{(\sim24)^2}{1000}=576\text{ mW}\$
  • 2.2 kΩ resistor \$\rightarrow\frac{(\sim24)^2}{2200}=261\text{ mW}\$

A resistor that would be safe would be 1 W resistors in this case, they are pretty bulky. I doubt that multisim selects those on default. 1/2 W and 1/4 W are way more common.

I don't have multisim available right now, but I think that the rated power is in the settings of the resistors, might be worth a look.

If I were to make this circuit, then I might have used 10 kΩ or higher. But it all depends on what you are actually trying to do.

  • The 1 kΩ resistor can be a 1 W resistor, or higher, not a 1/2 W or 1/4 W resistor.
  • The 2.2 kΩ resistor can be a 1/2 W resistor, or higher, not a 1/4 W resistor.

The power dissipated for a 10 kΩ resistor in the same setting is \$P = \frac{(\sim24)^2}{10000}=57.6\text{ mW}\$

  • With a 10kΩ resistor you can use a 1/4 W resistor, you can even go as low as 1/16 W resistor.

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