Consider this circuit:


simulate this circuit – Schematic created using CircuitLab

Ignoring the different threshold voltage, the (time domain) simulation shows a clear difference between Q1 and Q2. In particular, it takes about 300ms for K1 to go from "fully off" to (almost) "fully on" (and the current continues to rise slightly for some time after that), whereas K2 takes only about 60ms. In other words, Q1 has a very long rise time and could be said to "switch slowly", while Q2 "switches quickly" (at least compared to Q1).

voltage graph current graph

I know relays don't like being turned off slowly (contact arcing), and I suspect being turned on slowly isn't much better. (Note that the turn-on time for the relay itself is on the order of 5-10ms.) Therefore, it seems reasonable(?) that I would rather use Q2 than Q1 for this application.


  • Why is there such a significant difference between Q1 and Q2? Given only a data sheet, how would I go about estimating whether some other MOSFET is likely to act more like Q1 or Q2? What would I look for when selecting a MOSFET in order to get a shorter rise time?
  • I'm considering an FDP3682 in my actual application. Unfortunately, CircuitLab does not have a model for this component. Is there an easy way (without actually buying parts) to determine how an FDP3682 is likely to behave in this circuit?

Note: In my actual application, the gate input is driven by an RC delay, which I have approximated here by a sine function generator. (Naturally, this breaks down badly after the peak of the sine wave, but I'm only really interested in the behavior when the MOSFET switches 'on'.) The original version of this question shows the RC delay and includes additional context.


1 Answer 1


The BS170 is a small signal MOSFET with several ohms ON resistance. The IRF540 is more suitable for power switching. But you are using it in linear mode. You will probably get better results by adding a resistor from the source to GND, which will provide negative feedback. Depending on relay and drive voltage, something like 10 ohms may be a good place to start.

I just saw that V1 is 250 mHz, or a 4 second period. 500k and 10 uF is about 5 seconds TC, but actual delay will depend on many factors. Try a simulation, and provide more details on the relay, drive voltage, and desired operation. Run a time domain simulation and show the graph.

ps. I may have missed some details in your question.

Here is the time domain simulation for your circuit. It looks like the relay is activating after about 1.5 seconds and off when the input turns off after 2 seconds. Reducing R1 decreases the delay, as expected.

The FDP3682 seems similar enough to have a similar delay, but the actual circuit will need to be tweaked to obtain a more exact timing, as MOSFET thresholds vary considerably.

I have not tried a source resistor, but it might produce more repeatable results, although with different RC values.


Relay current

With 10 ohm source resistor and 200k for R1:


Relay current

  • \$\begingroup\$ Unless I'm really missing something, adding a resistor as suggested increased the ramp time the relay sees, significantly, which was exactly opposite the goal. Also you don't seem to have addressed the question, "is an FDP3682 a suitable replacement for the IRF540 in this circuit", at all. (Don't look too closely at what V1 is doing; that's just to see how much time passes between the "alarm" line going high and the relay activating, and also how long it takes the capacitor to discharge when the "alarm" line goes low.) \$\endgroup\$
    – Matthew
    Commented Feb 23, 2023 at 20:59
  • \$\begingroup\$ "Try a simulation"... try following the 'simulate this circuit' link? The Time Domain simulation is already set up. According to the sim, turn-on time should ~0.8s - 2.0s for the FDP3682's threshold voltage of 2V - 4V. The simulation has the IRF540 triggering at slightly less than 1.8s. I think most of your other questions are answered in the body of the Question; if not, please be more specific (and consider using chat). \$\endgroup\$
    – Matthew
    Commented Feb 23, 2023 at 21:07
  • \$\begingroup\$ Did you downvote my answer? I said I hadn't really looked hard at your question, and I have had a lot on my mind, and other things to do. I hope my edit helps, but I really need to know what more you needed. \$\endgroup\$
    – PStechPaul
    Commented Feb 23, 2023 at 22:44
  • \$\begingroup\$ Okay, maybe I see the confusion. I'm not talking about the delay between when V1 turns on and when the MOSFET "turns on". I'm talking about the delay between when the MOSFET starts to turn on and when it is "fully on". Note how the relay current in the first graphs goes from "off" to "on" much more quickly as compared to the second. (I wouldn't expect the MOSFET to significantly affect the RC delay... is that a wrong expectation? I'm also confused as to what's "not repeatable" here?) \$\endgroup\$
    – Matthew
    Commented Feb 23, 2023 at 22:51

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