I have a question considering LTSpice's voltage Controlled switch ! (http://www.linear.com/solutions/5735)

In my circuit I want to pass on the voltage from one circuit to another with the voltage controlled switch. The switch gets a control signal, which, if >= 1V (or 0.5V), turns the switch on and connects the 2 circuits.

This is currently modelled with a .spice directive. But as I cant do that in hardware, I need to figure something out!

I tried a NPN Transistor, but it didnt really work out as I had to apply the load of the latter circuit to the emitter, what in turn altered the output I was hoping to get !

In short terms: I need something switch that goes "on" when the control voltage is > 0.5 Volt and that doesnt alter the input voltage it is supposed to carry across !

Any ideas are greatly appreciated !

  • \$\begingroup\$ Wouldn't it be nice to have a VCCS in real life? \$\endgroup\$ Apr 6, 2017 at 6:08

2 Answers 2


Use a comparator (I recommend some small amount of hysteresis) set up to trigger at 0.5V and then use it to drive a low Rds(on) MOSFET. I cannot tell which channel mosfet would be the most useful for your application since you do not mention whether you are doing high or low side switching

  • \$\begingroup\$ Thanks for your answer ! I have a AND gate that gives me "1" when the switch is supposed to switch to "on" ! So i guess it's a N mosfet ? But how do I model the treshold voltage in LTSpice ? Do you happen to know that? Cheers ! \$\endgroup\$ Apr 5, 2017 at 14:37
  • \$\begingroup\$ I'm assuming you are familiar with comparator circuits so i will omit that part, otherwise I recommend you google them, they are quite a common type of circuit and plenty of information. To simulate the voltage that triggers the comparator you can use the voltage source in the piecewise linear mode \$\endgroup\$
    – Kvegaoro
    Apr 5, 2017 at 17:09
  • \$\begingroup\$ I'm familiar with comparators, fair enough! By any chance, do you have a rough estimate what the propagation time of such a comparator circuit would come up to (the rough order) ? \$\endgroup\$ Apr 5, 2017 at 17:36
  • \$\begingroup\$ A quick Digikey search says it can vary from 1.3ms down to 0.085ns \$\endgroup\$
    – Kvegaoro
    Apr 5, 2017 at 17:56

Your options are:

1) Use a relay, you will need sufficient current to drive the relay at the voltage the solenoid needs to switch on.

2) Use a mosfet, the gate is controlled by the gate to source voltage Vgs, it needs to be high enough for the mosfet to turn on.

3) Use an NPN transistor (for a low side switch) the current on the base pin needs to be high enough for the transistor to 'turn on'

  • \$\begingroup\$ Hey, thanks for your answer! I tried the NPN thing, which worked fine - on the emitter side, I got exactly the waveform that I wanted. But as soon as I connected a load (my 2nd circuit) to that emitter, there were all kinds of interferences and strange behaviours - maybe the emitter cant handle the load? :O \$\endgroup\$ Apr 5, 2017 at 17:33
  • \$\begingroup\$ If you like the answer, upvote and mark as answered. Make sure you ground things properly, have the right polarity on the NPN and have more than 0.7V of voltage drop across the NPN. \$\endgroup\$
    – Voltage Spike
    Apr 5, 2017 at 18:24

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