I am trying to simulate a circuit which should pull the supply down. circuit snippet

Problem here is, that the supply is a not current limited and the whole voltage is dropping accross the 0.1Ohm RC of Q4 and the voltage doesn't go down.

Is there a voltage component which behaves like the way I want to? Or what is the right way to simulate a pull down behaviour?

What I want to do, is shorten the supply with the GND when the transistor is in saturation.

Edit 1: For the comments and Arisers answer: After your input I saw that it would be better to name the question: How to design a non ideal voltage source and limit its current.

  • 1
    \$\begingroup\$ You cannot short out a pure voltage source without drawing infinite current. \$\endgroup\$ – Andy aka Oct 14 '19 at 13:22
  • \$\begingroup\$ Can you give more information about why you want to pull down the supply? There is likely a better method. \$\endgroup\$ – Justin Oct 14 '19 at 13:29

You have to put some amount of resistance in series with your voltage source. The simulator is giving you an "ideal" source, while any physical source will have some nonzero amount of source resistance.

You can also add a hard current limiter made from a current source and an ideal diode:


simulate this circuit – Schematic created using CircuitLab


It depends on what behaviour your real voltage supply might have.

If your real voltage supply is a battery it has a rather good defined internal resistance. LTspice allows you to model that if you edit the parameters of your voltage source.

If your real supply is a SMPS it gets more difficult. SMPS usually have shutdown procedures on overload. This must be modeled with some basic logic circuitry, e.g. using comparators and other means. You might resort to already modeled voltage monitoring circuits, but it might turn out complicated.

The third option, i.e. having a dual mode power supply with a constant voltage operating mode, which automatically switches over to a constant current mode on overload can be modeled the way Dave Tweed desribed.

But the first measure is to find out what the characteristics of your real PSU are.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.