# Minimize current spike during contactor turn on in an inrush controller

#### ak52

Joined Oct 15, 2014
230
Hello Experts,
When trying to simulate a inrush controller in LTspice, we see there is a sudden spike in the capacitor current , whenever the contactor(S1) is turned on.
I understand the positive spike, but why does the negative spike occur?

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#### LowQCab

Joined Nov 6, 2012
4,199
What problem are You trying to solve ?
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#### MisterBill2

Joined Jan 23, 2018
18,964
With an unmarked set of plots there is nothing to discuss.
Also, in many cases a simulator gets confused.

#### Danko

Joined Nov 22, 2017
1,842
I understand the positive spike, but why does the negative spike occur?
Only positive spike:

@ak52: Contactor like relay, changes its impedance instantly,
but not smoothly, during 100ms, like in your circuit.
So, it should be something like this:

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#### MisterBill2

Joined Jan 23, 2018
18,964
Just exactly where is that "negative spike" measured between?? When the contactor opens suddenly the voltage at the cap will drop a small amount because of the voltage drop across the 1000 ohm resistor. Recall that in this simulator, voltage sources can supoply infinite current because they have zero internal resistance. The simulator world is different from the real world.

#### ak52

Joined Oct 15, 2014
230
Just exactly where is that "negative spike" measured between?? When the contactor opens suddenly the voltage at the cap will drop a small amount because of the voltage drop across the 1000 ohm resistor. Recall that in this simulator, voltage sources can supoply infinite current because they have zero internal resistance. The simulator world is different from the real world.
The current that is measured is the current flowing through C1, Yes I see that when the contactor opens the voltage drops slightly, , but that does not explain why the current goes positive and then suddenly turn negative , whenever the contactor is switched on?

PS: Once i gave some internal resistance to the voltage source , the negative spike has gone, Could you pls explain this?

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