Current and voltage during the switching process of a DC solenoid valve

When switching on a DC solenoid valve, current and voltage curves are key parameters for assessing the behavior of the solenoid coil. In many applications, the coil is simplified as an idealized L-R series circuit—a combination of a fixed inductance (L) and an ohmic resistance (R) that describes the copper losses of the coil wire.

But what does this look like in reality? Let's take a closer look at the actual switching process.

1.    The idealized view: The L-R series circuit

In a simple model (L-R circuit), the current curve when switching on is as follows:

  • The current rises exponentially until it is limited by the series resistance.
  • This curve is reproducible and predictable – but only under the assumption that the inductance remains constant.

In practice, however, this is not the case, as one crucial factor changes dynamically: the magnetic core.

2.    The real dynamics of the magnetic coil

The solenoid coil of a valve contains a movable magnetic core that undergoes a defined movement during the switching process. This movement significantly influences the inductance of the coil and leads to a more complex current curve.

The actual process can be divided into five steps:

Step 0 – Retirement

  • The solenoid valve is switched off.
  • No current is flowing through the coil.

Step 1 – Switch on

  • The magnetic coil is supplied with voltage.
  • The current begins to rise in line with the idealized model.

Step 2 – Start of movement

  • At a certain point, the magnetic core begins to move.
  • This movement changes the magnetic properties of the system: the inductance increases.
  • Due to the induced counter-EMF (electromagnetic force), the current drops briefly despite the constant voltage.

Step 3 – End position reached

  • The magnetic core has reached its end position.
  • The movement ends, the inductance remains constant.
  • The current begins to rise again – this time similar to the idealized curve.

Step 4 – Maximum current

  • The current reaches its maximum.
  • This value is limited by the ohmic resistance of the coil.

 

Strom und Spannung während des Schaltvorganges eines DC-Magnetventils

Conclusion

The switching process of a DC solenoid valve is more complex than a simple L-R model suggests. The movement of the magnetic core has a significant impact on the current flow:

  • Dynamic inductance due to movement of the core
  • Temporary current drop due to counter-EMF
  • Current increase after movement ends until stable final value is reached

A deeper understanding of these processes is essential for the precise design and optimization of solenoid valve systems – especially for special solutions and demanding applications.