Protective circuit

A protective circuit is an additional electrical circuit that consists of various electrical components and ensures that damaging high voltages that arise when electromagnetic devices (e.g. relay coils ) are switched off do not occur. A protective circuit is intended to reduce the voltage peaks when switching off inductive loads so that no components are damaged. The height of the voltage peaks depends on the random switch-off time (phase position).

Protective circuits are common in vehicle electrics and in many areas of electrical control technology.

Basics

Relays have inductive properties

If an electrical coil is switched off, the magnetic field breaks down in it and a self-induced voltage is created . The level of the self-induced voltage depends on several factors. The number of turns of the coil has a particular influence on the level of the self-induced voltage. In the case of coils with a high number of turns, the self-induction voltage can rise up to ten to twenty times the value of the applied operating voltage . These high self-induction voltages can destroy electronic components or, in the case of switching contacts, lead to flashover and contact erosion.

Circuit variants

Components that do not influence normal operating processes, but can divert interference voltages or currents, are used as protective circuits. Protective circuits are also produced with an integrated status display.

There are the following circuit variants for protective circuits:

• Protective circuit with diode
• Protective circuit with diode and Zener diode
• Protective circuit with varistor
• Protective circuit with RC element
• Protective circuit with resistor

Protective circuit with diode

Circuit with diode

In this circuit variant, a diode is connected in reverse direction parallel to the coil. Since the polarity of the self-induced voltage is opposite to the applied voltage, the voltage is diverted via the now permeable diode and the current flows through the coil. Colloquially, this type of protection is also called "free-wheeling diode". The dimensioning of the circuit and the components used is not critical. The circuit is easy to set up. To prevent the protection diode from being destroyed by incorrect polarity , either reverse polarity protection diodes are installed in relays or the polarity of the connections is marked. The protective function is very reliable because it dissipates even low self-induction voltages. The voltage peak of the self- induced voltage is limited to the threshold voltage of the diode used. The disadvantage is that this circuit variant is only suitable for direct voltage and that there are high dropout delays of the relay contacts with relays. In the case of relay contacts, this can lead to a reduction in the service life of the switching contacts due to the extended switching arc duration. Therefore, this variant of the protective circuit is not so well suited for high switching loads.

Protective circuit with diode and zener diode

Circuit with Z-diode

A reverse-biased Zener diode is connected in series with a diode. If the induction voltage exceeds the level of the Zener voltage + threshold voltage of the diode, it is diverted via the two components. The very short drop-out delay and the simple circuit structure are advantageous here. The dimensioning of the components is not critical. However, this circuit is only suitable for direct voltage and there is no induction voltage damping below . Wrong polarity can destroy the protective circuit due to overload. ${\displaystyle U_{\text{ZD}}}$

Protective circuit with varistor

Wiring with varistor

In this circuit variant, the property of the VDR resistor is used to become low-resistance above a certain threshold voltage. This short-circuits the self-induction voltage. The uncritical dimensioning of VDR resistors is advantageous. They have a high level of energy absorption and are suitable for use with alternating voltage. The circuit structure is very simple and causes only a slight drop-out delay with relay coils. The disadvantage is that the damping effect only occurs above the threshold voltage . The maximum voltage peak of the self-induced voltage therefore depends on the characteristic (ignition voltage U _VDR ) of the varistor type used. ${\displaystyle U_{\text{VDR}}}$

Protective circuit with RC element

Circuit with RC element

The protective circuit by means of an RC element is a very simple, but nevertheless very effective circuit. This circuit is also known as a snubber or a Boucherot element. It is mainly used to protect switch contacts. When the resistor and capacitor are connected in series, the current can decay in a damped oscillation during the shutdown process. When switching on, the resistor prevents the full capacitor charge from being discharged via the switching contact. The protective circuit by means of an RC element is very suitable for AC voltage. Due to the energy storage in the capacitor, HF attenuation is achieved. In addition, there is an immediate shutdown limit. However, the circuit must be precisely dimensioned.

The resistance of the RC element is dimensioned according to the following formula: ${\displaystyle R}$

${\displaystyle R\approx R_{L}}$

The size of the capacitor can be determined using the following formula:

${\displaystyle C={\frac {4L}{R_{\text{Ges}}{^{2}}}}}$

The shutdown process can be determined and documented using an oscillogram. The voltage peak at the switching contact depends on the selected RC combination. If the capacitance of the capacitor is correctly dimensioned, only a small contact voltage is applied. If dimensioned correctly, the contact voltage is determined by the resistance of the RC element.

Protective circuit with resistor

Wiring with resistor

This is the simplest variant of a protective circuit, but it also has only a low protective effect. However, this protective circuit is suitable for both DC and AC voltage. The disadvantage is that when the coil is switched on, a continuous current flows through the resistor and that in a relay the resistor causes the relay contacts to switch off with a long delay. The resistance reduces the voltage peak to 2 to 6 times . The resistor is dimensioned in such a way that it is at least twice and at most six times the value of the coil resistance . ${\displaystyle U_{\text{Spule}}}$ ${\displaystyle R_{\text{Spule}}}$

literature

• Franz Pigler: EMC and lightning protection of control systems . Siemens Aktiengesellschaft, Publicis Corporate Publishing 2001, ISBN 978-3800915651
• Klaus Bystron: Power Electronics Technical Electronics Volume II. 1st edition, Carl Hanser Verlag, Munich Vienna, 1979, ISBN 3-446-12131-5

Individual evidence

1. ↑ Inductance and protective circuit
2. ↑ Valve connector with protective circuit and integrated status display  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.  
3. ↑ Moeller circuit manual for protective circuit
4. ^ Gregor D. Häberle, Heinz O. Häberle: Transformers and electrical machines in systems of energy technology. 2nd edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 1990, ISBN 3-8085-5002-3
5. ↑ Relay-small but powerful Panasonic Electronic Works  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.  
6. ↑ Higher Technical Federal, Training and Research Institute Bulme Graz-Kösting Protective circuit when switching inductive loads (PDF; 389 kB)
7. ↑ Markus Bichler: Technical solutions to reduce arcs  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 528 kB)  
8. ↑ Dieter Brockers: Lexicon resistances. Gino Else GmbH Electrotechnical Factory, 1998

See also

• Overvoltage protection