Inductive loads and interrupted currents are an explosive combination. High voltages, arcing, and HF broadband noise are some typical effects. The phenomena behind these transients are complex, but it is not difficult to understand the fundamentals and how to minimize the effects.
In many electronic applications, inductive loads are controlled with mechanical or semiconductor switches. One example of that kind of inductive load is a relay as shown in Figure 1 (left). A DC voltage is used to activate the relay when the switch is closed (ON). When the switch is open (OFF), the coil is de-energized.
In Figure 1 (right), we can see the ideal switch voltage waveforms when the switch goes from ON to OFF (Turn OFF) and OFF to ON (Turn ON). But real circuits are not so clean, especially when the inductance and/or di/dt are big.
When the switch is ON, current iL goes through the coil, so the energy stored in that coil is 0.5xLxiL2. When current iL is suddenly interrupted (opening the switch), the voltage Vswitch builds up as LxdiL/dt.
As an example, I prepared a simple circuit with a small 24V relay. In Figure 2, the switch voltage waveform is shown for the Turn OFF process. Two important effects can be observed: a) the peak voltage is more than 350V, as explained previously from the energy stored in the inductance; and b) ringing and HF noise are observed, with the possibility to create EMI problems around the circuit.
The inductance and the stray capacitances (capacitance C in Figure 2) from the relay and the layout of the circuit create the RF noise. The high voltage will be a severe problem when the switch is a semiconductor device, and the peak voltage is bigger than the breakdown voltage for the device. If a mechanical switch is used and big relays and/or big currents are involved in the problem, arcing can appear through the contacts.
When arcing appears, the voltage across the switch decreases, and the current increases. The process can be repeated, so several transients are created with strong energy (in some cases, mechanical switches can be damaged by the destruction of contacts).
To protect the switch from the high voltage transient or to avoid the arcing effect, two typical low-cost and effective solutions are: a) a big capacitor in parallel with the switch; b) a clamping device that reduces the current and voltage in contacts below the arc threshold level or maximum voltage for the switch to be protected.
In Figure 3, I introduced a varistor as a clamping device (V130LA10A from Littelfuse) to my previous example. This is a varistor with a maximum DC voltage of 175V and a typical capacitance of 1nF. Note how the transient is limited by the varistor and the HF ringing disappears. With this kind of solution, your product will be more reliable, and the possibility of suffering susceptibility problems in your nearby circuits is minimized.
My final advice: be careful of the switching activity with inductive loads. Inductive loads and di/dt is equivalent to high inducted voltages. Additionally, stray capacitance will create HF ringing. Take a look at the excellent information from Littelfuse devices (www.littelfuse.com) for detailed information about how to protect your circuits.
