Electromagnetic interference (MEC092E)

Triacs and SCRs are high-speed power switches. Their turn-on and turn-off times are very short (only few microseconds), and this can cause electromagnetic interference (EMI). EMI (also referred to as radio frequency interference, or RFI) is interference caused by the undesirable radio frequency signals produced by devices like thyristors. These signals can adversely affect the operation of many sensitive electronic instruments. Radio control systems as used in many mechatronic projects can be disrupted by interference from triacs and SCRs operating nearby.

When an SCR or triac is used in a circuit for power control, the high-speed current changes in the load generate a series of harmonically related radio-frequency signals. The magnitude of the fundamental signal is proportional to the magnitude of the controlled current, and, in many cases, it can be great enough to cause interference in AM radios and many other circuits that use low- and medium-frequency radio signals in their operation.


ln particular, the interference level is higher when the SCR or triac controls inductive loads as motors, solenoids, electromagnets, and so on. The snap action of the triac or SCR causes current oscillations in the circuit, generating a strong electromagnetic field.


A protective circuit that minimizes interference is shown in Fig. 1. The capacitor and the resistor form a “damping” circuit that reduces current oscillation and electromagnetic interference. Examples of the EMI process include the noise in an AM radio receiver and the static displayed on a TV screen when a circuit using triacs or SCRs is turned on.






lnterference can enter radio receivers, TVs, and other sensitive devices in two ways:


1. It can be picked up from radio frequency emissions in the atmosphere (radi-

ated emissions). In this case, the undesirable signal is generally very weak and

causes problems only if a radio or other sensitive piece of equipment is placed

near the interference source. It is also concentrated in the low band of the radio

spectrum, as shown in Fig. 2; the interfering signal is more powerful in the

low-frequency (LF) band than in the medium-wave (MW) band.






Radio receivers operating at higher frequencies, such as FM receivers and

VHF receivers (e.g., remote controls for garage doors and robots, and cellular telephones), aren’t affected much by this interference. This is due to the

lower power concentrated in this band as well as their basic operating principles.


2. In the second case, the signal is conducted into the Victim equipment Via its

own AC power line. To avoid this type of interference, it is suflicient to use an

L-C filter such as shown in Fig. 3.