The other day, I was casually scrolling through Google when I stumbled upon a flood of dirt-cheap RC snubber circuit modules on various online stores. That got me thinking—it’s high time we talk about these little circuits and their real-world applications.
This post will offer some insights on RC snubber circuits along with a few handy tips for inductive load suppression. Whether you are a newbie looking to learn the ropes or an expert in need of a quick refresher, there is something in here for you. Let us dive in…
On paper, RC snubber circuits function as protective measures in switching applications, utilizing a resistor and capacitor together to mitigate voltage spikes and transient noise. But the commonly available RC snubber circuit module, sometimes referred to as an RC absorption circuit module by certain vendors, only contains a resistor, a capacitor and a varistor—just three basic components.
According to most vendors, the prewired module is suitable for AC/DC 5-400 V inductive loads (<1,000 W) to protect relay contacts and triacs. I could not find an actual schematic of it anywhere on the web, but since it’s pretty easy to prepare it through physical inspection, I drew it myself. Here is that diagram.
Figure 1 The block diagram represents the RC snubber module circuit. Source: Author
The components in the module are:
- R = 220 Ω/2 W Resistor (MFR 1%)
- C = 104 J/630 V Capacitor (CBB22)
- MOV = 10 D/471 K Metal Oxide Varistor (10 mm/470 V ±10%)
The R-C values used in the snubber are by necessity compromises. In practice, the resistor value (R) must be large enough to limit the capacitive discharge current when the switch contacts close, but small enough to adequately limit the voltage when the switch contacts open. Larger capacitor value (C) decreases the voltage when the switch contacts open but it increases the capacitive discharge energy when the switch contacts close.
Furthermore, when the switch contacts are open, a current will be flowing through the snubber network. It should be verified that this leakage current does not cause issues in the application and that the power dissipation in the snubber resistor does not exceed its power rating.
A quick design insight
The optimal approach to determining the R-C values involves using an oscilloscope to trial various R-C combinations while monitoring spike reduction (or turn-off transient reduction). Then adjust the R and C values as needed until the desired reduction is achieved. Based on my practical experience, for most relays and triacs, 100 nF + 100 Ω values provide an acceptable suppression.
The above-mentioned RC snubber module, intended to be wired across a switching point as shown below, is a simplified resistor-capacitor snubber circuit made up of a resistor and a capacitor connected in series. Here, the resistor helps to absorb the energy from the voltage spikes, while the capacitor provides short-run storage for this energy. This way, the risk of a harm due to sudden change in electrical flow is minimized.
Figure 2 The RC snubber module is wired across a switching point. Source: Author
Most snubber circuits also include a metal oxide varistor (MOV) along with the RC circuit by placing the metal oxide varistor across the input line. An MOV is a specialized type of voltage dependent resistor (VDR) that uses a metal oxide, most commonly zinc oxide, as its non-linear resistor material.
The MOV will then protect the parallel circuit and the load. The MOV will set the maximum input voltage and di/dt through the load while the RC snubber sets the maximum dv/dt and peak voltage across the switching element like a triac; di/dt and dv/dt values should be considered when handling non-resistive loads.
At this point, it’s worth noting that when a triac drives an inductive load, the mains voltage and the load current are not in phase. To limit the slope of the reapplied voltage and ensure the right triac turn-off, a snubber circuit is usually connected in parallel with the triac. The snubber circuit can also be used to improve triac immunity to fast transient voltages.
Summed up briefly, the generic RC snubber circuit module covered in this post is suitable for certain circuits with inductive loads and switching devices such as triacs, thyristors, and power relays. When used, the two input screw terminals of the module are connected to the two contacts of the relay (such as common and normally open contacts), or it’s connected in parallel with the triac/thyristor (Figure 3).
Figure 3 The above image offers application hints for RC snubber modules. Source: Author
Inductive load suppression
Let it be known that inductive load suppression encompasses methodologies designed to mitigate the adverse effects of potential backlashes, which manifests when an inductive load—such as a solenoid or motor—is abruptly de-energized.
Moving on to additional guideposts for inductive load suppression, suppressor circuits are commonly used with inductive loads to control voltage spikes when a control output switches off. These circuits help prevent premature failure of outputs by mitigating the high-voltage transients that occur when current flow through an inductive load is interrupted.
The randomly selected sample voltage waveforms shown below illustrate this more clearly.
Figure 4 Here is a comparison between unsuppressed and snubber-suppressed voltage waveforms. Source: Paktron
In addition, suppressor circuits play a crucial role in reducing electrical noise/arc generated during the switching of inductive loads. Poorly suppressed inductive loads can make subtle noise that may interfere with the operation of delicate electronic components and circuits. The most effective way to reduce interference is to install an external suppressor circuit electrically across the load or switch element, as required by the setup, and position it in close physical proximity.
Listed below are some fine-tuned inductive load suppression application hints. The corresponding figures helps to visualize them.
- In most applications, placing a standard diode across a DC inductive load provides sufficient protection for DC or relay outputs that control DC inductive loads. However, if your application demands faster turn-off times, incorporating a properly sized Zener diode is a recommended approach.
- For relay outputs controlling AC inductive loads, an MOV can be paired with a parallel RC circuit. At this stage, ensure that MOV’s working voltage is at least 20% higher than the nominal line voltage.
- In DC voltage applications, the RC snubber network is typically wired across the relay contacts, whereas in standard AC voltage applications, it’s placed across the load. To reinforce the point, the RC snubber mechanism must be wired across the triac in phase control circuits.
Figure 5 The above image offers AC/DC application hints for inductive load suppression. Source: Author
Well, to wrap things up, RC snubbers help control voltage spikes and scale down noise in circuits, making them essential in power electronics. This quick guide provides only a glimpse into the complex topic, leaving plenty more to uncover—from diverse design configurations to their wide-ranging applications.
When dealing with power electronics systems, a thorough understanding of snubber behavior is essential for engineers and enthusiasts alike.
T. K. Hareendran is a self-taught electronics enthusiast with a strong passion for innovative circuit design and hands-on technology. He develops both experimental and practical electronic projects, documenting and sharing his work to support fellow tinkerers and learners. Beyond the workbench, he dedicates time to technical writing and hardware evaluations to contribute meaningfully to the maker community.
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