# Tag Info

20

It's actually a bit difficult to calculate the values required for an R-C snubber without knowing something about the amount of energy that needs to be absorbed, which is related to the load current and the load inductance. Often one or both of these values must be guessed, because hard data is not available. The idea of a snubber is that the capacitor ...

10

The design of this $RCD$ clamp requires the knowledge of the maximum peak current your controller can set up as well as the maximum voltage you tolerate across the MOSFET. I have presented all these equations in a seminar I taught at APEC in 2011 and entitled The Dark side of the Flyback Converter. The equations to determine the components values are there:...

10

"L1" is not an inductor, it's the world's cheapest looking magnetostrictive ultrasonic transducer. The construction is similar to a solenoid or other speaker, but there is a rod of magnetostrictive material in the center (which changes shape slightly when a magnetic field is applied, in this case lengthening and contracting). This rod is capped ...

10

The snubber provides a lossy path for the inductor flyback. It's essentially a high-pass filter that shunts the flyback spike past the opening switch back to the power supply, dissipating some of that energy in the resistor, as heat. The snubber can also be placed across the coil so the spike energy is shunted locally at the coil, although this wastes AC ...

8

Metal Oxide Varistors (MOVs) are cheap but will wear out and fail shorted. Properly rated capacitors as part ofa snubber will last indefinitely. Both will allow a significant voltage spike. See, for example, Electromagnetic Compatibility in Medical Equipment: A Guide for Designers ... By William D. Kimmel, Daryl D. Gerke MOVs are more appropriate to deal ...

6

I don't know where you got that circuit(r2,r3,c1), but IMO it is rubbish. Think of what happens when the foto-triac is not conductiong. In effect you can remove it and R2 from the circuit. Now you have the gate of your triac triggered via R3/C1, which delivers a phase-shifted current through the gate, 'conveniently' non-zero when the triac voltage is zero, ...

6

I think that using a snubber in your application is the wrong approach. You want to design something that clamps the transient that occurs when the switch driving the contactor coil opens. A voltage clamp is the better approach. Others have mentioned using a pair of Zener diodes connected in inverse-series. Although that is one approach, I don't like ...

6

simulate this circuit – Schematic created using CircuitLab Excellent answer for design of the R-C snubber. I would like to comment on relatively large capacitor value of 40uF. The reason for this comes from having to limit the voltage to a very low level (60V). Commercially available RCSs are typically built with 0.1uF C, and 47 - 220 ohm R. These work ...

6

This question misses the most important point. You really need to calculate how much energy is stored in the wiring inductance (LI^2) when you open the circuit. At 1000 amps, this could be quite a lot of Joules. This energy needs to be dissipated into something properly rated, both for the pulse energy, and the pulse power. Enough joules will turn any ...

5

Let's talk order magnitude for ease of understanding. Say your high voltage side is 50Hz mains power. In that case the reactance of the capacitor is: $X_C = \dfrac{1}{2\pi fC} = \dfrac{1}{2\pi\cdot50\cdot 47\cdot10^{-9}} \approx 68\text{k}\Omega$ As the capacitor's reactance is much higher than R3 and therefore we can safely ignore R3 for sake of this ...

5

Arcing can still cause physical damage to the switch contacts. And if the contacts happen to be close enough while arcing, they can actually fuse together from the heat generated.

5

You don't want just capacitors, you want to install an RC snubber on the transformer secondaries. The reason is that when the secondary voltage drops far enough that the diodes stop conducting, you would get significant ringing due to the interaction between the diode's open-circuit capacitance and the transformer's inductance. Here is an appnote on ...

5

1st Read the MOC3021 datasheet, you have a correct circuit for inductive loads. Not only the snubber, also the trigger circuit is different 2nd: the snubber goes connected across the triac, not between live and neutral. 3rd: Triac isn't suposed to drive capacitive loads, so why don't you use a phase angle control instead using capacitor in series, as this ...

5

Basically this is a non-problem. You don't need to do suppress the free-wheeling ringing due to the inductor and drain-source capacitance of the open-circuit MOSFET (more likely than the diode) in a buck converter because nothing bad happens if you leave it alone. The voltage in both polarities is never bigger than the voltage due to switching so the MOSFET ...

5

There are three main causes for what you are seeing, which one or combination is dependant on specifics of your setup Probe pickup There is the possibility these spikes are not real & are artefacts of how you are probing. If you are using a x10 or x100 probe then the clip is EARTH referenced. If you are connecting this to the SOURCE of the lower FET ...

5

Before even getting to the calculation part, it's important to point out that you've mixed up stray capacitance (what you would use for snubber calculations in this case) with interwinding capacitance (which is totally irrelevant for determining your snubber values). While interwinding capacitance does play a role in what kind of harmonics are conducted back ...

5

You can roughly estimate the power loss in the resistor by calculating the power needed to charge and discharge the capacitor at the switching frequency of the converter: P=CV²f. With 330pF, 16V and 2.1MHz, this results in about 180mW being dissipated in the resistor. The formula can be derived like this: The energy stored in a capacitor is Ecap=1/2 CV². ...

4

One of the problema with RC snubbers is that they leak current. This doesn't really matter if the leakage current is much lower than the load requires to work, but otherwise it does. In the circuit diagram in the question there is a 10nF capacitor in series with the load. If we neglect the load and the series resistor, at 50Hz this capacitor acts as a $X_{... 4 You have to be very careful about probing such circuits. It is easy to get large surge voltages in the ground system into the scope measurement point. I expect that what you are seeing on the scope is not really what is happening. You will need to separate the current path for the gate drive from the path for the inductor. Use separate ground wires between ... 4 Without a schematic, it is difficult to judge. A$RC$snubber is different than an$RC$damper. What is possible is to have the parallel combination of a damper and a TVS as shown below: The$RC\\$ network is there to damp the resonance of the leakage inductance and the capacitance lumped at the drain node. Sometimes, the leakage contribution is so ...

4

This is what I found in the document that relates to your question: - To minimize the antenna-like radiation of this electromagnetic energy from wires connected to the inductor, the flyback diode should be connected as physically close to the inductor as practicable. This approach also minimizes those parts of the circuit that are subject to an ...

4

If you want phase control, you can't use a zero-crossing optocoupler. They are designed to prevent phase control to minimize switching noise (EMI). Try a MOC3052-M instead. You will need to lower R3 significantly to get 10 mA. And don't aim for just 10, aim for 15 or more. My version of Jasen's quote -- aim between the goal posts (great quote, I may borrow ...

4

it's a actually high pass, thus it's only usable at the low frequencies. at higher frequencies there is too much heating in the resistor.

3

A diode is essentially essential in that application. Why? - see "Why use a diode" below. The Jaycar specifications have been badly scrambled. See **Jaycar's mangled specs:"" at the end of this answer. The relay sold by Jaycar is a BESTAR reed relay with 1050 Ohm coil for 12V operation. Data sheet here There are a series of relays with the same contact ...

3

My guess is that all the 10nF or 100nF recommendations are for zero-crossing TRIAC applications, and that one has to be really careful to not apply that recommendation blindly to relay-based solutions. So, I propose the following possible solutions: 1: RC snubber designed according to the calculations in How to calculate resistor and capacitor size for ...

3

As far as snubbers go... If you want to absorb as much noise power as possible then C in the snubber should be large enough that its impedance is near 0 and much smaller than R at the frequency of interest. R should be matched to the noise source impedance if you want to absorb the maximum amount of power from that source. 1/(2 * pi * 185MHz * 120pF) = 7....

3

What you're talking about is called a "freewheel diode", which gives the inductive voltage spikes a place to go when the motor is stopped. The anode of the diode needs to go on the negative wire and the cathode of the diode (with the stripe) needs to go on the positive wire of the motor. This way the diode will not conduct during normal operation, but when ...

3

OUTCOME REPORT: Okay, I think I've sucked all the learning juice out of this exercise, thanks to @AndyAka and @winny for their help and encouragement :) Here's before the snubber, switch-node in yellow, output in blue: And here it is after: The values I used were 1nF and 470R for anyone just looking for a recipe to try :) My key takeaway interpretations ...

3

Circuit protection isn't my forte but since I was involved in the development on Relay for Intermittent Wiper Function I will chip in. Is the 100mA PTC fuse appropriate for the circuit? It seems appropriate. Is the TVS a good idea or unnecessary with the LM2931 already having 60V load dump protection, -50V reverse transient protection, internal ...

3

simulate this circuit – Schematic created using CircuitLab Figure 1. OP's circuit redraw with positive voltages on top and negative on bottom. simulate this circuit Figure 2. Using snubber diodes. In this configuration the maximum voltage that can appear across the contacts is 12 V + 2 x 0.7 V = 13.4 V. simulate this circuit Figure 3. Back to back ...

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