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Need help with snubber network

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king nero

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I have got a motion sensor which drives the coil of a small 230 VAC relay. This relay switches a transformator+rectifier for several garden lights at 12 VDC.

Whenever the coil is de-activated (motion sensor's end of signal, a set time), it sends spikes back into the motion sensor which resets it and makes it do strange things.

I'm more mechanical minded, but need some help with this one. Would a R and a C put parallel over the coil help this?

If so, could someone point me to where to find the necesssary info to calculate the values of both, or is there a rule of thump to do so?

many thanks in advance...
 

cameo_2007

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yes,its recommended to put an RC snubber across the ac coil.
its more of a trial and error calculation and find out the right value for it.Anyways you can start with these values and see the perfomance.

R - 470 Ohms/5 Watt
C- 0.1uF X-Type capacitor

let me know the result and by the way how is the coil driven by the sensor ?
 

king nero

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Will get these components and will let you know the results.

How the coil is driven? does the scheme attached is good enough for an answer? As I don't really understand your question.



With:
L and N: 230 VAC mains network
P1: motion sensor
S1: manual switch
F1 and F2: fuses
T1: transformator

Not that it matters, but my first post was incorrect: no rectifier, garden lights are AC powered and each has a small rectifier built in (they're LED lights)
 

cameo_2007

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ok.
I assume that the sensor gives an output of 230V.
But this connection will definitely invite trouble as the sensor is very much prone to noise from switching the coil directly.
just put that RC snubber and find whats happening.Will try something else if its not ok.
 

king nero

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Yes, sensor output is line of 230 V.

tried with 470 Ohms and .1uF - didn't work.

Would playing with these values help, or should I try driving the transformator directly instead of using a relay?
 

cameo_2007

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no.then no point in trying with snubber.

will the motion sensor be able to drive the transformer directly? do you have the datasheet for that sensor..If it can , you cn try that.
 

umery2k75

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The best solution is to put a diode across coil of your relay. Actually when the coil is de-energize the magnetic field of relay collapses which builds up high voltage.The freewheel diode will help to short the current through the DC coil itself.But your coil is AC.AC has no relation directly with your DC supply of the sensor, this should not reset the microcontroller of your sensor.It doesn't matter whether how many spikes you get on your AC line.Please confirm if your relay is DC or AC.What is P1?
 

FvM

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It's very clear from the circuit, that the coil is an AC drive. A standard solution apart from RC snubber is a varistor, used with most AC driven contactors in automation.
 

king nero

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umery2k75 said:
Please confirm if your relay is DC or AC.
king nero said:
I have got a motion sensor which drives the coil of a small 230 VAC relay.
umery2k75 said:
What is P1?
king nero said:
With:
L and N: 230 VAC mains network
P1: motion sensor
S1: manual switch
F1 and F2: fuses
T1: transformator
cameo_2007 said:
will the motion sensor be able to drive the transformer directly? do you have the datasheet for that sensor..If it can , you cn try that.
Don't have the data sheet, but if memory serves me correctly it can handle about 1-2 amps, which should be enough for the transformator. Will check with an amp meter what the transformator needs...
Anyway, thanks for the values of the R and C, it was worth a shot...
 

FvM

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Actually, there are two inductive loads in your circuit possible causing overvoltages, the relay coil and the transformer itself. Athough the latter is dampened by the connected load, it may still cause overvoltage and contact arcing by it's leakage inductance. So it may be necessary to reduce overvoltage at the transformer primary as well. It's also not clear, if P1 uses an electronic switch (triac) or a relay internally. Only a relay should be expected to cause switching noise at all.
 

king nero

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P1 most likely uses a triac to switch the load. It's a cheapo, common motion detector.
 

cameo_2007

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umery2k75 said:
AC has no relation directly with your DC supply of the sensor, this should not reset the microcontroller of your sensor.
hello there,
how do you claim this.
from where is the DC derived from ?
 

umery2k75

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@cameo_2007
Your AC power is first rectified into pulsating DC. Then after filteration you get a DC supply, then you use regulators. I'm talking of a linear regulated power supply, in this case.SMPS decision on it is beyound the scope of this topic.These units are suspectible for normal working for less than 10ms, if you cut off the main AC momentarily, there internal supply is capable to keep the circuit alive for shorter amount of time. Usually, these are the figures in ms, depending on the device. So I don't think such a minute disturbance glitch in AC would causes the fluctuation in DC output of the sensor.


A Varistor is a good idea, as given by FVM,but I have not seen protecting device connected across the AC relay,we use in DC only. AC is already pulsating, I don't think there's any need to put any component block, that would kill kick back voltage of relay.
 

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