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Choke selection for AC line

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May 27, 2010
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Hi all,

I has been searching since hour and couldnt find what i wanted to learn.
I will make a circuit, input of the circuit will be 230VAC(rms), 50Hz. I want to use Choke against to disturbance on the line. Can anyone tell me, what kind choke should i use, and how can i determine it's inductance and rated current?

Thanks a lot

The type of choke depends on what the disturbance you are trying to eliminate is. The idea of a choke is to let one signal through (the 50Hz AC power) without hindering it but to block the passage of other unwanted signals. You need to specify what kind of signals you are trying to block and also you need to specify the current carrying capacity of the choke. That depends on the load (current in Amps) you are passing the AC on to.


Sorry if it sounds newbei, i just wanted to use choke, to have proper %50Hz sinus, and prevent harmonics.
I also read it is good against emc problems.
The load can have max 600mA(rms). It is a inductive load, but i dont know inductance of the load(solenoid).

Thank you.
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If it's an inductive load it should ignore most incoming interference and produce no constant EMC from itself. Sounds like what you really need is an X2 rated capacitor across the AC rather than an inductor in series with it.


there will be also triac and optocouplers, voltage of ac load will be decided via triac.
Do you think, i dont need any choke? TRiac will be triggered by microcontroller(microcontroller is not on my circuit, it is other circuit).

i decided to use two varistor one is for input of circuit against to overvoltage second paralell to load against inrush current. Am i doing right?
thank you

Triacs will introduce switchig transisents so in that case you should use a filter. I would be inclined to use a ready made one such as these examples:

To stop inrush current you have to wire something in series with the incoming power so it limits how much can flow. Look for NTC thermistors or something like these:

For over voltage you wire across the incoming lines so it can clip the excess and convert it to heat. More examples:

thank you, it will be my first circuit, every knowledge is usefull to me.
Could you tell me, what is difference between,
current limiter and surge absorber, varistor? When would i need surge absorber varistor, it is different than overvoltage varistors i assume.
i was thinking to use this before you told me about current limiter


A current limiter is basically a resistor you wire in series with the incoming current. You could use a normal resistor but then it would always be limiting the current when you really only want it to work for a moment at the time you switch on. The special property of surge (inrush) limitiers is their resistance drops sharply when they warm up. When you first switch on they are assumed to be cold and have a relatively high resistance so the current is kept low but the internal heat they generate warms them up and makes the resistance drop, making them look more like a short circuit. This is why they are called NTC devices, NTC = Negative Temperature Coefficient. So they are designed to run warm or hot in normal operation.

A surge absorber (aka varistor or MOV) is also a resistor but this time with a different property. It's value is normally very high so little current flows through it but at a certain voltage the resistance drops sharply. In a circuit where something before the absorber can limit or cut the current, wiring one across the voltage makes it conduct if the voltage is exceeded so it can't go any higher. So you normally pick a 'clamping voltage' a little higher than the highest voltage you expect to see in normal use, the absorber then does nothing but if the voltage increases it (hopefully) safely conducts it away. The mechanical design of the absorber is such that it can handle very brief high current surges, often in the thousands of amps range, without damage although it can only sustain that kind of current for a tiny fraction of a second.


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