Hi, I am designing an LC filter to go on the 24 Vac input power lines before the rectifier and switching regulator to filter out noise.

With a typical LC filter though, the inductor is in series with only on one line of the 24Vac. But it seems that noise could get through on the other line. Will it have the same response if I put a series inductor on both lines and the parallel capacitor after that (see attached picture)? I'm getting different results in LTSpice when I simulate that, but it also depends on where I put the ground in the simulation. If I put the ground on the lower side of the AC voltage source, then I get weird results. If I put the ground at the output of the circuit on the bottom of the load, then I get more reasonable results. But I get different results with 1 or 2 inductors (either 1 on 1 line or 1 on both lines).

In the attached schematic, L3 and C1 form the typical LC filter. But to filter noise out on the bottom line of the AC voltage source, I put in L5. L1 and L2 are just a common mode choke. C3 and R1 are for damping at resonant frequency. L4 is not used (shorted out). I get reasonable results with the GND where it is in the picture. When I move it to the lower terminal (-) of the AC voltage source, then I get strange results (frequency response).

Where should I put the ground and should I only put L3 in or both L3 and L5 to filter out line noise?

Thanks.

 

What's your goal? Just playing with a simulator program?
Did you forget a transformer has inductance?
Did you think about filtering the power cord on the primary side?
Do you have a good grip on the difference between Neutral and Ground?
Does the simulator know these things?

 

Search the web for power line filters from Corcom, Filter Concepts, Schurter, and go through some data sheets to get a feel for how these things are done. Generally speaking the inductance is a common mode choke, a dual-winding inductor that has high common mode inductance and reasonable series mode inductance. There can be one or more of these, surrounded by differential and common mode capacitors. Unless you really want to build one, the easiest way to get there is to buy a 120 VAC part and use it. It's performance doesn't change with the lower voltage.

ak

 

What's your goal? Just playing with a simulator program?
Did you forget a transformer has inductance?
Did you think about filtering the power cord on the primary side?
Do you have a good grip on the difference between Neutral and Ground?
Does the simulator know these things?

I'm designing a real circuit. Well that's true the transformer has inductance. So you think no LC filter is needed or still need a C but no L? Noise could get onto the lines between the transformer and the board though although they are short but motors and pumps are starting and stopping so a lot of transients on the line voltages.

I understand neutral is the current carrying conductor that is connected to earth ground near the electrical box. But noise can be on the neutral line as well I think. In a 3 phase system we have L1 and L2 hooked to the transformer primary generating the 24 vac sometimes (no neutral). The ground in LTSpice should just be the reference with which to compare the voltage being measured with. In my case I'm measuring transfer function output over input voltage with respect to ground. I think I have my LTSpice ground in the right place but not 100% sure.

But say I did need an LC filter, would two inductors be used? Would I use superposition and basically have the equivalent of two LC filters where the capacitance is half the value?

 

I'm so old that I don't know what, "superposition" is!

I only know that you have 5 lines to work with (6 if you count the, "ground" of the DC supply voltage) and transformers have capacitance from primary to secondary. The first thing to do is decide what your reference point is, and that can make all the difference!! Then you filter in series with a common mode choke like AK recommended and then capacit the secondary to the common point to drain away high frequency, common mode signals that got through the transformer. After that idea, you work on, "how much attenuation". If you don't have it fixed with a common mode choke and 3 capacitors, you go get your simulator.