# Combining LC filters

I would like to add an EMI filter to a high power LED driver. The datasheet of the driver suggests an LC filter for the power line as shown below. However I would like to use more than one driver connected to the same power line, how would I properly combine these?

Should I use a single inductor - capacitor combination for all drivers? Does the inductance and capacitance multiply, or does it stay the same for any number of drivers? Or perhaps separate filters for each driver is the way to go? In this case adding the inductors is clear - one for each driver's Vin, but what about the capacitors? One common capacitor for all inductors?

As you probably already see, noise filtering is quite new for me, so any help will be appreciated!

EDIT: It's step-down DC constant current driver.

• Is this a DC input voltage? Do you have a link to the data sheet? I would have put the Cap on the other side of the inductor... Oh the intent is to keep the "crud" from the LED driver off of the power line. Sep 15, 2014 at 13:55
• Yes, forgot to mention, it accepts DC voltage. I'm actually using an analog dimmed version, for which they suggest capacitors on both sides, but I used this picture or the sake of simplicity. Here's the link to the datasheet: recom-power.com/pdf/Lightline/RCD-24.pdf Sep 15, 2014 at 14:33
• I would put a Pi filter on the power pin. Sep 15, 2014 at 15:17
• @IhavenoideawhatI'mdoing, OK so mostly your question is one filter for each unit or one bigger filter where the DC supply is. Do you have EMI issues that you need to address, or is this more of a prophylactic addition? The filter corner frequency will be set by L and C (1/(2*pisqrt(LC)) C has to be sized to handle the maximum voltage, and L the maximum current. So just one filter would have to have an inductor that could handle all the current. Sep 15, 2014 at 17:15
• Basically yes, one filter vs filter for each driver and how would the values of L and C change, if at all. It's more of a preventative measure, I don't currently have problems but the drivers will be used in a high EMI environment and there may be issues in some situations. That's exactly my problem, initial idea was to add a single filter but I don't have room to add such a large inductor, so I need ideas how to connect separate filters if possible. Sep 16, 2014 at 5:40

## 1 Answer

Since no one reading this was sure of the answer, I contacted the manufacturer of the drivers and asked for their assistance. The best solution here is a separate filter for each driver. Even better if it's a pi filter, as Matt Young proposed. The values don't change for any number of drivers. It is possible to design a common filter, but harder to do so.

Thanks for everyone who contributed!