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Firnagzen

Sun Apr 29 2007, 05:35AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

I was looking through the HvWiki earlier, and came across something at the bottom of the Solid state tesla coil page. It says that the secondary side grounding can be achieved through a EMI filter to the mains grounding.

Thing is, I've never heard of an EMI filter, and searching "EMI filter" on google just gives me a lot of websites and companies selling the things. Could someone elucidate me on this subject, or direct me to a site which would be helpful?

Thanks.

Ps. What kind of ratings would be needed for a EMI filter for a tesla, anyway?

...

Sun Apr 29 2007, 05:44AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

We usually rip them out of old junk, or buy them (since they are dirt cheap). I like the ones out of computer power supplies, as they provide a nice looking power connector for the project, and give you filtering

They are just a few caps/inductors, I don't think their values are too critical.

Firnagzen

Sun Apr 29 2007, 05:53AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

Um, but what does the filter do? And can I use one for a tesla coil? Would special ratings be required?

Thanks.

Ps. Hmm, I've got an unused computer power supply knocking around. How to identify the filter section?

Ken M.

Sun Apr 29 2007, 06:16AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

From what I've gathered is, they are used to prevent feedback from the coil to the ac lines that might harm sensitive electronics, the ones from PC PSU's are right where the 3 prong plug is at.

Firnagzen

Sun Apr 29 2007, 06:39AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

Hm.

I've just had a look at that PSU, and what I've found is this: On the 3 prong plug, I found what (appears to be) a resistor connected across the two prongs and a 2.4uF 250V capacitor.

I'm somewhat doubtful that any of these is/are an EMI filter, and I've had a look at the circuit board, too. There's nothing which looks like an EMI filter. (I had a look at a site selling the things for some pictures. They all seem to be some weirdly shaped enclosed modules, right?)

So it seems like I'll have to buy one, any tips as to how to pick an EMI filter for grounding the secondary? (ratings, etc?)

Thanks.

cjk2

Sun Apr 29 2007, 06:14PM

Registered Member #51 Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263

What you have above is actually just the first stage of an EMI filter. A typical filter used in a SMPS will be a CLCL circuit or a CLC circuit. What you have above is the first C in a CLCL circuit. If you look at the board that was in that PSU case, you will find at least 1 more inductor(2 windings on a powered iron core) and a high grade capacitor.

Heres what a CLC circuit typically looks like.

Firnagzen

Mon Apr 30 2007, 09:55AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

Dang. And I clipped the wires to get the picture. Looks like I'll find another one. Thanks for the help, though!

On a slightly different note, how do I apply one to a tesla coil, anyway? As I understand it, the filter goes between the coil secondary's ground wire, and straight to mains ground. But that's two wires accounted for. There are five in the above schematic!

By the way... kpF? Kilo pico farads?

uzzors2k

Mon Apr 30 2007, 02:07PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

The other 4 wires are AC input and output. Line/Neutral in and Line/Neutral out. kpF <= roflol, yeah I think he means 3nF.

Dr. Shark

Mon Apr 30 2007, 02:42PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

That would leave him with the secondary ground directly connected to mains ground and the filter not connected at all. Seriously, I also don't get how this would be connected but I _guess_ the secondary would not be connected to ground at all but to the power rails via the two 3nF capacitors. This way any interference would have to go through the choke.

IamSmooth

Mon Apr 30 2007, 06:15PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I connected one for my coil. The EMI filters have amp ratings so make sure it can handle the current you plan to draw through your transformer.

I have my transformer ground connected to a rod in the ground. The input of the EMI is connected to the hot/neutral and ground. The output of the EMI is connected to the input to the transformer.

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