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Old 19th August 2005, 06:42 AM   #21
flg is offline flg  United States
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Location: North East
After searching for a reasonable inductor for my Pi filter and finding the prices a little steep, I tried 1 thing that seemed to work for low currents(a few amps). People like Coilcraft make these common mode inductors. They are designed to bridge a power supply input and block EMI (Unwanted high frequency). They are specially wound to to influence the AC line with something less than 100uH. Virtually nothing to the 50/60Hz line. But, any signals appearing at the AC input in common mode, see's many mHs. I think the particular 2 of the 3 they offer were something like 4.5mH and 1.5mH. They do offer 1 with a 20mH common mode L but the current/saturation is to low for this type of application. The winding of these things is a little weird. If you can measure L, there are various ways of series/parrellel that produce unexpected results but it worked. They should not be to hard to find at the Coilcraft www site.
Another idea might be close to what I read here earlier??? the PC power supply people use something like a 50mH at the input to improve power factor. When I saw them I grabbed a few but I havent tried to test them yet. They should be good for about 2-3A and these power supplies are junk to most hackers.
I did however give up as I wanted more I for my design. I went to E-Bay and found a 20mH 10A hammond unit for $15 +SH.
FYI,. I'm using 2 - 39,000uF going into the inductor and 2 - 39,000uF coming out for a cool unmeasureable 47 single ended Volts...
"It was the perfect high end audio product: Exotic, inefficient, expensive, unavailable, and toxic." N.P.
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Old 19th August 2005, 08:53 AM   #22
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Default And what did the Master do?

This is what Mr Nelson uses himself in the XA 160 amps.

A ferrite core. And it looks three times as small as the air core I use myself.

(Sadly this is not my own amp. Still saving)
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Old 19th August 2005, 09:29 PM   #23
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And what did the Master do? Post #22
This is what Mr Nelson uses himself in the XA 160 amps.

A ferrite core. And it looks three times as small as the air core I use myself.

They look like standard iron cores. Those will be good enough for me.
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Old 19th August 2005, 09:51 PM   #24
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An inductor with more DCR has a lower q, the resistance damps the ringing. If your inductors and capacitors start really resonating youve got trouble, as mentioned by PRR.

Heres a fairly relavent link on the Damping of Power-Converter Front-End Averaging Filters

This was helpful. Earlier I mentioned shunting to ground but your link provides the right formulas. Moreover, I had not known about the CR leg that comes ahead of the C shunt. I will study this more over the weekend. Thanks.

After dealing with this power supply stuff, my guess is that those guys at NASA probably do know more than me.
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Old 29th March 2008, 10:55 PM   #25
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Location: Tyrone Ga. U.S.A.
I recently bought a few 250va toriodal autoformers to play with.
Just for fun I hooked one up as a choke in a choke input filter.
I know it wasn't suposed to work but under a 1A load it preformed just like a "real" choke. Then I removed all the windings a real pain in the *** . The 3" core I uncovered was
wound from a continious strip of steel. Then I wound 58 turns
of #16 copper wire around it and measured ~10 mh.

Using a 18v center tap transformer, a pair of Shokey diodes, this
10mh inductor and a .25F cap I measured 7v into a 1ohm load.
When allowing for the voltage drop in the diode and sourse resistance in the inductor this matches the 9v X .88 = 7.72v
expected. The #16 wire did get slightly warm at this 7A load
them hot when loaded to 12A. but not bad for a salvaged
transformer core and ~ 8 feet of #16 copper wire. This may not
work with othe transformer cores.
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