KISS 135
KISS 135
Phasing in
Lundahl’s Double-Coil Cut-C Core Chokes
like the LL1638
by John Byrns
I will try to give you a little simple insight into what is going on with the two different connections of the Lundahl two winding choke. Note that for ease of understanding the discussion below ignores all "stray" capacitances, although they will exist in a real circuit.
First consider a choke input filter of the ordinary sort where the purpose of the choke between the rectifier and the first capacitor is to smooth out the current delivery from the rectifier to the first filter capacitor. We can take this choke and cut the winding into two parts as in the Lundahl choke, and if we then reconnect the two cut ends with a jumper we will once again have a working choke input filter.
Next, observing that the current is the same everywhere in a series circuit, and ignoring the effects of stray capacitance, we can rearrange the series circuit consisting of the transformer/rectifier, two choke windings, and filter capacitor/load so as to place one of the two choke windings in the negative lead of the power supply, as long as we maintain the same direction of current flow through all the series components as it was in the original arrangement. So we have moved one winding of our two winding choke to the negative lead and we still have a working choke input power supply, with half the total winding in the positive lead, and half in the negative lead.
Next, revisit the original connection of our two winding choke where the two windings were directly connected with a jumper. Consider what happens if we reverse the connections to one of the two windings. What we end up with is no longer a choke with a large value of inductance; the choke now has only a small inductance dependent on the leakage inductance of the two windings, or how tightly they are coupled, in series with the resistance of the wire in the windings. With this negligible value of inductance we no longer have a choke input filter, we essentially have a capacitor input filter. I don't have one of these Lundahl chokes to try it with, but when you reverse the connection of one winding like this the DC output voltage under load should increase considerably, assuming tight coupling between the two windings. So now all we have is a capacitor input filter and we might just as well throw out the premium choke and save some money.
But now consider what happens if we keep the choke anyway, and using our series circuit rule slide one of the windings to the negative end of the rectifier, keeping the series current flowing in the same direction through the translated winding as it was with the two windings directly connected in the positive lead. While we still have a capacitor input filter, the choke has now become a "common mode" choke. What does a "common mode" choke do for us? It allows a "common mode" voltage to be impressed on the transformer/rectifier block without affecting the output of the supply, in other words we can grab hold of the transformer and rectifier and drive it, including both the positive and negative terminals together, up and down without affecting the output of the supply.
The bottom line is that with half the winding in the positive lead from the rectifier, and the other half in the negative lead, whether you have a choke input filter, or a capacitor input filter with a common mode choke, depends on the relative phasing of the two windings on the choke. The key to all this was given in this quote you posted earlier:
"The two chokes as connected [in the differential connection which the Lundahl spec sheet describes as “serial connection for improved common mode rejection”] have their two windings effectively connected in series aiding. In other words, the DC flux from both windings adds in the core. Because of this connection the only common mode inductance that the choke will provide is that due to the leakage inductance between its two windings." (1)
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More on these hookup possibilities at
KISS 134: Noise Reduction with Lundahl’s Double-Coil Cut-C Core Chokes
John’s web pages with his own fascinating designs are at: http://users.rcn.com/jbyrns/
(1) Quoted from a letter by Doug Bannard to Andre Jute.
THE VOLTAGES IN THIS AMP WILL KILL YOU.
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All text and illustration is
Copyright © 2006 John Byrns
and may not be reproduced except in the thread KISS xxx on rec.audio.tubes.