In Reply to: Better read more books, bud. posted by cheap-Jack on December 6, 2006 at 08:40:51:
Figure 1 depicts capacitance between the windings of a common mode choke (the subject of question 1.) By definition, common mode signals are identical at both terminals; they both ride up and down together (remember, we're talking about common mode signals only.) The capacitance - all the little capacitors - see no change in voltage across them. There is no charging/discharging of the capacitance between windings and it has no affect on the common mode performance of the choke. Resonant frequency and Q do not depend on the capacitance between windings as far as common mode signals are concerned.Figure 2 depicts capacitance to ground (the subject of question 2.) The capacitance forms a low pass LC filter in combination with the inductance of the choke. This is generally what the choke is used for anyway; for common mode rejection caps to ground are typically placed after the choke and sometimes before. There are a couple of interesting points:
- The capacitance is 'seen' by both common mode and differential mode signals, however the inductance of the choke is much greater for common mode signals, so the corner frequency of the LC filter is completely different for common mode and differential mode signals; very much lower for common mode.
- To first order, the capacitance to ground does not influence the resonant frequency or Q of the choke.
Figure 3 depicts capacitance between individual turns in each winding as well as between layers in the winding. This is the capacitance that is responsible for self-resonance; it is effectively in parallel with the inductance and forms a parallel resonant filter. At frequencies above resonance the filter appears a capacitive not inductive and the impedance falls with increasing frequency. That is in contrast to the rising impedance of an ideal choke.
Since this capacitance is the one that is responsible for self-resonance, it is also the one that most influences the Q, MUCH more than those discussed above.
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In answer to c-J's quiz question, loosely wound turns are used to minimize the capacitance between turns as described above. Whether or not there are just a few turns depends on the intended use. At very high frequencies the capacitance between turns needs to be very low and a few turns will yield enough inductance. At lower frequencies more - and consequently tighter - turns might be used.
-- Dave
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Follow Ups
- Answers to quiz questions - Dave Cigna 18:10:14 12/06/06 (53)
- Re: Answers to quiz questions - dave slagle 11:32:12 12/07/06 (2)
- Re: Answers to quiz questions - Dave Cigna 16:28:03 12/07/06 (1)
- Re: Answers to quiz questions - dave slagle 18:14:39 12/07/06 (0)
- Are you talking about CM choke or coil winding in general? - cheap-Jack 08:12:02 12/07/06 (13)
- Re: Are you talking about CM choke or coil winding in general? - Dave Cigna 16:16:40 12/07/06 (12)
- Sorry be "rude" again, I want more relevant answer than this. - cheap-Jack 09:22:41 12/08/06 (11)
- Re: Sorry be "rude" again, I want more relevant answer than this. - Dave Cigna 17:36:11 12/08/06 (10)
- Here is what I get from your 'teaching'. - cheap-Jack 19:58:32 12/08/06 (9)
- Ok - Dave Cigna 15:13:26 12/10/06 (2)
- The concept is simple, but .. - cheap-Jack 09:53:21 12/11/06 (1)
- Re: The concept is simple, but .. - Dave Cigna 16:40:48 12/11/06 (0)
- How can you be so confused? - Russ57 08:35:08 12/10/06 (4)
- Re: How can you be so confused? - dave slagle 18:34:06 12/10/06 (0)
- I double Dog dare you... - NoWay 17:56:12 12/10/06 (2)
- I guess we can say - Russ57 11:58:05 12/11/06 (1)
- Re: I guess we can say - NoWay 14:26:16 12/11/06 (0)
- As Henry would say... - Dave Cigna 06:33:22 12/09/06 (0)
- That's some fine Mole Whacking there Dave - NoWay 03:28:22 12/07/06 (0)
- Great job - Russ57 19:05:38 12/06/06 (34)
- Leave lip service behind, please. - cheap-Jack 09:05:48 12/07/06 (1)
- No reference that would satisfy you - Russ57 10:49:19 12/07/06 (0)
- Re: Great job - fatbottle 01:48:29 12/07/06 (0)
- Re: Great job - dave slagle 06:01:56 12/07/06 (21)
- Yes, balanced power irons do NOT guarantee symmetry. - cheap-Jack 08:21:36 12/07/06 (20)
- Re: Yes, balanced power irons do NOT guarantee symmetry. - fatbottle 08:30:09 12/07/06 (19)
- Wrong shot! Dave Slagle is the "expert" to answer you. - cheap-Jack 08:36:32 12/07/06 (18)
- 'Yes, balanced power irons do NOT guarantee symmetry' - fatbottle 08:44:17 12/07/06 (17)
- "Not too clever " of you either to call me "expert" - cheap-Jack 08:59:20 12/07/06 (16)
- A schematic for the expert - fatbottle 10:00:32 12/07/06 (14)
- Have you read Thomas Mayer 's posts? - Russ57 12:08:56 12/08/06 (9)
- Re: Have you read Thomas Mayer 's posts? - dave slagle 15:32:08 12/08/06 (7)
- Re: Have you read Thomas Mayer 's posts? - fatbottle 16:58:07 12/08/06 (6)
- Re: Have you read Thomas Mayer 's posts? - dave slagle 19:12:42 12/08/06 (5)
- Re: Have you read Thomas Mayer 's posts? - fatbottle 15:03:08 12/09/06 (4)
- Re: Have you read Thomas Mayer 's posts? - dave slagle 18:21:44 12/10/06 (3)
- Re: Have you read Thomas Mayer 's posts? - fatbottle 03:50:28 12/11/06 (2)
- Re: Have you read Thomas Mayer 's posts? - dave slagle 07:53:30 12/11/06 (1)
- Re: Have you read Thomas Mayer 's posts? - fatbottle 11:37:21 12/11/06 (0)
- yes - fatbottle 13:31:05 12/08/06 (0)
- Sorry to be "rude" again. - cheap-Jack 09:14:41 12/08/06 (3)
- $650 an hour Expert hahahahaha !!!!!!!!! - fatbottle 10:31:59 12/08/06 (2)
- Don't you know you're a kiddie mental case. - cheap-Jack 08:46:11 12/14/06 (1)
- I'd rather be a kiddie mental case than a stalking troll - fatbottle 10:17:33 12/14/06 (0)
- Please explain why balanced power irons do not guarantee symetry - fatbottle 09:13:23 12/07/06 (0)
- I have been using balanced power for years - Russ57 04:44:12 12/07/06 (8)
- Re: I have been using balanced power for years - fatbottle 05:23:35 12/07/06 (7)
- In case you don't know, ... - cheap-Jack 08:49:28 12/07/06 (6)
- That is your opinion but it is 'apple and orange' as you would say - fatbottle 09:22:55 12/07/06 (5)
- I'll add one small but important point - Russ57 16:43:42 12/07/06 (4)
- Re: I'll add one small but important point - dave slagle 18:32:34 12/07/06 (2)
- Yea, unbalanced centre-tapped power won't help much. - cheap-Jack 08:52:36 12/08/06 (0)
- What would you propose as an experiment? - Russ57 04:39:08 12/08/06 (0)
- Re: I'll add one small but important point - fatbottle 17:03:01 12/07/06 (0)