Re: More Coupling...

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-uswest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi Paul, 

Tesla list wrote: 
>
> Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> <paul-at-abelian.demon.co.uk>
> I have some more questions for you! 
>
> You mentioned 1uF + 18k ohms in series with the secondary - can I ask 
> what these are for?

This is a simple rc noise filter also used to swamp any resonance which can be
significant at these levels as expressed in a post by Terry back in Nov. 97.
Today, I removed the rc filter and measured only a slight variation of 0.002V,
so the filter isn't causing the problem. 
>
> The DMM may suffer a 0.2V or 0.4V shortfall in its AC readings due to 
> forward drop across internal diodes - are you able to compare the DMM 
> with say a 'scope reading to establish whether this is the case?

Yes - today I checked it against a scope measurement at a low AC voltage and
found 0.54V difference, but I trust the DMM more than I do the scope (won't go
into details). I'll take my DMM into work tomorrow and check it with other
equipment. 
>
> Were you able to continuously monitor the primary current during the 
> measurement run? I'm wondering whether the current changed, perhaps 
> due to the resistance warming up.

No. I believe the resistor heating up did cause a primary current change during
the course of the measurements. Today I checked the coil this time using a
toaster. To keep the toaster elements at a stable temp, I placed a fan to blow
into the heating elements. Measured a K of 0.172 at 1.5" (p above s). Then back
to the hair dryer (I'm finding that the hair dryer is giving the most stable
current readings over the course of time). (I also used a vacuum cleaner and
iron). The hair dryer is the best. With the hair dryer, K is 0.174 (slight
change from yesterday and today's tests indicating most like cause is meter
reading). 
>
> Other than the above suggestions, the possibility remains that some 
> inductively coupled conductor loop is present, which would disturb the 
> induced voltage on the secondary.

Well, a 4-bay light fixture was just off to one side and above the secondary.
Before anything today, I moved the coil assembly center to the garage with
nothing near by. The above mentioned readings were taken from there, so the
light fixture wasn't causing misreadings. There's nothing else around. 
>
> Can you measure the secondary inductance in-situ, with the primary open 
> circuit, to see if it remains at the nominal 87.6 mH in the current 
> test setup?

The measured value was back in MN (in CA now). I had access to an LCR there,
but not here. No f-generator either. 
>
> I've been looking at the acmi code to see if I can account for the 
> discrepancy. The trouble is that acmi calculates the self inductances 
> by summing all the mutual inductances between the turns in a winding, 
> and the result is good to a percent or so. The same summing routine is 
> used for the mutual between two windings, but the code doesn't 'know' 
> the turns in question are in different windings, so its hard to see 
> how it can be as much as 17% out. It might be argued that the mutual 
> involves on average a longer range of coupling than the self 
> inductances, but then if the coupling was over-estimated at longer 
> ranges, the trend would be for reduced discrepancy when the coils 
> overlap more - the opposite to what we see.

Paul, let me check the meter at work. Bert's measured values seem to agree with
acmi and Johns program is also very close. I'm suspecting the secondary meter
readings to be off by about 0.4V. 

I think acmi will be an excellent tool. (Is this acmi similar to Marks program
MANDK?). I am also surprised how well Johns program is calc'ing so close
considering a dry formula (no idea what it is). I think after I check a
suspected meter reading error, I'll redo the tests again using the hair dryer
and grab a second Fluke for concentric Amp Volt readings. 

Take care, 
Bart