Total inductance for this setting??

[rascalseven]

Hi,

Below is a drawing I made of the 5kHz boost portion of the Pultec EQP-1R filter design.

Instead of the 1A's multitap inductor, the 1R's hi-boost section uses two separate inductors (50mH and 100mH) in various series and parallel configurations to achieve different inductances for the five different frequency choices.

I understand that in series these make 150mH and in parallel they make 33.3mH, but I don't undertand the 5k setting. Can someone show me how to calculate what the straight wire (red in the drawing) in parallel with the 50mH inductor does to the overall inductance? Without the wire it would be 150mH, but with it.......????

Thanks so much.

Peace,

JC

 

[PRR]

> what the straight wire (red in the drawing) in parallel with the 50mH inductor does to the overall inductance?

If these are two separate inductors: 100mH.

If they were on the same core: roughly zero inductance. Some leakage inductance but a fraction of the normal inductance.

 

[crazydoc]

[quote author="PRR"]If they were on the same core: roughly zero inductance. Some leakage inductance but a fraction of the normal inductance.[/quote]
Why? The current will still pass through the 100mH inductance. Say if you have a center-tapped power transformer, and short one half, does the other half not put out any voltage? (I guess I should go try it :grin: )
I'm certainly not saying you're wrong :green: - just that I don't understand.

Edit: I hooked 13.5VAC to the primary of a 117VAC/24VCT (to limit the current when shorting the CT), getting 1.4V/1.4V from the secondary. When I shorted half the secondary, the voltage on the other dropped to 1V. The volrage across the primary (13.5) was essentially unchanged, though I could see the needle move slightly.)

rascalseven - sorry to butt in on your thread like this - I'm not much help. It looks to me like the red wire just shunts the 50mH inductance, giving 100mH for that setting.

3k 0.02uF 150mH
5k 0.01uF 100mH
8k 0.0062uF 50mH
10k 0.0062uF 33mH
12k 0.005uF 33mH

is what it looks like to me. Pretty nifty switching scheme - I could never think of something like that.

 

[rascalseven]

So...... here's the HF+ section of the schematic I have:

Did they just wire it this way to minimize wiring?

Thanks so much for your help!!

JC

 

[NewYorkDave]

You have a 50mH and a 100mH in series. They add to 150mH. If you short out the 50mH, you have 100mH total. Easy.

The second case that PRR is describing is when two coils are wound on one core, well-coupled, acting as a transformer. If you short the secondary of a transformer then the primary does indeed drop to the leakage inductance, a small value. If the two coils are wound in such a way that the inductive coupling between them is very small, however--lots of space between the coils--then you should still have the full inductance value of the coil that's not shorted out. Or fairly close to it, anyway.

 

[PRR]

> I guess I should go try it-

An excellent idea. Don't trust "experts". Prove it for yourself.

> I hooked 13.5VAC to the primary of a 117VAC/24VCT (to limit the current when shorting the CT)

That doesn't really limit the current, or simulate what would happen in a tapped choke in an EQ network where there is a lot of resistance. Put about 500 ohms in series with the primary, or enough to slightly reduce the unloaded voltage. Short one winding again. All the windings will drop to radically low voltage.

In a typical iron-core transformer (or tapped choke), each winding is coupled 99% to all other windings. Whatever loads one winding will affect the others to about 99%.

> 1.4V/1.4V from the secondary. When I shorted half the secondary, the voltage on the other dropped to 1V.

Because you drove the primary from a very low-Z source, what you have really measured is the relative winding resistances (as transformed by turns-ratio). Tt would be typical for secondary resistance to be similar to primary resistance (as transformed by turns-ratio). Your results say that the (transformed) secondary resistance is somewhat higher than primary resistance, probably to reduce idle losses yet keep cost low.

 

[crazydoc]

Thanks NYD and PRR. I used a 300 ohm resistor (only one I had of sufficient wattage) is series with the primary. Shorted 1/2 the secondary - the other half droped from 14 to under two volts, and the primary dropped to 20V (I wold have guessed this would be the ratio of the series resistance to 300 ohms - about 60 ohms -but the actual DC resistance of the primary measures 20 ohms). When I shorted the whole secondary the primary dropped another 5 volts.
Now I just have to get a grasp on this...

 

[CJ]

Inductance is related to turns as a quadratic function.
In other words, Inductance varies as the square of the turns,
So lets say I wind ten turns, the inductance is 10² or 100, times a constant, which we won't worry about.
So if we wind 20 turns, we now have 20² or 400 times a conastant.
So the inductance will go up a lot quicker than the turns we wind, IF, the wire is wound on the same core.
But hooking up two inductors of 10 turns each, wound on identical cores, you will only double your inductance, where as we quadrupled it when wound on one core.
This is why when winding Pultec inductors you do not get much inductance til you reach a certain point, then all hell brakes loose and you really have to check your inductor every few turns or so to get the next tap.

This is also why you get 600 ohms from a primary of a transformer when you hook up two 150 ohm windings in series, Twice the turns will give you 2² , or 4 times the impedance, which is directly related to inductance.