Joined: 09/08/2007 Location: Australia Posts: 3338

Posted: 11:18pm 21 Jan 2020

Sorry Nick, forgot to answer your questions.

The way its wound is fine. It could have been wound with solid round wire, but the main aim is to get as much actual copper onto the core as possible. Rule of thumb: Work out what wire gauge is needed to carry the current without it burning up. Stuff as many turns as possible onto the core. Then tweak the air gap for optimum inductance versus saturation limit.

Don't worry about the choke wire being a thinner gauge than the transformer primary wire. The transformer may have ten metres or more of wire on it, the choke maybe one metre length of wire. The voltage drop loss along the primary is far more serious than the voltage drop across the choke. So we can run the choke up to a higher working temperature with thinner wire without losing significant efficiency.

If there is no deliberate airgap the magnetic path length directly effects the inductance. A U and an I core will give a higher inductance per turn (Al value) than a pair of U cores. That can make a significant difference when designing a transformer. As soon as you gap the core for a choke, even minimally, the introduced air gap effects things so hugely you can ignore the ferrite path length altogether.

Its kind of like having a dc series circuit and measuring the current with a one ohm resistor. Then putting a 2K resistor in series. The permiability of ferrite might be around 2,000 with respect to air. So it does not matter if we use one U core (1 ohm) or two U cores (2 ohms) and then introduce a series air gap of 2K ohms. The air gap just totally dominates things. So the Al value only has meaning when we have no air gap.

So for chokes we need to think in terms of just the air gap.Cheers, Tony.