Not sure what's actually shown in this ballast diagram - Page 1
Forum section: electronics / projects

author: Ben321
author_rank: Frequent Contributor
timestamp: December 10, 2017, 08:03:25 am
content: So I was looking at this metal halide ballast and noticed that in the circuit diagram shown an official manual for the ballast, it shows part of the transformer appears to be an air-core inductor. I screen capped it and uploaded it to Imgur for posting here.As you can see, the upper part of the transformer in the diagram appears to be iron core (2 parallel lines next to the inductor symbol) but then the bottom part of the primary winding appears to be air core (, no double parallel line, just like with a tesla coil). Yet if you look at the actual ballast in the picture below, the entire thing has an iron core. Again, I screencapped this from the website selling the ballast, and uploaded to Imgur for posting here.So can somebody here at EEVBlog explain what exactly the circuit diagram is trying to show with the core of the transformer?

author: SeanB
author_rank: Super Contributor
timestamp: December 10, 2017, 09:20:51 am
content: This is a basic ferroresonant transformer, with the secondary side ( blue red) being designed to supply a current into the lamp load. The primary side is designed so there is a section of the core ( the section between orange and black) to have a core that has shunts, so that part runs in close to core saturation, with the capacitor providing both phase correction and circulating the near resonant current, so that the flux it induces into the secondary is constant, irrespective of load, so the power going into the lamp is constant. thus part of the core is essentially nearly totally uncoupled from the rest, and the magnetic shunts complete the rest of the core so it will not saturate in normal use. All of the windings have a ferrous core in them, just that parts are not fully connected in the same magnetic circuit, and some parts are designed to operate at a high field near saturation for regulation purposes.

author: Ben321
author_rank: Frequent Contributor
timestamp: December 10, 2017, 12:41:38 pm
content: Quote from: SeanB on December 10, 2017, 09:20:51 amInteresting. However from my understanding the secondary is actually not between blue and red. The secondary coil goes between the unnamed wire at the very bottom of the secondary (which is internally connected to the orange wire tap on the primary) and the red wire at the top of the secondary. The blue wire is a tap on the secondary that is required for correct operation of the ignitor circuit. Once the current is flowing through the ionized gas, the igniter no longer has current flowing through it, and so the blue wire is unimportant in the long run. So I think for analysis of the ballast circuit shown, you can just pretend the blue wire and the ignitor don't even exist. It appears that the actual path the lamp current takes is from the red wire at the top of the secondary, through the bulb, back along the white neutral wire, then up through that small segment of the primary between white and orange, and finally back up to the unnamed wire on the secondary.I think the below picture (which I made by editing the first picture I posted in this thread) shows what actually needs to be analyzed here.I'm also not sure how important the capacitor is to the actual operation of the ballast. I read another website where it said the capacitor is only used for power factor correction, and the lamp would still be correctly current limited if the capacitor were removed. It mentioned that there is another design though in which the capacitor is in series with the lamp, in which case the capacitor is required for providing correct current limiting. Though I do find it a bit strange that in the ballast diagram I'm looking at here the capacitor isn't directly connected across the primary, but rather is connected between the white (neutral) wire and a tap on the primary that is very close to the top of the coil, but not at the top of the coil. I wonder what difference those few turns at the top of the coil make.