Hi,

I have an old BTH choke/ballast for 80W fluorescent lamps that my Grandad gave me a while ago. It has 4 terminals labelled 1,2,3 and 4, I'm assuming it's for the original 5' 80w lamps so I've wired it up before using only terminals 1 and 2 with live going into terminal 1 and 2 going off to the lamp with a standard switchstart circuit and this works perfectly fine.

I'm wondering though, what are terminals 3 and 4 supposed to be used for? I did wonder if they were for cathode heating so I connected them across where the starter switch would normally go and all that happened is the ends of tube glow very bright, much brighter than they should do. One other thought was that it's the secondary winding for a semi-resonant circuit but this thing's got to be 1950s and I didn't think such circuits existed back then.

Most of the print has rubbed off the ballast over the years but all I can make out is 'BTH TAPPED CHOKE MRJ 304' and 'For 80 watt MCF lamps, 200 - 250 volts, 50 cycles'. There's something else on there that looks like it might have been a diagram but it's impossible to see.

I'm hoping that someone on here might be able to give me some help 

well this is just guessing. but those could be auxilary connection points for neutral wire. I have some old helvar ballast that had 1 aux connection point for neutral. If you have multimeter you could measure resistance between different terminals, that would be great help to see what for those two are. Also could you post a picture of that ballast.

Thanks for the reply, here is a picture:


I put an ohm meter on it and terminals 1 and 2 are connected together and the same for 3 and 4 but 1 and 2 are not connected to 3 and 4, I get the same reading of 9Ω between terminals 1 and 2 as I do for  3 and 4.

well then it could be double coil ballast. similar like this: http://www.lighting-gallery.net/gallery/displayimage.php?pos=-42169 but maybe someone with little more knowledge could confirm this.

Thanks for the info, does that mean I should connect live to 1 and neutral to 3 and connect the lamp across 2 and 4?

If it is a double coiled balalst then yes. But, as bluelights said in the comments of that picture I linked. If the polarity of ballast windings are incorrect. "the voltages will cancel out, putting full 230V across the lamp, the only thing to limit the current would be winding resistance and leakage inductance between the windings - so the lamp would be destroyed"

I have no idea how to find out correct polarity, maybe somebody else could give some advices to that.

To find out if this is a double coiled ballast, you could measure the current of the circuit, if your multimeter is capable of that. based on my logic, If only one coil of double coiled ballast is used current should be bigger than normaly would be for 80w lamp. I would remember that 80W FL used similar ballast than 80W MV so current should be same(+ ballast losses?) but could somebody confirm this?