magnetic ballasts as chokes?

I could save you all the irritation if only my meters could read Henrys, but I guess it'd be nice to get it out there regardless:
I found this lamp at Goodwill, thought it'd look great hanging in the middle of my bedroom; very dungeon-ish.  It has spent the bulk of its life as a bug-zapper, so it had this socket that accepted only 15-watt-florescent bulbs.  Maybe a little too spartan.  So I ripped out the guts (and that cruel little fence) and installed an E27 Edison socket (how many things have we named after him?  How many things did he name after himself?).
I had a few transformers left over, and one of them, sporting two leads, was in series on the hot side of the bulb-- it was labeled “Listed Ballast 140 H”.  My meter can pull off an Ohms reading if going downhill with a breeze and it said the ballast had 26 Ohms of resistance.  It looks about the right size, I can see the E-I lamination-- is this something I could use as a choke for a tube power supply?
I couldn't find the part number and reactive ballasts for florescents are described as inductors. 
I'm planning on getting my very own meter-that-can-do-stuff, but off the top of anyone's heads, is this something that would work?
The thing I'm working on now doesn't take much current; I'm not expecting to run a console off something that was ballast for a 15-watt bulb, but, eh?  Maybe I'm just excitable.  Yea, free choke, didn't have to wade through a HAM fest (though I wouldn't mind doing that right now).
I guess while I'm on the topic, does anyone know of a HAM fest-ish thing going on in Las Vegas, NV?
Thanks!

lernith said:

I could save you all the irritation if only my meters could read Henrys, but I guess it'd be nice to get it out there regardless:
I found this lamp at Goodwill, thought it'd look great hanging in the middle of my bedroom; very dungeon-ish.  It has spent the bulk of its life as a bug-zapper, so it had this socket that accepted only 15-watt-florescent bulbs.  Maybe a little too spartan.  So I ripped out the guts (and that cruel little fence) and installed an E27 Edison socket (how many things have we named after him?  How many things did he name after himself?).
I had a few transformers left over, and one of them, sporting two leads, was in series on the hot side of the bulb-- it was labeled “Listed Ballast 140 H”.  My meter can pull off an Ohms reading if going downhill with a breeze and it said the ballast had 26 Ohms of resistance.  It looks about the right size, I can see the E-I lamination-- is this something I could use as a choke for a tube power supply?
I couldn't find the part number and reactive ballasts for florescents are described as inductors. 
I'm planning on getting my very own meter-that-can-do-stuff, but off the top of anyone's heads, is this something that would work?
The thing I'm working on now doesn't take much current; I'm not expecting to run a console off something that was ballast for a 15-watt bulb, but, eh?  Maybe I'm just excitable.  Yea, free choke, didn't have to wade through a HAM fest (though I wouldn't mind doing that right now).
I guess while I'm on the topic, does anyone know of a HAM fest-ish thing going on in Las Vegas, NV?
Thanks!

Click to expand...

That will work, but how well?  This is an AC inductor, with no air gap. When you run it with DC, its inductance drops significantly because it goes quickly into saturation. That's the whole problem with DC chokes: you need iron to corral the magnetic field, but you need air to increase the reluctance.

Ah, there's the rub!  I was trying to figure out how it boasted 140 Henrys....

The other problem is that such inductors often have huge parasitic capacitance - since they're only made for working at very low frequencies, no effort has gone into trying to minimize the apparent capacitative influence from one end to the other.

But as always - try it, and see what happens!

Jakob E. 

> “Listed Ballast 140 H”.

That can't be a Henry; in context it is probably a UL spec. Paragraph 140 sub-paragraph H?

> 15-watt-florescent

I will assume you live in 120V 60Hz land. (Rather, that the bug-lamp came from such power.)

15w at 120V is 15W/120V= 0.125 Amps. Somehow the tube and coil together act like 120V/0.125A= 960 ohms. Sanity-check: 120V^2/960= 15W.

The tube has negative incremential resistance, but always needs gas-voltage to conduct at all. And tube voltage must be less than line-voltage (for simple ballasts like these). Simplify recklessly. 120V total, 90V across tube, 30V across coil. The coil seems to act like 240 ohms.

> 26 Ohms of resistance

This is tentative confirmation. Small power and low-audio windings tend to have DCR 2%-10% of AC impedance at lowest working frequency. Since the point here is to waste-off some voltage, we can accept a higher DCR than we'd like in an audio chore; the price-cut market demands it.

The alternative reading, "140H", says 140H*60Hz*6.28= 52K impedance. It is not believable they could wind 52K AC and 26r DC, 0.000,5% parasitic resistance.

0.64H at 60Hz is 241 ohms, it appears the coil for a 120V 15W fluorescent tube could be on the order of a part-Henry.

Someone who actually understands highly non-linear gas tube operation will point out many false simplifications, may even know what Henry is in there. I'm just saying.

Iron-core inducance is NEVER a fixed number. Inductance can vary 10:1 before you even hit saturation, just with change of level or frequency.

As a ballpark check: put the choke and 470 ohms in series across your signal generator. Meter the voltage on the resistor. Sweep from 20Hz to 200Hz. You should get full voltage at 20Hz and less at 200Hz. The point where voltage is 0.707 of max is where L=R. If I'm right, just above 100Hz. Try again but at much lower/higher level, 100mV or 10V.

15W dissipated in the tube does NOT imply 125mA tube current (unless the tube is dropping 120V, which seems unlikely in normal operation).

I would assume that the tube voltage drop was actually less then ~60V or so and that you might be seeing 250mA tube current which would imply a choke impedance of 240ohms and a line current/voltage phase angle of ~45 degrees. 

Regards, Dan.

> 15W dissipated in the tube

Not what I meant. The -whole fixture- pulls roughly 15W at 120V. True, the power-factor and waveform are awful, and we don't know how far off; this is just a number to guide you when loading fuse-circuits. A hundred bug-lights is marginal for a 15A 120V circuit.

> I would assume that the tube voltage drop was actually less then ~60V

I assumed 90V, which may be high; I ignored the 1.414 term because I'm lazy and felt an approximation would rule-out the "140 H" marking as "Henries".

> The coil seems to act like 240 ohms.
> imply a choke impedance of 240ohms

We seem to come to a similar result. Part-Henry is more likely than "140 H".

I really can not think of an audio use for this readily-available choke. If you paid me to use one, I'd find a place to put it (DC-bleed on capacitor coupled headphone amp?) but I don't see an obvious good use. Midrange EQ-coil, but it is an awful big lump for that.

Enjoyable reading-- thanks for all the replies.  I am approaching Socrates-one-more-thing-I-don't-know-nirvana.  Onward!