Flourescent lamp problem

[zipper]

Wow this is really taking off guys! I had not expected such a huge response & was ready to concede (especially since this isn't my cooker hood to start with). But now it has become too interesting to let go & I would like to get to the heart of the matter...How to test a ballast choke!
I understand a ballast in basically a simple inductor & it's purpose is (at least initially) is to produce a voltage spike & thereafter limit current in the lighting circuit once fired. But it seems to me like it's still only a fairly basic coil of copper around an iron core & as that it should be measurable in some form. But then again maybe i'm wrong. Between posts I dismantled an old bathroom lamp I know for sure works. I measured the choke on that for continuity, guess what...zero on a 20M scale. Not content with that I ramped my megger down to 250v & zapped the terminals, result...open circuit. So what else could be inside there that blocks continuity?

 

[333rocky333]

I measured the choke on that for continuity, guess what...zero on a 20M scale.

Is that not good continuity

 

[zipper]

Of course I meant infinity on a 20M ohm scale, not zero ohms. Please excuse my overeagerness (quick to type/slow to think) sorry!
I also got an open circuit reading with a 250v megger test. Then again seeing as it's DC test voltage I suppose that isn't a conclusive sign the choke is actually defective. Or is it?

 

[333rocky333]

I knew what you meant, just joking

Apart from burnt out ones, or where the connecter has broke, where the windings join, I have never had a unit fail.

Then again I have never tested one for continuity.

 

[zipper]

Glad you were only pulling my leg! As I said at the outset, lighting isn't my thing. But I can't help thinking I am overlooking something simple when it comes to establishing if a choke is actually good or bad. I have roamed the net looking for a valid test procedure, but to no avail. To be honest repairing the cooker hood has taken second place. It could be as simple as a bad contact in the tube fitting that's causing the problem, I will check that later. Right now I am more curious about the elusive inner workings of the choke.

 

[Adam_151]

Not something utterly daft about the GS38 shrouds preventing the probes from getting on the terminals properly, is it?

 

[zipper]

Hi Adam, I don't think so but I will look at the hood again tomorrow (in daylight & without the aid of alcohol). Right now I am more interested in finding a test procedure for establishing if a choke is good...or no-good. There must be one someplace.

 

[333rocky333]

I think maybe they only altered the windings to lower EMC and there still just a winding inside.

tridonic chokes are 230 v 50 hz rated.
The line current to run a 11 watt tube is about 0.08 amps.
maybe you could work out the resistance.

 

[zipper]

Hi Rocky, I run that down to approx 290 ohms. I don't have that on either of these Tridonic chokes...both read infinity/open circuit. I suspect a choke might work on a self inductance principle, but being somewhat weak on the lighting side I'm not that sure. Thinking about it in wider terms, does a choke need load to function correctly?

 

[zipper]

Easy to tell it's Saturday night...I missed a decimal point! Of course that should have read approx 2900 ohms (not 290).

 

[bernardgreen]

The DC measured resistance of a choke will be less than the AC measured impedance of the same choke.

Without a load there is no current flow in the choke so no magnetic field created so no opposition to flow so the answer is YES... chokes do not work without a load. ( like a flow restricter can only restrict when there is flow to restrict )

 

[zipper]

No-load...no-light, that explains it all.
Choke schematics are frustratingly misleading as they show a continuious winding wrapped around an iron core. Something akin to a transformer without a secondary winding. It also explains a method to check if a choke is ok or not...test it with a known good load.
Then the light fault must be down to a break in a tube cathode or maybe just a bad connection on the female pin connectors in the lamp body. I am pleased we got to the bottom of the issue...I have enjoyed this discussion a lot. Regards to all

 

[bernardgreen]

No-load...no-light, that explains it all.
Choke schematics are frustratingly misleading as they show a continuious winding wrapped around an iron core.

That is exactly what a choke is. Any change in current in the coil creates a change magnetic field. This change in the magnetic field creates a voltage in the same coil but with a polarity which opposes the flow of current. With a steady DC current there is no change of magnetic field so no opposing voltage is generated.

 

[RF Lighting]

This is a ballast for a 26W PL lamp, tested on low ohms continuity.

On IR at 500VDC it shows as a dead short.

 

[Steve]

put it away, showoff.