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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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2nd Jun 2019, 11:57 pm | #1 |
Pentode
Join Date: May 2006
Location: Poole, Dorset, UK.
Posts: 129
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Fluorescent ballasts
What's in old style Fluorescent ballasts? In particular, any idea of the inductance/current rating foe one ex-125W fitting that had 2 in series, marked '7.2uF'.
I wondered if these would be useful for valve HT smoothing, etc. 73 JohnB G8JMB |
3rd Jun 2019, 6:57 am | #2 |
Nonode
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
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Re: Fluorescent ballasts
The marking on the component that you have suggests that it is a capacitor and not an inductance.
MOST fluorescent lamp circuits use a simple series choke to limit the current. 8 foot 125 watt lamp circuits are often slightly more complex and consist of a choke AND a capacitor in series with each other and with the lamp. The capacitors in old fluorescent lights are often in marginal condition and I would think twice about re-use. New ones are cheap. The series chokes are often fit for re-use and can be used as smoothing chokes for HT supplies. They are not, in theory, ideal for this purpose but in practice often work well. A lot of home built valve equipment used lamp ballasts as smoothing inductances, they were cheap to purchase new due to the millions manufactured, and cost nothing if scavenged. A series choke for a 125 watt lamp should be good for at least 750ma. This being the current it passes in normal use. A lower current is better to limit saturation of the magnetic core. Chokes for 125 watt lamps are usually a simple coil with only two external connections. Other types exist. For smoothing HT supplies, a simple choke with only two connections is wanted. Avoid the more complex types with multiple connections, they may contain other components. All this presumes the traditional style of fluorescent light. Some modern types use electronic ballasts, a sub-species of switched mode power supply. These are unsuitable for anything other than use as intended. |
5th Jun 2019, 10:48 pm | #3 |
Pentode
Join Date: May 2006
Location: Poole, Dorset, UK.
Posts: 129
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Re: Fluorescent ballasts
It's actually labelled 'CHOKE' but no inductance values given
John |
5th Jun 2019, 10:55 pm | #4 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,263
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Re: Fluorescent ballasts
I've seen a few fluorescent lights with 2 chokes (ballasts) in series, I always thought it was to make them easier to fit in the fixture than one big one.
A choke and capacitor in series I'm guessing helps with the power factor, I'm no expert.
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Kevin |
6th Jun 2019, 7:25 am | #5 |
Dekatron
Join Date: Jul 2007
Location: Penrith, Cumbria, UK.
Posts: 3,684
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Re: Fluorescent ballasts
Is this for a semi-resonant effect to get the striking voltage up on longer tubes as opposed to straightforward power factor correction functionality in the lesser sizes?
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Regds, Russell W. B. G4YLI. |
6th Jun 2019, 7:28 am | #6 |
Dekatron
Join Date: Jul 2007
Location: Penrith, Cumbria, UK.
Posts: 3,684
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Re: Fluorescent ballasts
In the ones I've dealt with the cap is shunted across the fitting for PF correction. I once found, to my cost, that my brand new 'Eagle' mains-carrier intercom set didn't work when the kitchen fluorescent lamp was on and I discovered it was this PF correction capacitor that was sinking the signal!
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Regds, Russell W. B. G4YLI. |
9th Jun 2019, 10:39 pm | #7 |
Dekatron
Join Date: Jan 2003
Location: Bradford on Avon, Wiltshire, UK.
Posts: 3,301
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Re: Fluorescent ballasts
Perhaps 7.5 uF is the recommended value of capacitor to use with the choke.
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"Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly." |
10th Jun 2019, 10:26 am | #8 |
Rest in Peace
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,378
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Re: Fluorescent ballasts
My aunty had a fluorescent light which eat tubes regularly. The light contained a capacitor and choke in series. Some calculations seemed to show that the capacitor was the current controlling component. Why the tubes failed with one black end, I never found out.
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10th Jun 2019, 11:13 am | #9 |
Nonode
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
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Re: Fluorescent ballasts
There are numerous fluorescent lamp circuits, the simplest and most common is a single choke in series with the lamp.
This circuit is the norm for lamps 5 feet or less in length. A capacitor is generally connected between the supply wires to improve the power factor but plays no direct part in operating the lamp. Some 8 foot and some 6 foot lamp circuits use a choke and a capacitor in series with the lamp. Both these components are vital to correct lamp operation. The value of the capacitor tends to reduce in time and this results in a reduced output and problematic starting. A minority of circuits use two chokes in series, this is indeed to ease manufacture. Used to be common in the 5 foot "arrowslim" fittings that used a 5 foot 50 watt lamp that was only one inch in diameter, rather than the then usual one and a half inch tube diameter. A small minority of special fluorescent lamp circuits use two chokes in parallel. Usually for lamps of unusually high wattage as used for cosmetic sun tanning, ultra violet water treatment, and dye-line printing. It is simpler to use a parallel pair of standard and mass produced chokes than to manufacture specials for a limited market. 8 foot lamps operate at a rather high voltage, a fair proportion of the 240 volt supply, leaving only a limited voltage to drop across the choke and capacitor. They require a full 240 volt supply for reliable and stable operation. They wont operate well or reliably on a 220 volt supply, and wont work on 200 volts. Therefore never popular in mainland Europe. More of a UK thing. They work well in the USA on 277 volt circuits. |
10th Jun 2019, 1:57 pm | #10 |
Octode
Join Date: Jul 2008
Location: Gloucestershire, UK.
Posts: 1,043
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Re: Fluorescent ballasts
Hello,
The 7.2uF capacitors used in the 8' fittings are within tolerance of the tubular 8uF electrolytic smoothing capacitors used in many early 1930s radios and are usually 440V ac rated. The earlier ones are usually rather too large but some of the newer ones are similar in size. I would be vary of used ones as some are self healing ones and can eventually suddenly fail. In the picture below I show two new 7.2uF capacitors. The larger one is from 1982 and the smaller one from 1995. With the thread cut off the top the smaller one makes quite a realistic replacement when the original electrolytic has been replaced. Yours, Richard Last edited by Mr Moose; 10th Jun 2019 at 2:04 pm. |
10th Jun 2019, 2:12 pm | #11 |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,185
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Re: Fluorescent ballasts
Just to make sure, the capacitors used in fluorescent fittings are NOT electrolytics. Just impregnated paper capacitors. An electrolytic, even a bipolar one, would not be a good choice for this application.
On the other hand, capacitors from fluorescent fittings could indeed be used to replace old electrolytics in early radio sets. Just make sure to use polypropylene ones or restuff, since the paper types can always fail leaky or shorted. |
10th Jun 2019, 10:56 pm | #12 |
Octode
Join Date: Apr 2016
Location: Ventnor, Isle of Wight, & Great Dunmow, Essex, UK.
Posts: 1,377
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Re: Fluorescent ballasts
The original 8 foot fitting was 125W and used a choke and a series 7.2mfd capacitor.
At some point 8 foot fittings were downgraded to 100W and the series cap was dispensed with. A 100W tube will not start in a 125W fitting and A 125W tube will not run in a 100W fitting. However, if you remove the capacitor from the 125W fitting, it will then run a 100W tube- the ballasts would appear to be the same. The opposite is also true- add a series capacitor to a 100W unit, and it will then run a 125W tube! Many switch start 6 foot twin fittings had one lamp run direct in series with a ballast. The other lamp used a different ballast with a series capacitor. I would not use capacitors from old fittings as they were by far the most common reason for failure of a 6 or 8 foot fitting. Cheers Nick |
11th Jun 2019, 9:33 am | #13 |
Dekatron
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,081
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Re: Fluorescent ballasts
..Nick's summary hints at why my twin 8' fittings all have one ballast marked 125/100w and another marked 100w. If i pay attention to how the capacitor is wired no doubt it will become clearer. (An arrangement to set them out of phase is probably in place) When scavenged they came with a mixture of tubes.
Dave |