Fluorescent tubes in series off one larger wattage ballast?

Discussion in 'Electricians' Talk' started by Conrad T, Mar 25, 2020.

  1. Conrad T

    Conrad T New Member

    I was wondering if this could work?
    I’ve got 2 30W uvc tubes but can’t find 900 ml batten fixings anywhere. 30w ballasts aren’t easy to find either and are expensive, probably because everything is being replaced with Led.
    Now I know you should always match the ballast with the tube But (the word that means disregard everything I’ve just said) I’ve been rummaging around my attic and have come across a 70w ballast and T8 connectors , the output of this ballast is about twice the voltage that each of the individual tubes require at the correct current. So my question is : Could I run these tubes in series off the single ballast, my assumption is the potential drop should be equal across the two resulting in both lamps being driven at the correct power. Or will the second lamp not strike so break the circuit? I am overdriving them at the moment but concerned it’s going to dramatically shorten the life of the tubes and worried it may cause them to produce some shorter wave radiation leading to production of ozone, the tubes are doped as far as I can tell but thinking the overdrive may damage or just overrun the protection built in at manufacture.
    Yes I’m making a uv sanitiser, I’d buy one off eBay but they’re most likely to be counterfeit blue lamps.
  2. MGW

    MGW Well-Known Member

    With a magnetic ballast even running a 58W tube with a 65 watt ballast causes problems, but with electronic ballasts often they are marked for a range of lamps, with two tubes on same ballast common with fly catchers, I found only some starters work, and sorry that long ago can't remember which, as to why not a clue, I really don't know exactly how a starter or electronic ballast works over the general principles.

    So the magnetic ballast does two things, one it causes a high voltage peak when starting, and two it limits the current when running, the latter is very voltage dependent, in one job our boss decided to light some tunnels using 110 volt fluorescent fittings, we did a quick calculation, 58W at 110 volt around 0.5 amp so should be able to run 32 fittings from a 16 amp supply, did not want to run close to limit, so used 25 fittings per 16 amp supply. However they tripped out, on getting the clamp on ammeter found drawing around 20 amp.

    On opening the unit found it used a auto transformer, with input either 110 volt or 127 volt, swapping to 127 volt stopped them tripping but last few would not start so last 5 set to 110 volt, no more problems.

    However it showed how important voltage is, before voltage with Europe was harmonised to 230 volt, if you used a Dutch 220 volt fluorescent on 240 it used way over its rating so may say 58 watt on the tube, but used more like 70 watt. And of course the life of all components were reduced, when the HF electronic ballast came out it was found a 58 watt tube used more like 55 watt and lasted longer with higher light output, but all HF ballasts are not the same, early ones designed to work from 12 volt used the heaters which were not switched off, the poor quality HF transformer has tapping either end, so typically 7 volt across heater to start, once running dropped to 2 volt, latter versions did not use heaters, both pins connected together, so even with HF there were some that used less power to others, but a 8W fluorescent tube in a caravan used less than a 10W bulb and gave more light, so no one worried.

    Odd but the physics 'A' level exam text book completely missed out the ballast, seemed wrong to me to give a wiring diagram that would never work, but there you are.
  3. peter palmer

    peter palmer Well-Known Member

    Lots of multi tube fittings have tubes in series with one ballast but require series starters to get them to strike, if you put normal starters in them then they don't work at all and also if one tube blows then they all go out.
  4. Bob Rathbone

    Bob Rathbone Well-Known Member

    I would not bother with inductive ballasts, the HF electronic ones are cheap enough and consume less energy. It has been common practice in the past to run 2 20watt lamps on a 40 watt ballast in series, it always worked until one lamp developed a fault.
  5. Conrad T

    Conrad T New Member

    Thank you everyone, it appears the answer is yes, I’ll rewire them and save the life on the tubes, just in case this Covid thing goes on a bit longer, hoping it’s not going to need the 9000hrs life expectancy of a properly run tube. Stay at home keep the olds safe. It’s about time millennials learned how to change a fuse lol
  6. MGW

    MGW Well-Known Member

    No I don't want to change a fuse, the problem with a fuse is you need a new one when it goes, so prefer a trip, so if I make an error I can reset it, and don't need to go out to buy new fuse.

    OK with semi-conductors often a trip is too slow, and we have to use fuses, but a fuse should only rupture if there is a fault, with our ring finals the over load may not be due to a fault, but simply too many items being used, so trip far better.

    The problem is old people like me often live in houses built before the 90's and still have fuses in them, old fuse wire I had a card at the fuse box, but when it was decided ordinary people were not clever enough to work out which was 5, 15, or 30 amp on the card and so should use cartridge fuses, I did not have a spare 5, 16, and 32 amp fuse in stock, they cost a few quid each, (£2.82 screwfix price) not the few pennies for a card of fuse wire, so only bought one if one ruptured, and not easy to change, you had to dismantle the fuse carrier and rebuild it, the old Wilex fuse box did not have flip open carriers, is was a conversion from original re-wire types.

    I converted old house to MCB plug in units, but in theory one should test the loop impedance first, if a 32 amp fuse goes slightly over the loop impedance it may make a few milliseconds difference in tripping time, but with a MCB if the magnetic part is not tripped, then there is a huge difference in tripping time when using the thermal part of the trip.

    As to fuse in a plug, what is needed is good teachers, my son now in 40's when at school was to be taught how to change a fuse, so teacher as common then asked the class if anyone knew how to do it, son said yes, and proceeded to explain how to select fuse depending on watts and if motor or resistive load, OK today we only use a fuse to protect cable, but this was before we joined the common market, however the teacher was teaching the class to always replace with a 13A, this was same teacher who said there were two types of transistor does anyone know what they are, son answered bipolar and field effect sir, which was correct, but teacher said no you have NPN and PNP at which my son burst out laughing.

    Even a couple of years ago the physics 'A' level book showed the wiring of a fluorescent light without including the ballast, it's not the 'millennials' that need teaching, it is the school teachers who try to teach things which they don't know themselves, there is no point keeping children in school longer if the teachers are telling them the wrong things.

    My dad started his apprenticeship at 14, 5 years with one firm then 4 x 6 months with other firms called the journeyman, so 7 years in all, so he was fully qualified at 21, if you keep children in school until 18 then to complete apprenticeship by 21 you have to cram 7 years learning into 3 years, and things are more complex today.

    When I was an apprentice I was taught about dash pots and star delta starters, auto transformer starters and resistor starters, so up to 7 stage starters, today it is all soft starts and inverters, far more complex today and less time to learn it, I did not need to learn how to program a PLC when I started, my son now like I did before retiring walks around with a laptop in tool bag as can't find fault without it.

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