Cadmium Resonance Lamp Power Supply

Discussion in 'The Projects Forum' started by Ehurlow, Oct 26, 2018.

  1. Ehurlow

    Thread Starter New Member

    Oct 25, 2018
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    0
    Hello!

    I'm a chemistry grad student working on a photochemistry project for which I need to get a cadmium resonance lamp functioning as it has a spectral emission around 225 nm that would be useful for my reaction. The lamp (16W Philips 93107E) was originally packaged with a power supply, but the lamp I purchased from eBay did not come with one. I've tried tracking down the power supply online via its model number (Philips 59003BT/62), but I haven't found anything. I've also contacted Philips Lighting customer support but was bounced around for over an hour with no results.

    Luckily, I found a document (which I've attached) that goes over the technical aspects of the lamp and power supply. Here is an excerpt from the document (p.2):

    "Although some of these lamps could be connected to a 220 V A.C. supply,
    with of course suitable current limitation, for easy ignition it is better to supply
    them with a higher voltage. A transformer having an open voltage of 470 V
    at a primary voltage of 220 V is therefore supplied together with the lamp.
    The transformer is an autotransformer of the low power factor type. This
    means that provision is made for a considerable spread of the lines of force,
    which has the same effect as if a choke were connected in series with a
    normal transformer. The current is thereby automatically limited. Consequently
    the working current is practically equal to the short-circuit current, which
    amounts to about 0.9 A. Thanks to the high open voltage the tube ignites
    without the oxide-coated cathodes having to be pre-heated."​
    Here is a figure showing the circuit diagram:
    Resonance Lamp Power Circuit.png
    The lamp is stated as running at 25 W (p.3, Fig. 6, entry 3).

    I have talked to our school's electronics shop and they said they would be able to figure something out, but gave me a quote of "probably less than a grand." Right now our lab doesn't have the resources to fund such an undertaking, especially because this is a bit of a side project. Would anyone be willing to help me figure out how to get this lamp running? Is there a modern product that would work as a power supply? If not, how difficult would it be for a novice to build a such a device? The lamp we have is an antique, so I would like to be as cautious with it as possible. The opening line in the above quotation states that the lamp could be run off 220 V with proper current limitation (0.9 A I'm assuming?), but it seems it would need to be pre-heated. This seems a bit risky to me.

    If there is any more info that would be helpful, I'd be happy to provide it.

    Thanks so much for your help!


    Evan
     
  2. crutschow

    Expert

    Mar 14, 2008
    21,713
    6,261
    It appears the lamp is marginal to ignite at 220V so that's why they use a transformer with a 470V secondary.

    Since a standard transformer is not current limited, you will need to limit the current.
    For this, add a large choke (inductor) in series with the lamp.
    For 0.9A @ 470V it should have an inductance of 1.66 Henry @ 0.9A and 50Hz.

    To minimize the chance of damaging the lamp I would put a 1 A, 600V fuse in series with the lamp.
    You misread the data sheet.
    It says "Thanks to the high open voltage the tube ignites without the oxide-coated cathodes having to be pre-heated."
     
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  3. Ehurlow

    Thread Starter New Member

    Oct 25, 2018
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    0
    Fantastic!
    Thank you very much, I'll begin looking into building the device.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    They use a 470V transformer, but a 440V transformer may be easier to find, and that likely will also work okay.
    In that case you would reduce the inductor value to 1.55 Henry.
     
  5. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    809
    If you use a 440 V transformer and a 1.5 H inductor in series with the tube for 25 V and 0.9 A for the tube, you are going to have a large amount of iron and copper. The transformer would need to be rated at about 200 VA for an autotransformer or 400 VA for an isolation transformer for it to handle the required current - in this case entirely a function of the size wire used for the windings. The inductor will be big and heavy (e.g. the Triad Magnetics C-17X which is 1.5 H at 300 mA weighs 1.6 pounds). It will also need wire suitable for the current AND a big chunk of iron for the core to prevent magnetic saturation. It may be hard to buy a suitable inductor off the shelf.

    There has to be a better way, but I can't think of anything available off the shelf that will do the job. Most of the common lamp types that are vaguely similar (mercury vapor, fluorescent, high intensity discharge) operate at much higher voltage and lower current.

    If a "push button start' where you might have to push the button a few times to get the lamp to ignite were acceptable, something like a 48 V 1 A transformer in series with a suitable inductor might work. A push button switch right across the lamp would short-circuit the lamp, and when the switch was released, provided it happened at the right time in the AC cycle, you'd get a big voltage spike that should start the the tube. Higher transformer voltage and inductance would improve current regulation. A resistor in series with the push button might be necessary to limit the current with the lower voltage transformer and smaller inductor.
    This is more or less the way the old fluorescent lamps that needed a glow-type starter worked except the starter self-heated due to current through the heaters in the tube, then opened with a thermostatic contact.
     
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  6. Ehurlow

    Thread Starter New Member

    Oct 25, 2018
    3
    0
    That would be totally fine.

    Would this be the correct circuit diagram for what you've described?

    circuit with switch.png
    What would constitute a proper inductor in this case?

    I have a variac available to use if that would help.
     
  7. crutschow

    Expert

    Mar 14, 2008
    21,713
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    Another alternative is to connect the lamp to the 220V line with a 0.9A, 0.75 Henry inductor (@50Hz) in series.
    Then momentarily shorting the lamp to start it, as epp suggested, may work.
     
  8. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    809
    This is not looking good in terms of finding an off the shelf inductor with adequate current rating that isn't huge and very expensive.

    For a 48 V transformer I think 220 mH should be about right. With a variac available it might be better to use a lower inductance such as 150 mH to allow higher current. Without knowing more about the tube characteristics there is an aspect of guesswork. I need to think about this a bit more.
     
  9. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    809
    One of my concerns with trying to use a low voltage transformer is that the tube voltage may be quite high for some time after initial strike until the cadmium is vaporized - perhaps higher than the transformer secondary voltage.
     
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