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Industrial duty dimmer

Discussion in 'Electronic Design' started by Jaggy Taggy, Mar 26, 2005.

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  1. Jaggy Taggy

    Jaggy Taggy Guest

    For a museum exhibit I need a dimmer.
    The electrical requirements are light, I want to regulate a small aquarium
    pump with this dimmer, so any good old dimmer will do, really.
    But it needs to be very reliable because it would be very difficult and time
    consuming to get to it once it is installed in the final exhibit.

    Where do I find such a dimmer??

    Uwe
     
  2. Fred Bloggs

    Fred Bloggs Guest

    Could it be found at an industrial electrical supply store? Forget the
    aquarium- SED has the perfect exhibit, it will be the world's smartest
    moron, Larry Brasfield.
     
  3. You need one designed for a motor, so it isn't a "dimmer". Check out
    the variable speed controls for ceiling fans, but be careful. If you
    slow the pump down too much it may fail to start running again if the
    power cuts out for more than a few seconds. Why can't you put a "Y" on
    the air hose and use a small needle valve to vent some excess pressure?
     
  4. John Larkin

    John Larkin Guest

    Have you tried an ordinary dimmer with the pump? That may not work.

    John
     
  5. Chris

    Chris Guest

    Second the motion. Most of these small aquarium pumps are actually
    driven by a small solenoid that vibrates a bellows. For that kind of
    setup, you would need a 20 watt variable frequency drive rather than a
    line voltage phase control dimmer. ;-)

    In addition to that, the R-C/potentionmeter/diac/triac setup that is
    the circuit for these small dimmers requires a minimum load current to
    operate properly. The small aquarium pump might not be enough of a
    load, and its power factor isn't going to help things, either.

    If you insist on "better living through electricity", find a small
    motor-driven air pump (think automotive?) and use PWM or other means to
    vary motor speed. But it would be best to follow Mr. Terrell's advice
    and tee off a bleeder valve to vary the amount of air. That's an
    inexpensive, reliable solution.

    Good luck
    Chris
     
  6. Jaggy Taggy

    Jaggy Taggy Guest


    The air pump I will have to use is a 2 to 3 watt aquarium pump which indeed
    uses a small solenoid which vibrates the bellows.
    I tried to run one of these things off of a regular $5 dimmer and it worked
    fine, but as I said 'momentary' performance does not impress me when failure
    means having to fly to Europe to fix it, that is where my wont for a
    reliable solution comes in.

    Using the aquarium pump continously and bleeding off the excess is an
    interesting thought.

    Controls like this, either a valve or the pot of a dimmer, see an incredible
    amount of abuse in a science museum environment and the final solution
    should be the one which results in the most rugged 'visitor interface', and,
    in spite of its perceived ruggedness, still the one the maintenance people
    can easily fix.

    If needed I can get around the minimum load requirement by using an
    additional dummy load resistor in conjunction with the regular dimmer.

    I am aware of PWM but that doesn't seem to be a solution in my case.


    What is the variable frequency drive Chris is referring to??


    Thanks for the postings so far, this is helpful to me

    Uwe
     
  7. I read in sci.electronics.design that Jaggy Taggy
    Try a 100 ohm wirewound or cermet variable resistor.
     
  8. Mark Jones

    Mark Jones Guest


    Also consider that as the pump ages, its capacity will decline.

    I like the idea of a "hidden" bleeder screw or valve.
     
  9. Rich Grise

    Rich Grise Guest

    Every vibrator-style aquarium pump I've ever seen has an adjustment
    right on the pump. I've seen pumps in fountains and such that seem
    to run continuously forever without human intervention. Is this what
    you're going for? Why is this such a big deal? Is this thing
    supposed to have some kind of customer-adjustable knob or something?

    Just get an ordinary aquarium pump, set it to the right air flow,
    and leave instructions, "When air flow changes, turn this knob. If
    you can't bring air flow to spec, replace pump and adjust new pump
    accordingly."

    If you're new to electronics, and are just itching for some kind of
    demo masterpiece just to show off, don't put it in a public-accessible
    place until it's been tested literally to destruction. This should not
    take long - I know: BTDT.

    Good Luck!
    Rich
     
  10. Guest

    no good at all reliabilty wise, not for your spec.

    Really the only logical option is a bleed valve. A metal one.
     
  11. Chris

    Chris Guest

    Hi, Uwe. A couple of points:

    * Sorry -- the 20 watt variable frequency drive business was a bit of
    a joke (note the "winky" ;-) ). VFDs are frequently used for speed
    control of multi-horsepower AC motors.

    * You've got a point -- if the museum patrons are gooig to be tweaking
    this continuously, a small needle valve might not be a good idea. They
    wear out fairly quickly. In addition, they can be ruined easily by
    cranking them past full off.

    * I did a quick Google scan, and saw a couple of manufacturers of
    aquarium air pumps that have models with electronic air speed controls.
    These could possibly be the R+pot-C/DIAC/triac controls in the cheapie
    lamp dimmers, and one description mentioned that it had a bellows-type
    motor. So, in fact, what you're talking about with a dimmer-type
    controller might be possible.

    * One issue with these lamp dimmers I didn't mention has to do with
    the asymmetry of the DIAC triggering voltage. This results in a DC
    component in the load voltage. While this isn't a problem with lamps,
    it can cause issues with motors or inductive loads like solenoids. It
    heats the coil, and can result in damage. It's something that has to
    be taken into account.

    * I believe a power resistor in parallel with your pump is a good
    idea, but you shouldn't need more than a 680 ohm 25 watt resistor for
    120V, or 1500 ohm, 50 watt for 240V. Since lamp dimmers are in series
    with the AC load, this should provide enough load current to make sure
    your dimmer works properly.

    * You mentioned that this business might be shipped to another
    country. You should be aware of voltage differences (if you get a
    120VAC pump and send it to a country with 240VAC, the pump will have a
    very short, warm and exciting life. Also, if you're building it in a
    60 Hz environment, and you ship it to a 50Hz environment, you'll have
    less air volume. Remember, bellows stroke frequency will be dependent
    on line frequency).

    * Given that this is going to a museum, you've got a couple of more
    issues, if the museum patrons are going to be playing with this
    control. In many countries, exhibits with electrical controls may be
    classified by the local regulatory agencies as machines. If so, you
    may have to deal with the requirement for low voltage (24V max.) panel
    controls. This would obviously mean a line voltage lamp dimmer with
    line voltage on the pot would be out.

    * You also have to deal with the potential for patrons to be downright
    malicious, at least by accident. Can you imagine what would happen if
    a 3rd grade urchin spills a Juicy Juice on a typical dimmer control?
    Bzzzt. Ouch.

    * From a practical standpoint, too, you have to look at the life cycle
    of the pot or dimmer control you're using. Most dimmers are basically
    made to be tweaked a couple of times a day for 10 years or so, meaning
    about 50,000 cycles. One busload of 3rd graders could run through that
    in one day, especially if your exhibit is fun and exciting to more than
    one of the little urchins. Imagine the delight of the museum tech
    having to change the dimmer every month. You might want to look at an
    AC control with remote pot, and use one of those Allen-Bradley machine
    panel potentiometers with washdown capability. The pot itself is
    mounted behind the panel mounted shaft, and joined with a flexible
    coupling, which eliminates the side-to-side torque that usually results
    in premature pot failure. Since the only moving part is on the front
    panel, it should be fairly easy for the museum tech to replace. I
    guarantee that this will be a predominant failure mode here. Get the
    one with the small terminal block under the pot for the three wires.

    You can buy or borrow a lamp dimmer and actually try your idea.
    Measure the DC resistance of the solenoid coil. Then, after you hook
    up the parallel load resistor, run it straight off line voltage for a
    while with the cover off. Get an idea how hot the coil becomes at full
    voltage. An educated (but careful) finger is probably good enough
    here. Then hook up the lamp dimmer in series, and see what happens.
    Measure the DC component by putting a DVM across the load, set for
    highest range DC. Measure maximum DC voltage (this will usually occur
    between 5% and 40% of maximum control voltage). Figure out how much DC
    power is going to be dissipated. Try leaving the dimmer at that
    setting for twenty minutes, and then see if the coil gets hot. Try
    several power settings, and see. See if yuo can make the thing stall
    and get stuck at lower voltage. It's a start, anyway. After you've
    done this, you can look at different types of AC controls that will
    work for you. There are some which work with inductive loads, some
    which have forgiving minimum current requirements, and can be used with
    an external speed pot. There are also others which provide an isolated
    low voltage for a remote control pot which complies with EU strictures
    on low voltage controls. I'd need a lot more information before
    specifying something myself.

    <dreamland>Of course, if I wanted to nuke this problem, and money was
    no object, only reliability, I'd use a 24VDC motor driving a
    peristaltic pump, and use a PWM motor controller with the Allen Bradley
    washdown machine panel pot as the user control. You can get everything
    you need except the Alen Bradley pot, including the speed control, from
    the Cole-Parmer catalog. Make sure you use the most expensive, most
    durable tubing available.</dreamland> (This is very expensive, and
    should be viewed as humor, unless you've got a lot of cash to spend.)

    Where I live, the local science and industry museum has a tech
    department, and has people who are paid to do this kind of thing. I
    know if I had a problem of this type, I'd want to call them up and ask
    a few good questions. They also may be able to come up with more
    gotchas off the top of their heads. If you're a member of your local
    science museum patron support group and you ask nicely, you might be
    able to buy one of the techs or engineers a good lunch, and in
    exchange, be able to ask a few questions and pick their brain about
    your project.

    Good luck
    Chris
     
  12. Jaggy Taggy

    Jaggy Taggy Guest

    Chris, thank you for your very thoughtful answer, you hit on most of the
    pertinent questions I can think of.

    First a few points regarding the effort which often goes into these
    controls. They easily cost hundreds of dollars since you can never let the
    public handle a potentiometer directly, they are always isolated and
    protected from radial and axial loads and from turning past their stops.

    In addition to all these precautions I design the knobs in such a way that
    the public does not really have much leverage on the whole assembly, for
    example by using a flat knob with a shallow depression for your index
    finger, hard to exert much force that way.

    And then you obviously need the best pot money can buy.

    And then you need to make it easy to change because it will fail anyways.


    The pumps in our case are pretty much determined. I looked for the pump with
    the right amount of flow and a very low noise level. I have been using this
    particular one for years and will try not to change it. I supply my exhibit
    with a big transformer so I can continue to use this type of pump even in
    Germany with 220V!


    The pump will last about a year under continuous use. I used an arrangement
    to bleed of the excess in an earlier version and thought the electronic
    control would allow to change the airflow similar to a bleeder arrangement
    but prolong the life of the pump since it might not run continuously,
    statistically speaking the visitors might leave it in the off position half
    of the time.

    Also, in a bleeder valve (I designed my own back then, not a needle valve
    but a ported cylinder which does not need any stops, you can keep turning
    it) it is not that easy to make a channel which will gradually and evenly
    bleed off air over 180 or 270 degrees, we are talking a very small groove
    here...

    Early tests seemed to indicate that a dimmer arrangement with this pump
    might work, but for how long??


    The discussion here seemed to question the reliability of this arrangement
    and since then I have been thinking again about the bleeder arrangement.
    And indeed I think I will go with a modified bleeder valve, which I can
    build as beefy as necessary, it can have a continuous turn, no stops
    necessary, and I managed to find an arrangement which allows the entire
    bleeding valve to be removed as a unit from the outside while no screws are
    visible, another requirement of our particular set-up.


    And Chris, finally, referring to the suggestions in your last paragraph, I
    am one of those people one often calls for advise in these matters. I know
    this must be a disappointment to you but this is how I earn my keep and it
    is quite a challenge given all the other constraints and the publics
    ingenuity to disassemble the exhibits to come up with good solutions.

    We will see how long this one will last in Germany....

    Anyways, thank you and the other posters for the input, I think I got a good
    approach now.

    Uwe
     
  13. Chris

    Chris Guest

    Jaggy Taggy wrote:
    Not a disappointment at all, Uwe. Enjoyed talking with you.

    By the way, there are a number of relatively inexpensive aquarium pumps
    which will last longer than the one year of continuous use you're
    getting. However, you have to question whether it's worth the
    increased price. Possibly you could research a recommended replacement
    for your customers, and allow them to buy the standard replacement or
    the upgrade from you.

    Good luck, and happy Easter.
    Chris
     
  14. Chris

    Chris Guest

    Jaggy Taggy wrote:
    Sorry, Uwe. One more think. You might want to look at a "footswitch
    potentiometer", which uses spring action to always return to one
    detent. I think kids might get a bit of a kick from it, and it will
    definitely save wear on your pump by _always_ returning to value.

    It might be best to build a little box (like a shoeshine stand) out of
    the wall of the display, and put a sign pointing to the footswitch pot
    in a prominent place.

    http://www.linemaster.com/varior.shtml
    http://www.linemaster.com/hercpot.shtml

    The orange NEMA job (the Linemaster Hercules) looks mighty industrial,
    too.

    Good luck
    Chris
     
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