Industrial duty dimmer

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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

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

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.

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?

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

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

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

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

I read in sci.electronics.design that Jaggy Taggy wrote (in ) about 'Industrial duty dimmer', on Sat, 26 Mar 2005:

Try a 100 ohm wirewound or cermet variable resistor.

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

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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

• You mentioned that this business might be shipped to another country. You should be aware of voltage differences (if you get a

• 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.

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. (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

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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

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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.

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

Good luck Chris

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