# constant current or constant voltage?

Hello, I'm looking into driving about five separate piezo ultrasonic water misters and one of them have the rough requirement of 24 volts. my question is if i wanted to drive all five at the same time I'm thinking I would want a 24 volt constant voltage source, is this accurate?
Bonus question: what dictates whether a constant current or a constant voltage source is needed?

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jfryar302723 days ago

You are correct! 24V will work, all the way through all five.

You may have already heard of the water analogy, but here it is again: Electricity is the same as the flow of water. Voltage is the pressure of the water, while current is the amount of water(There is also resistance, capacitance, and such, but unneeded here). If you are trying to fill a water balloon with water, you have to know the rated pressure of the balloon, otherwise, it would pop! For this example, we will use 24 PSI (Pounds per Square Inch) - the same as your piezo elements, just in voltage.

To fill a 24 PSI balloon up, you will need 24 PSI. If you wanted to fill up two balloons at 24 PSI, you would still use 24 PSI - just twice as much water. I think you can figure out what happens for five balloons.

I hope this makes sense, but if not, let me know and I'll try to change my answer.

Jack A Lopez4 days ago

You did not say if it was AC or DC. Although, the word "constant", kind of implies DC, it might be worth your time to examine the hieroglyphics on device, or the documentation that came with, for to see if there are some scribbles about this.

Also I think I owned (still own? but where did I put it?) one of these ultrasonic mister gizmos, and I seem to recall it wanted 24 volts AC, or the scribbles on it said it did.

Obviously you have questions, both specific and general, about the care and feeding of electrical devices.

The specific question involves some specific ultrasonic water mister gizmo, of which you own five of these.

Actually I think Steve gave you the short answer, just wire all five in parallel, and give them the same voltage.

Actually giving them the same voltage is which is kind of unavoidable when they're wired this way, given the fact that circuit elements wired in parallel share the same voltage. You know: It's like a law of nature. It's a corollary of Kirchhoff's circuit laws. Duh.

The more general question, what you have labeled as "Bonus question", is, "what dictates whether a constant current or a constant voltage source is needed?"

I claim I can answer both questions at once, with a truly simple answer.

I say, in general, in all cases, you should feed your electric device the kind of power it wants.

You might not be happy with this advice, because it requires a somewhat animistic view of electrical devices; i.e. you must think of your electrical device as animate, as a thing with desires and emotions. I mean, if you were raised to think of devices like toasters, and vacuum cleaners, and cell phones and stuff, as inanimate lumps of matter, then you might feel silly about asking questions like, "What does the device want?"

You know, you do not ask what a soccer ball wants. You just kick it around. Or maybe sometimes you pick it up and throw it? However this is usually not good advice for how to treat your toaster or cell phone.

In a sense, your electrical devices do want things, like electricity, delivered in a certain special way.

Other devices want interaction. They want to be played with. For example, your phone: in a sense, it really wants you to push its buttons.

;-P

In terms of emotion, I say that machines can be happy, to the extent they are existing in a manner consistent with the designs of their creator. For example, a light fixture is happy when it is emitting light, and an electric mixer is happy when it mixing cake batter. Similarly, these same devices can be thought of as unhappy in their failure modes. Clearly a dim, flickering, light fixture, is suffering and unhappy. An electric mixer that just makes buzzing noise, without turning the blades, is clearly unhappy.

When your devices are turned off, waiting to get turned on again, well obviously they're just resting. Some device manufactures explicitly call this, "sleep mode"

I think the animistic view is acceptable, and helpful, for people without years of study in the fields of physics or electrical engineering, because the only other possible explanations, that I am aware of, are explanations in terms of physics, or electrical engineering.

So you should feed your device the kind of power it wants.

Yet, how do you discover what kind of power your device wants?

Well, one way is by reading the documentation that came with, and following the recommendations of the device's manufacturer.

Like they say in the late-night infomercials: It really is that easy!

But I get the impression that you're one of those people who just refuse to be satisfied by easy explanations, like the one about the sky being blue or gray during the day, and black at night, because it just is.

Like, you're one of those people who wants to know why.

Yet it would take multiple lifetimes for me to explain everything about everything about electrical devices. However, maybe, just maybe, I can give you some brief explanation, restricted to the narrow topic of devices that want DC electricity, and maybe that will be enough to satisfy your curiosity, for now.

Why do some DC powered devices want constant voltage? Why do some want constant current?

Well, for a DC powered device that is complicated, like for example, any device that contains at least one integrated circuit, almost always that device wants constant voltage, and I think the reason for this is historical, and related to the physics of batteries, because a battery is very approximately a constant voltage source.

So most DC powered devices want constant voltage, and for complicated devices, this want is a consequence of the biases of the designer; i.e. he or she is contemplating powering it with a battery, or something like a battery.

Also I think electrical engineers instinctively distrust constant current sources, because of some counterintuitive math associated with these. Specifically what I am thinking of is an ideal constant current source connected to an open circuit; i.e. a load with infinite impedance. This circumstance is a physical impossibility because the amount of power needed to push constant current through infinite impedance, is also infinite. In contrast, a constant voltage source connected to an open circuit, i.e. connected to nothing, or empty air, is no big deal. The current that flows is zero, and the power supplied and dissipated is also zero.

Getting back to the real world, specifically for simple devices, like a LED, or a heating element, the want for constant voltage or constant current, is a consequence of the physics of the device itself.

LEDs are kind of the well-known example of a DC powered device that wants constant current to make it happy and radiant, rather than constant voltage, which tends to make it overheat and die quickly. The reason why has something to do with the physics of the semiconductors that make up the LED, specifically the shape of the I-V characteristic, and the way this curve changes with temperature.

But in simple terms, LEDs are just happier when you feed them constant current, because giving an LED constant current is is giving the LED what it wants.

4 days ago

Constant current-ness is used internally as a property of many special types of cicuit, but constant current supply of any great power are usually reserved for things with very low resistance (or, if you prefer, very high conductance), like LEDs, like magnetic wires

3 days ago

Yeah. It is like the constant current source wants a short circuit, or something close to this, as its load, or rather this is the circumstance in which it does the least amount of work.

I guess here I am imagining power sources are like people, who also are more comfortable when they do not have to work very hard.

BTW, I can point to a better example of a constant current source being a weird, unnatural thing, and that is what you get when draw a Thevenin equivalent circuit (voltage source in series with a resistor) and a Norton equivalent circuit (current source in parallel with a resistor), side by side.

Conveniently, Wikipedia already has a picture of this, here,

https://en.wikipedia.org/wiki/File:Norton-to-theve...

The thing that strikes me about this picture is, for the no load case, the resistor in the Norton circuit is dissipating power, specifically P = Ino^2*Rno.

I realize these two circuits are equivalent, as seen from the terminals where a load is connected.

However the Thevenin circuit seems to me to be a better description of a real physical thing, like for example, a AA cell, sitting on table, with no load connected to it.

I mean, a real life battery cell does not, should not, dissipate power as heat, when nothing is connected to it.

iceng4 days ago

Always enjoy these representations :-)

steveastrouk4 days ago

You want all 5 in parallel with a constant voltage source

RJF4 days ago

When describing voltage, current, and resistance, a common analogy is a water tank. In this analogy, charge is represented by the water amount, voltage is represented by the water pressure, and current is represented by the water flow.

A mister is built so the voltage will put out so much sound, as the voltage is lowered the sound will lower. The flow or 'i think' what would be related to the hose size will be needed to drive the mister. Each mister will create a 'resistance' like kinking the water hose a bit. As more misters are added on they will each take water to work and 'kink' the hose a bit to the next one. Voltage, Resistance and Current are related to a math formula called ohms law. I=V/R, i is current, V is voltage and R is resistance. Changing these items in the formula will possibly change the others.

Been a while since someone told me the water analogy, so folks feeding back can correct or expand on it. RJF