Rheostats and dimmers

I have wondered for eons how a rheostat saves energy. I hope someone can clear this up for me.  My current understanding is that a rheostat provides resistance, and that decreases the electricity that reaches the light bulb, and so the bulb is not as bright. But the resistance in the rheostat is also electrical use, and isn’t the reason the light bulb does not get as much electricity because it was taken by the rheostat?

I just searched the internet, and found a site where this question was under discussion, along with  modern dimmer switches, which I thought were the same as rheostats, but in the modern dimmer switch, some users were saying that the switch is electronic, and it cuts off a certain part of the electric wave. It says the modern dimmer switch also heats up. That discussion was putting forth various explanations, no attempt of forging a cohesive explanation where everyone finished up understanding the concept.  Seems like this should be, like gravity, not subject to opinion.  The device consumes a quantity electricity and it converts it to heat.  Right?

And how can a device “cut off” an increment of the wave, or a section of it?  Where does that go?  It has to be accounted for in the equation doesn’t it?

I am confused and would like to understand, so I brought my question home to permies where a lot of people know a lot of stuff, are kind and are often willing to explain things to folks who don’t understand yet.

Thanks!

Thekla, I think you have mostly answered your questions already. You're on the right track.

Rheostats are resistance loads -- the same as toasters, heaters, things like that. They have a bunch of conductive materials that convert a portion of the energy in an electrical circuit into (mostly) heat. And so they reduce the energy available to other items in the circuit -- dimming an incandescent light bulb, for example. No energy is saved at all; there is just more heat produced in one place and less visible light (or whatever) in another.

A dimmer switch is a very different beast. It is a super high speed electronic switch that can turn the power on and off faster than the eye can see. Sort of like an old movie theatre film, which flickered a little but you still saw the characters moving -- even though it was a moving film made up of static images. But when the switch is off, it's off - no power is consumed. And that can happen more than 30 times per second before you would even perceive a flicker.

I hope I have made this simpler and not more complicated. I can get into the whole DC/AC thing, and get as technical as you want, but I hope this makes sense.

From Ohm's law, we can say that Voltage divided by Resistance equals Current passing through a circuit. So, the higher the resistance, the lower the current. Voltage times Current equals power, so the lower the current for a given voltage the lower the power passing through the circuit. This is it in a nutshell.

thank you both!

I will go ahead and ignore people who tell me it saves energy to use a rheostat to dim the lights!

Is it a rheostat that is used in something like a toaster oven, that accounts for being able to turn the heating element, higher and lower?

As for the “modern dimmer switch” I understand what you said, Douglas, but I don't understand its applications or usefulness.  It would not dim a fluorescent light fixture would it?

Thekla McDaniels wrote:Is it a rheostat that is used in something like a toaster oven, that accounts for being able to turn the heating element, higher and lower?

It is usually a thermostat that controls oven elements, so as to keep the temperature as stable as possible. It will have a minimum temp limit at which it comes on, and a maximum limit at which it will shut off. It is an on\off situation.

A better example for your original question would be a hotplate or stovetop (old fashioned, not modern), as they do use a rheostat to control the temp. With full power running through the element, imagine how much energy it is putting out to make the element glow orange hot. Now, if you increase the resistance of the rheostat to turn it down to simmer, is the rheostat glowing hot putting out the balance of that previous energy? No, so the circuit should be using less power, bearing out Ohm's law.

A rheostat is an adjustable resistor.  An easy way to think about electricity is to think of it like water.  Electrical current is like the volume of water flow and a resistor is like a restriction in the pipe.  So, as you adjust the restriction (rheostat) you adjust the flow.  More flow to the light bulb means a brighter light and more current (electricity) used; just like a wide open valve means more water flow and more water used. A rheostat adjusted to a high resistance is going to restrict flow and reduce electricity used; just like a valve that is mostly shut will stop most of the water from flowing and just allow a trickle through.

The rheostat is not using up electricity just like the valve is not consuming water.

I wonder if there is some terminology confusion.

A rheostat, as I understand it, is a variable resistor. The resistor dissipates some of the power in the circuit as heat, and so dims (say) a light bulb by lowering the available voltage.

To my knowledge, no modern appliance uses a rheostat. I only see them in low voltage DC electronics. Electronic switching is used (the most basic is a silicon controlled rectifier, SCR) -- the same basic technology used in a dimmer switch. These are safe and energy efficient.

Naturally there are fancier versions of these now, but the principle applies.

Thekla McDaniels wrote: It would not dim a fluorescent light fixture would it?

A dimmer will only work on lighting systems that are designed for it. Certain fluorescent and LED bulbs can be controlled with a dimmer and they are labelled for that.

For hard-wired fluorescent fixtures, I just replace the old tubes with LED tubes. More efficient, and I like the quality of the light.

I got around this issue completely by buying lamps with multiple switches, that turn on one, two, three, or four bulbs.  I utilize compact florescents, that I believe can not be dimmed.  So, with two of these lamps in different locations, I can maximize or minimize the light level simply by the number of bulbs that are fully on.

This works especially well for the middle of the night if I need to get up to go to the toilet, but don't want to be exposed to bright light.  The first of the bulbs is a little 7W CFL, whereas the rest are 25W, so the dim light helps enough with navigation that I am not tripping over anything, but doesn't ruin my night vision.

Michael Qulek wrote:I got around this issue completely by buying lamps with multiple switches, that turn on one, two, three, or four bulbs.  I utilize compact florescents, that I believe can not be dimmed.  So, with two of these lamps in different locations, I can maximize or minimize the light level simply by the number of bulbs that are fully on.

This works especially well for the middle of the night if I need to get up to go to the toilet, but don't want to be exposed to bright light.  The first of the bulbs is a little 7W CFL, whereas the rest are 25W, so the dim light helps enough with navigation that I am not tripping over anything, but doesn't ruin my night vision.

I use similar techniques -- levels of lighting that suit the task rather than one single, bright fixture. This saves a lot of energy.

Dim "navigation lighting" so I don't run into things or fall down the stairs. Task lighting that illuminates only what I'm working on. And then whole-room lighting for the occasions where I need it.

Often the navigation lighting is provided by a little 1-AAA LED flashlight (rechargable) that's always in my pocket. It has a momentary switch on the end, so I can flash a few seconds of light when I need it. Handy as heck!

Douglas Alpenstock wrote:

Michael Qulek wrote:I got around this issue completely by buying lamps with multiple switches, that turn on one, two, three, or four bulbs.  I utilize compact florescents, that I believe can not be dimmed.  So, with two of these lamps in different locations, I can maximize or minimize the light level simply by the number of bulbs that are fully on.

This works especially well for the middle of the night if I need to get up to go to the toilet, but don't want to be exposed to bright light.  The first of the bulbs is a little 7W CFL, whereas the rest are 25W, so the dim light helps enough with navigation that I am not tripping over anything, but doesn't ruin my night vision.

I use similar techniques -- levels of lighting that suit the task rather than one single, bright fixture. This saves a lot of energy.

Dim "navigation lighting" so I don't run into things or fall down the stairs. Task lighting that illuminates only what I'm working on. And then whole-room lighting for the occasions where I need it.

Often the navigation lighting is provided by a little 1-AAA LED flashlight (rechargable) that's always in my pocket. It has a momentary switch on the end, so I can flash a few seconds of light when I need it. Handy as heck!

I have a friend who uses the "new-fangled" programmable, color-changing, LED lightbulbs to great effect. He works odd hours for an ambulance service, sometimes sleeping during daylight hours and waking in the middle of the night. He has his "nighttime program" with dim red light to not mess up his night vision and sleep rhythms, his "wake-up to get ready program" using a ramping-up brightness to daylight spectrum, and his "off-duty program" with warmer white light for after work, days off...

Douglas

That rechargeable flashlight sounds perfect for night use, maybe with a red filter on it for nighttime if i am going outdoors!

I have read recently of how much human use of white light at night disrupts other organisms: birds, insects, plants, amphibians etc etc etc….