Electrical question-potentiometer

[Starkiller42]

I would like to put a potentiometer in series with the fan on the system I'm designing, but I don't know what impedance pot to get. 1K, 5K, and 10K are my options.

What are you guys using, and what would you recommend?

Or should I just get a dimmer?

Thanks in advance!

[Exahertz]

More than likely these guys are not putting a pot in series with the fan as the pot would be enormus! the way the fan speed is controlled is through pulsed width modulation (PWM) either by use of some kind of analog to digital converter connected to one of a PIC micro controller's inputs that pulses a transistor to drive the fan or in a very simalar method using an ASIC or simple circuit to PWM the fan

[runmc]

I would like to put a potentiometer in series with the fan on the system I'm designing, but I don't know what impedance pot to get. 1K, 5K, and 10K are my options.

What are you guys using, and what would you recommend?

Or should I just get a dimmer?

Thanks in advance!

Is it a 12V fan ?

[Exahertz]

your best bet (if avalible) is to make a switch that selects either the 5vdc of 12vdc potentials. and if your slick, using diodes you could use the potential between the 5v and 12v and get 7vdc

[Starkiller42]

Lol sorry, I should have specified; I'm using a 120VAC fan. *whacks self*

[Xeon th MG Pony]

you don't use a pot, you use a rheostat, it is sort of like a pot but there is a very big difference.

Though as Exahertz says a PWM controller or a simple Volt selector switch will be easiest. This is the easiest method and what I use, For my water cooling system I use the -12, 0V, +12 so I can drive it at either 12V or 24V.

[Xeon th MG Pony]

Lol sorry, I should have specified; I'm using a 120VAC fan. *whacks self*


Now you are in a new ball game.

for small load fans a dimmer can work though it is much safer and reliable to get a motor speed controller off ebay as they are pretty much dirt cheap from there.

[Starkiller42]

Hmm, I did a search for "motor speed controller" on ebay, and I found almost no AC ones whatsoever. Can you give me a few other search terms that might yield results?

[Exahertz]

Though this is a "fan" thats being used in an A/C related unit, the fan's mechanical and electrical functionality resenbels that of your conventional celing fan. thus any single phase 120vac motor controller will work as long as its rated spec can handle an equal or grater number of amps than the motor its self will consume.

i've seen dimmer switch style celing fan controllers that would work quite nicely for your application

[wdrzal]

I've also seen DC driven ceiling fan motors...............off of AC current and a recctifier. infinate adjustability to max motor rpm.

[Starkiller42]

Well, then I'll just pop on down to my local Big Box Mart and find a dimmer-style ceiling fan controller. Thanks everyone!

On a side note: anyone know a supplier of knobs, like the big black ones you can find on older audio equipment? I'm going for that style, and having a dimmer knob stick out would be a complete mood-killer ;).

[wdrzal]

ceiling fans that have controllors areusually DC motors, I thought I said that :shrug: ....or have 3 speed motor, pull ,once ,twice or three times on chain;) .

you can't slow a single speed electric motor with a reostat(like for incandesceint lights) except "you can use a modern dimmer triac but with inductive loads ,it must switch off each half cycle of the power,(sine wave).

what makes a USA compressor run slower in europe???????????????

frequency or HZ "try a frequency drive", cheaper to get a different motor that runs at correct speed or is a muti speed(3 or 4 speed) use approiate taps or all,just not at the same time.

if small amperage motor try a TRIAC, remember your load will be inductive.

[Starkiller42]

Well usually means that there might be a few AC ones, so I was kinda hoping for that ;)

Either way the best thing might just be to go down there and see what the person in the fan department has to say.

As for the half-wave triac, are you referring to using a diode in series with it? If so that would be simple enough to wire, assuming I can find one that will handle the volts/current.

[ak_47_boy]

rheostat aka light dimmer

[wdrzal]

rheostat aka light dimmer

modern dimmers are variac's, not reostates that give off the extra voltage it dimmed the resistive load(the lights) as heat. variacs don't do that.

[serialk11r]

You said its AC right? How about a variable transformer?

[Starkiller42]

Aren't variable transformers pretty expensive, though?

[serialk11r]

Aren't variable transformers pretty expensive, though?

You can get cheap ones off ebay (small ones rated to a few amps or less) ;) AC-AC conversion is so simple I don't see why you would need to waste a ton of energy with a rheostat or something like that.

[Starkiller42]

Wow, I just went and checked ebay; the transformers are a lot cheaper than I remember them being! Last time I looked was maybe a month ago after seeing a really nice load tester using one. Probably just luck of the draw, whatever's on the market at any given time.

[wdrzal]

you can't simpley lower voltage on a ac motor, usually manufacturies specs are around 5% to maybee 10% of rated voltage.

[Starkiller42]

Dangit! Say walt, doesn't that mean that the triacs aren't usable, since lowering voltage is precisely what they do?

I suppose it would be a good time to ask this too: what kind of fans are you guys using for your condensers? I'm looking through McMaster right now, and they must have several hundred fans...

[SexyMF]

There is a terrific amount of misinformation here.

A rheostat is a potentiometer for higher voltages/currents.

A Variac(tm) is an autotransformer - just a transformer with a single winding. Works the same as a potentiometer.

Light dimmers are not Variacs. They are controlled rectification circuits which use thyristors.

Yes, you can use a rheostat. Your fan will probably work down to 70V.
Yes, a light dimmer will work fine. These handle light inductive loads.


Sounds like you need to do some of that reading you are always telling everybody else to do Wdrzal.

[serialk11r]

you can't simpley lower voltage on a ac motor, usually manufacturies specs are around 5% to maybee 10% of rated voltage.

Okay, then how would a rheostat work? They both control the voltage. A rheostat will also reduce the voltage. Transformer is a hell of a lot more efficient though.

[gsmaster]

A nice & cheap way to make an AC fan a "two speed" fan is to use a simple diode in series that will cut the positive or the negative half-sinewave so the rotation speed will be approximately the half. To get full speed, a switch that shorts the diode must be used.

This is the way most cheap hairdryers work


CAUTION: Use a diode and switch that can withstand the current and voltage.

[Xeon th MG Pony]

What I said was correct, carbone film compared to wound up NiChrom is a big differance in my books. but then he said it was for a much larger voltage as in A.C. mains thus that should no longer be considered in my books.

Light dimmer would work, but not the most ideal option, motor speed controllers work to some degree on the same idea, but are geared more for inductive loads thus better option. VFD unit is ideal solution how ever can be costly.

You can simply lower the voltage on motors but with negitive side effects, thus isn't realy an option.

So where is this miss information? I'm not seing it, and I mean that seriously, what do you think is miss information here??

[n00b 0f l337]

I use solid state fan controllers personally, they work great.

[- Feedback]

AC motors don't operate well on variable voltages. They have to be designed to take the inefficiency of such operation. In other words they must have much more ventilation than simular power AC motors.

Condenser fans typically must operate in high temperature conditions. They are often the failure point of a system when they overheat and burn out. Running an AC motor on reduced voltage for this application is begging for trouble.

[Starkiller42]

Well then, what fans are you guys using and/or would recommend to me? I take it most of you use DC fans; while I would like to avoid putting in a power supply, I could if necessary.

[wdrzal]

Dangit! Say walt, doesn't that mean that the triacs aren't usable, since lowering voltage is precisely what they do?

I suppose it would be a good time to ask this too: what kind of fans are you guys using for your condensers? I'm looking through McMaster right now, and they must have several hundred fans...

They don't lower the voltage at all, they switch it on and off. when on 120v when off 0v.

break one line on the fans wire and install a light switch, now turn it on,100% rpm. now flip the switch quickley...... fan maybe 80% of full rpm but when on still gets 120v, switch even slower fan when getting power still gets 120 volts ,it the on to off time that controls speed,this is how triac's work but switch very fast,each half cycle 60hz means switching 120 times a second, the motot recieves full voltage just not 100% of the time. now if you could flip a switch that fasts you would have same effect


they look just like a dimmer, you will probably only find true reostat dimmers(resistive load only) as dimmeres at a store or shop that has very old stock.

all modern stores will sell a triac as a dimmer,they are much more power efficient since when off they are off, reostates lower voltage and give that extra voltage of as heat.

[Exahertz]

it is possible to change the voltage when pulsing AC. AC is a sine wave function, that means the wave is assending and decending from
0 -to- 120 -to- 0 -to- neg 120 -to- 0
it is possible to pulse on the top highest and bottom lowest parts of the wave, that will give you 120 volts, however it is also possible to pulse in the middle of the wave as well, giving you a selectable voltage anyware from 0 to 120.

for example:

the wave is at zero and starting to rise, the switch is on, as the wave reaches 60v the switch cuts out, now as the wave goes over its peak and starts to fall, the switch turns on again at 60v and stays on, even after the wave has fallen below zero the switch stays on, when the wave falls to negitive 60v the switch cuts out, now as the wave goes over its peak and starts to rise, the switch turns on again at negitive 60v.

looking at it on an O-scope the wave looks like devil horns (a sine wave whare the middle portions of the perabala are at zero), but the output is no grater than 60vac.

[serialk11r]

it is possible to change the voltage when pulsing AC. AC is a sine wave function, that means the wave is assending and decending from
0 -to- 120 -to- 0 -to- neg 120 -to- 0
it is possible to pulse on the top highest and bottom lowest parts of the wave, that will give you 120 volts, however it is also possible to pulse in the middle of the wave as well, giving you a selectable voltage anyware from 0 to 120.

for example:

the wave is at zero and starting to rise, the switch is on, as the wave reaches 60v the switch cuts out, now as the wave goes over its peak and starts to fall, the switch turns on again at 60v and stays on, even after the wave has fallen below zero the switch stays on, when the wave falls to negitive 60v the switch cuts out, now as the wave goes over its peak and starts to rise, the switch turns on again at negitive 60v.

looking at it on an O-scope the wave looks like devil horns (a sine wave whare the middle portions of the perabala are at zero), but the output is no grater than 60vac.

Parabola? Sine waves aren't parabolas...
Sorry I've been uh, too concentrated on math these days.

Anyways back on topic.
So AC fans must be PWM controlled? That might explain why the wood lathes at school use a PWM controller for the AC motor...

[wdrzal]

A 120 volt sine wave has 170 volt peaks and valleys, 120v is the RMS

[serialk11r]

A 120 volt sine wave has 170 volt peaks and valleys, 120v is the RMS

Good point.
Oh a little off topic but you know how the RMS is considered to be 0.7 the peak? Well the area under the curve of the sine function from 0-90 is 57.29.... which is 180/pi. So wouldn't the RMS be 2/pi times the peak voltage (0.63something)?

[wdrzal]

Below is from a thread 6 months ago, we were talking about measuring power iirc. note the last post from detriotAC. whats usually known as RMS ,because of post 20, detroitAc elaborated a little more, ending the false impressions some have of a sine wave.


12-03-2006, 09:53 AM #18
wdrzal
Xtreme X.I.P.


Join Date: Mar 2005
Posts: 3,876 JINU you are completely wrong about voltage calculations. and owe a apology


A single phase sine wave for alternating current has a wave that over time passes zero. so a true RMS (root mean square) voltage of 120 vac will have a170 volt peak then cross 0 then have a 170 volt valley each 1/60 of a second for 60HZ power.60 cycles per second or it changes direction 120 times a second.

So back to the measurement ,As it crosses zero, averaged by time ,1/60 sec gives you the "True RMS" (root mean square) which is the average voltage over time (1/60sec)for 60 HZ power.

This is what is used for the formula and is correct and accurate.

All most all multimeter's give you a RMS reading,But unless you have a meter that says "TRUE RMS" is not actually measuring the peaks & time ,only estimating so the volt reading may be a little off. You will pay more for a better meter that reads "true RMS".

EXAMPLE : a true RMS meter will read actual peaks and valleys of the sine wave and average them over time whether it is 60 HZ (USA) or 50 HZ (Europe)

Reading the "TRUE RMS" is the correct reading for his calculations.

That was actually a great example of how to calculate electrical loads.
__________________

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12-03-2006, 01:23 PM #20
SexyMF
Xtreme Member


Join Date: Nov 2005
Location: Auckland, New Zealand
Posts: 578 wdrzal:

Actually you have fallen into a classic trap. The average of any sinusoidal wave is zero. There is as much negative as positive. The RMS is referring to the equivalent heating effect - ie DC. Hence why the DC and ACrms through a resistance offer the same heating effect.

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#21
DetroitAC
Xtreme Member




Join Date: Jun 2006
Location: Detroit, Michigan
Posts: 183 Quote:
Originally Posted by SexyMF
wdrzal:

Actually you have fallen into a classic trap. The average of any sinusoidal wave is zero. There is as much negative as positive. The RMS is referring to the equivalent heating effect - ie DC. Hence why the DC and ACrms through a resistance offer the same heating effect.

Well mate, you've fallen into the classic trap of trying to show someone how they are wrong, while being wrong.

It's the ROOT of the MEAN of the SQAURED. In other words all values are first squared, hence all values are positive.

Technically you square the individual values, integrate over the time period, divide by the period, and take the square root. (From my college circuits textbook: Electric Circuit Analysis, 2nd Ed, Johnson, Johnson, Hilburn p.388

[- Feedback]

Triac drivers are better than a rheo or variac, but they still aren't that good with AC induction motors. They are still going to run inefficiently and therefore heat up more.

[Starkiller42]

Fellas, fellas, we're getting a little far afield here; a little too into the nitty-gritty and not enough practical application.

Can we please stick to the topic at hand: which fans so you use and or would recommend, and what methods of controlling them are there?

[serialk11r]

Below is from a thread 6 months ago, we were talking about measuring power iirc. note the last post from detriotAC. whats usually known as RMS ,because of post 20, detroitAc elaborated a little more, ending the false impressions some have of a sine wave.


12-03-2006, 09:53 AM #18
wdrzal
Xtreme X.I.P.


Join Date: Mar 2005
Posts: 3,876 JINU you are completely wrong about voltage calculations. and owe a apology


A single phase sine wave for alternating current has a wave that over time passes zero. so a true RMS (root mean square) voltage of 120 vac will have a170 volt peak then cross 0 then have a 170 volt valley each 1/60 of a second for 60HZ power.60 cycles per second or it changes direction 120 times a second.

So back to the measurement ,As it crosses zero, averaged by time ,1/60 sec gives you the "True RMS" (root mean square) which is the average voltage over time (1/60sec)for 60 HZ power.

This is what is used for the formula and is correct and accurate.

All most all multimeter's give you a RMS reading,But unless you have a meter that says "TRUE RMS" is not actually measuring the peaks & time ,only estimating so the volt reading may be a little off. You will pay more for a better meter that reads "true RMS".

EXAMPLE : a true RMS meter will read actual peaks and valleys of the sine wave and average them over time whether it is 60 HZ (USA) or 50 HZ (Europe)

Reading the "TRUE RMS" is the correct reading for his calculations.

That was actually a great example of how to calculate electrical loads.
__________________

.................................................. .................................................. ....


12-03-2006, 01:23 PM #20
SexyMF
Xtreme Member


Join Date: Nov 2005
Location: Auckland, New Zealand
Posts: 578 wdrzal:

Actually you have fallen into a classic trap. The average of any sinusoidal wave is zero. There is as much negative as positive. The RMS is referring to the equivalent heating effect - ie DC. Hence why the DC and ACrms through a resistance offer the same heating effect.

.................................................. .................................................. ....


#21
DetroitAC
Xtreme Member




Join Date: Jun 2006
Location: Detroit, Michigan
Posts: 183 Quote:
Originally Posted by SexyMF
wdrzal:



Well mate, you've fallen into the classic trap of trying to show someone how they are wrong, while being wrong.

It's the ROOT of the MEAN of the SQAURED. In other words all values are first squared, hence all values are positive.

Technically you square the individual values, integrate over the time period, divide by the period, and take the square root. (From my college circuits textbook: Electric Circuit Analysis, 2nd Ed, Johnson, Johnson, Hilburn p.388
Okay nice argument, but what I was asking was, IF the graph is a true sinusoidal curve, then wouldn't the RMS be the absolute value of the area under the curve (2piX/180) where X is the peak voltage (in volts duh) divided by the wavelength (180 in this case)? That simplifies out to being piX/90 which is quite far from the 0.7X that people commonly use (as in, the RMS is 0.7 times the peak voltage).

[Derek]

What I got to control the fanspeed on my AC fan was meant for ceiling fans. Basically looks like a dimmer but it's a bit bigger (the stuff behind the dial that is) and does a wonderful job. Been using for several years now with the same fan and no problems at all.

You can find them on ebay for uber cheap.

[SexyMF]

So where is this miss information? I'm not seing it, and I mean that seriously, what do you think is miss information here??

Don't worry it is not you Xeon. It is wdrzal. He will never admit he is wrong. From many posts that is apparent.


I'll rest easy knowing that I have an BE in electronics, augmented with single phase and three phase electrical qualifications.

wdrzal I think you offer nothing to this forum other than safety lectures, sarcasm and poor electrical information.

Talk to the hand. I'm not even going to bother with you anymore.

[wdrzal]

.7 x 170v=119 volt when 120v is correct,sounds like a close "rule of thumb"

I think it depends on frequency, iirc in very very high frequencies, the wave is a parabolic shape. can't remember how high the frequency is. iirc that statement is true but don't bet any money on that do the research.my memory is a little fuzzy on that.

ps; electrical and electronic engineering are different animals. qoute and refernce your facts instead of saying , I got this degree, your wrong.

[wdrzal]

sexfmf ,,,,,,DetroitAC quoted the definition and gave the reference book name title and page number.

Again from detroitAc backing me up:

It's the ROOT of the MEAN of the SQAURED. In other words all values are first squared, hence all values are positive.

Technically you square the individual values, integrate over the time period, divide by the period, and take the square root. (From my college circuits textbook: Electric Circuit Analysis, 2nd Ed, Johnson, Johnson, Hilburn p.388


ps; if I make a mistake or recall something wrong ,I always addmit it and applogize for the mistake, Benn a long time since I was in school and post from memory,everybody makes mistakes, I know I do.

[wdrzal]

how a triac works:


http://home.howstuffworks.com/dimmer-switch3.htm

[SoddemFX]

Five seconds to google to show the proof of where it (0.707) comes from:

http://www.ece.unb.ca/tervo/ee2791/vrms.htm

Principle is ~high school level (16yrs), maths is ~college level (18yrs) so i'm sure a quick wikipedia search would help to understand better if the maths is confusing to anyone.

I'll rest easy knowing that I have an BE in electronics, augmented with single phase and three phase electrical qualifications.

Thats all well and good but i don't think you should use qualifications to justify yourself, especially on a forums...

Tom

[Starkiller42]

Thank you, Derek, for remaining on topic.

As for the rest of you (sexyMF, wdrzal, and soddemFX), shame on you! I've clearly asked twice (and quite nicely, I thought) for everyone to remain in topic and actually provide usable solutions instead of getting into to math behind it all. You seem to have completely ignored my request.

[wdrzal]

If you don't uderstand the basics you will go thru your whole life asking?????? shame on you for taking the easy route,instead of studying the fuction and limits,you simpley want a part# from home depot.

before working with DEADLY electricity,you need to know all the basics and be able to measure and calculate loads,otherwise your guessing and haved learned nothing.Do you want to bet your home and family on that. You shouldn't mess without ,at the very least, learning the basics of electricity 101.

That goes for everyone

[Starkiller42]

I, for one, am quite familiar with "Things that go Zzzap" 101. I need to, as I'm am studying to become an electrical engineer. Maybe other people might not know, and if they were to have asked a question, then I wouldn't have minded at all if someone had answered.

However, as I am, for instance, fully educated on such things as RMS voltages and mosfets, and most of the arguments here seem to deal more with a desire to get in the last word, I asked that the basic mechanics of the devices be kept to a minimum, and the topic be kept on useful solutions.

You see, walt, my problem is that I already know the basics; what I don't know is the part numbers. That's why I'm asking. So next time, please don't assume that people haven't put in the study time, or that we are taking the easy way out. It's nothing short of insulting.

[Exahertz]

I like these kind of arguments. they are sometimes very informitive. and out of intrest, i have just prited and read a few articles on SIN, TAN, COS and COTAN. along with RMS, variable freq drives, triacs and a few other devices. I feel more knowlaged on AC motors now, im sorry that nobody had your part number, but if your on a track to becoming an EE, wouldnt you have known what part you needed? To be honest, "some" of this information here is pure gold. and the rest is just parts of the argument...

Math, hands down, prooves everything, beyond reasionable doubt, and those who know math, like to see math used.

[Xeon th MG Pony]

Then if you have don the basics why did you need to ask a basic question?

[wdrzal]

I like these kind of arguments. they are sometimes very informitive. and out of intrest, i have just prited and read a few articles on SIN, TAN, COS and COTAN. along with RMS, variable freq drives, triacs and a few other devices. I feel more knowlaged on AC motors now, im sorry that nobody had your part number, but if your on a track to becoming an EE, wouldnt you have known what part you needed? To be honest, "some" of this information here is pure gold. and the rest is just parts of the argument...

Math, hands down, prooves everything, beyond reasionable doubt, and those who know math, like to see math used.

Your catching on to my method of teaching, instead of giving a part number I mention the correct words so you can be in the ballpark to do your own research. while doing that research it always leads to other must know or interesting parts or principals you where not aware of. Keep in mind your not copying a unit,which a lot was done,it's easier but you learn much less. your are designing/engineering one,so you must make it safe to operate.

That means considering All the maximum operating limits and electrical codes and not exceed them. When you build something from a pile of parts, YOU :poke: are responsible for any damage or accidents caused by not takeing safety into consideration.Just as LAW ,not knowing is no excuss.

[Starkiller42]

Xeon, I'll presume that was directed at me.

It's simple, really, I've learned about all sorts of components and circuits from your basic resistor to pnp MOSFETs. I can construct basic circuits myself, and often decipher what a circuit does by looking at it.

But I have not yet reached the point in my education where we start gaining practical, real-world experience/knowledge about many things, of which I would include the "best way" to control the speed of a fan. So no, while I know the basics, and thus can understand how and or why a proposed solution might be useful, my lack of real world knowledge is what keeps me from knowing that in the first place. That is the reason that I asked. All of the reading in the world cannot truly compare with real world experience.

Another way to look at this is that my curriculum has largely been "component" based; learn about the basic components that most devices are made out of, and then later learn more about those devices. I am at that awkward point where I know a lot about the components, but have yet to learn much about the devices that they can be/ are used in. I hope that makes sense.

And Exahertz, I'm glad that you have broadened your horizons through this conversation. Maybe I just didn't get enough sleep or something, but I was really peeved when several people blatantly ignored my pleas for help and instead just continued to argue, which was increasingly less informative and more about (what I felt, at least) people's ego's.

[- Feedback]

I think I should point out that PNP and NPN are bipolar transistor types. Mosfets have P channel or N channel.

Another thing is you can't use any electrical device in larger circuits reliably unless you understand what each component does, how it does it, and what it's limitations are. There are a lot of rules of thumb to deal with too. One is that every IC should have a .1 microfarad bypass capacity between each Vcc/Vdd to ground in as short of a distace from the leads as possible. Certain applications dictate even better (http://www.williamson-labs.com/480_rlc-c.htm) bypassing effect.

There are no easy way to learn all of these things. It takes a lot of time, effort, breaks, caffeine, and hair to get this stuff down.

[Starkiller42]

omg - feedback, your right about the transistors. Lol, I guess I've been trying to suppress my transistor physics class---my professor was pretty terrible.

But you're exactly right on the learning process. That's exactly the path I've been following. And that's why now, I'm asking about the next level.