Rheo Vs. Pot
TattooGeek
Potentiometer. Are they interchangeable? I am going to be using it
(them?) to regulate fan voltage for a PC. TIA - TatttooGeek
Tom Biasi
The terms that you are using describe uses for a variable resistor not
devices within themselves.
Do you wish to control the potential difference (Voltage) or the current ?
Did you try doing a search for both of those words ?
Regards,
Tom
"TattooGeek" <te...@destroybabylon.com> wrote in message
news:dee97878.02040...@posting.google.com...
Peter Bennett
wrote:
>I would like to know the difference between a Rheostat and
>Potentiometer. Are they interchangeable? I am going to be using it
>(them?) to regulate fan voltage for a PC. TIA - TatttooGeek
A potentiometer has three terminals - both ends of the resistor
element, and the sliding contact.
A rheostat only has two terminals - only one end of the resistance
element is accessible. A pot can be used as a rheostat by ignoring
one end terminal. (I also think of rheostats as higher power devices)
--
Peter Bennett VE7CEI
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html
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John Popelish
A rheostat is a variable resistor (two terminals). A potentiometer is
a voltage divider with a sliding tap (3 terminals). If your rheostat
has 3 terminals, it is really a potentiometer. If you need only two
terminals, either will work, assuming the wattage and current limits
are not violated.
--
John Popelish
cpemma
news:dee97878.02040...@posting.google.com...
> Potentiometer. Are they interchangeable? I am going to be using it
> (them?) to regulate fan voltage for a PC. TIA - TatttooGeek
The definitions given are accurate, but it's become very common for any
wire-wound pot of 3W or more to be called a rheostat, (or even a
loudspeaker volume control as Maplin in the UK class them ;).
For your needs a low resistance (about 50R depending on the fan) and
reasonable wattage (3-5W typically) is the key, more info on sizing and
wiring at Rheostats: http://www.cpemma.fsnet.co.uk/rheo.html
But do look at solid-state control, so much more second millennium ;)
N. Thornton
> > Potentiometer. Are they interchangeable? I am going to be using it
> > (them?) to regulate fan voltage for a PC. TIA - TatttooGeek
> For your needs a low resistance (about 50R depending on the fan) and
> reasonable wattage (3-5W typically) is the key, more info on sizing and
> wiring at Rheostats: http://www.cpemma.fsnet.co.uk/rheo.html
>
> But do look at solid-state control, so much more second millennium ;)
Hehe, yup, tho a rheostat will do it, very simply too. More efficient
would be just to switch the fan between 5 and 12v, and perhaps 3.3v
too, to give you 2 or 3 speeds.
I know some people have also used 7v (12v to 5v rails), but thats not
really recommended, can occasionally cause problems.
Regards, NT
cpemma
news:a7076635.02040...@posting.google.com...
> > > I would like to know the difference between a Rheostat and
> > > Potentiometer. Are they interchangeable? I am going to be using it
> > > (them?) to regulate fan voltage for a PC. TIA - TatttooGeek
>
> > For your needs a low resistance (about 50R depending on the fan) and
> > reasonable wattage (3-5W typically) is the key, more info on sizing
and
> > wiring at Rheostats: http://www.cpemma.fsnet.co.uk/rheo.html
> >
> > But do look at solid-state control, so much more second millennium ;)
>
> Hehe, yup, tho a rheostat will do it, very simply too. More efficient
> would be just to switch the fan between 5 and 12v, and perhaps 3.3v
> too, to give you 2 or 3 speeds.
Some fans won't start on 5v so thats a bit dodgy. And 3.3V is even
dodgier, apart from not being on the 4-pin molex connectors.
Besides, going down to 5v is too big a drop IMHO. 7-10v gives a major
noise reduction without totally crippling air-flow and pressure.
> I know some people have also used 7v (12v to 5v rails), but thats not
> really recommended, can occasionally cause problems.
The only *real-world* danger with the 7-volt trick is that fan speed
sensors won't work (the hardware monitor chip wants a 0v "low" level, not
+5v) and a +5v signal could damage the mobo chip. If you've no speed
signal connected, no worries.
N. Thornton
> > Hehe, yup, tho a rheostat will do it, very simply too. More efficient
> > would be just to switch the fan between 5 and 12v, and perhaps 3.3v
> > too, to give you 2 or 3 speeds.
>
> Some fans won't start on 5v so thats a bit dodgy. And 3.3V is even
> dodgier, apart from not being on the 4-pin molex connectors.
>
> Besides, going down to 5v is too big a drop IMHO. 7-10v gives a major
> noise reduction without totally crippling air-flow and pressure.
That's quite true. This puter is using a 12v fan running on 5v, it
runs at around 1000 rpm, and its fine, nothing gets hot. And its
overclocked too. The sound of silence :) But I accept not all
machines would be happy with that.
> > I know some people have also used 7v (12v to 5v rails), but thats not
> > really recommended, can occasionally cause problems.
>
> The only *real-world* danger with the 7-volt trick is that fan speed
> sensors won't work (the hardware monitor chip wants a 0v "low" level, not
> +5v) and a +5v signal could damage the mobo chip. If you've no speed
> signal connected, no worries.
I thought the problem was that in some situations the computer would
not draw enough 5v load to keep the PSU going, and either the 5v rail
could rise, or the PSU shut down. There is also the possibility of
introducing significant rubbish onto the 5v rail when the PSU 5v
supply is hardly loaded. Motor current is not clean after all. Maybe
you know more about that.
The fan sensor question, I guess one sacrifices that, if you've got a
fan sensor to start with. Depends on the value of your machine I
guess.
Regards, NT
default
usually a coil of wire with a moving contact, when you move the contact you
change the effective length length of the coil between the terminals of the
device. In this way you vary the resistance of the rheostat.
It looks something like this:
A ----/\/\/\/\/\/\/\/\/\/
/|\
| The terminals are A and
B
|
B
A potentiiometer is basically a "variable voltage divider", in case you
did'nt know a voltage dividor is a cicuit the consists of two or more
resistors in series.
When you have on resistor connected across a power supply of say 12V, then
you will have about 12V across that resistor regardless of its resisitance
the resistance will determine the current ). This is because Kirchoff's
voltage law (KVL) says than the sum of voltage drops around a cicuit must
equal the supply EMF, simple conservation of energy in action. Where else
can the other voltage be across ( yes it can be across the wires connecting
the resistor to the power supply, but the resistance of these wires is
assumed to be neglible, even so it is very small, that is why I say
approximately 12V, actually 11.9V the other 0.1V is across the wires, you
see shortly) However if you have say two resistors in series, then the
voltage will divide between the two resistors such that each gets just
enough of the total voltage to pass the total currrent according to Ohms law
( R=V*I). The sum of the voltages across the two resitors must equal the
supply voltage. This is know as a voltage dividor, and the voltages will
divide, according to the ratio of the resistances. This simply means that if
the two resistors are equal then the voltage will divide equally between the
resistors. So it you had two 1Kiloohm resistors connected in series with a
12V supply, you will find exactly 6V across each resistor. If you made the
one resistor bigger, and or the other smaller the voltages will adjust
accordingly with the bigger of the two resistors having the bigger voltage
and the smaller of the two having the smaller voltage, however the sum of
the two voltages will always equal the supply voltage. A potentiometer has
three terminals. Inside there is a coil or track of wire having a fixed
resistance ( this will be the value marked on the side of the pot) There is
a third leg which connects to a wiper that slides across this track just
like the wiper in the rheostat. Here is a diagram:
A -----/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/----- B
/|\
|
|
C
When you move the track to the left the resistance between A and C decreases
and the resistances between C and B increases. and vice versa when you move
it in the other direction. This is in essence a variable voltage dividor,
when you apply a voltage of say 12V between A and B, you will obtain a
varable voltage out between C and B, if you choose B to be the common or
reference ( usually -ve).
Perhaps this visual way of looking at it helps explain things.
11V 8V 6V 4V 1V
2V ----------/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/----
-- 0V(GND)
/|\
| Vout = approx 7V at
this point
|
+7V
In reality the decrease in voltage will not be so "linear", but will most
probably change suddenly at certain places, meaning you might have to have a
very fine hand when adjusting. Another thing is that you can use any value
of pot because this effect depends on the ratio of resistances, the bigger
pots will draw less "bleeder current".
You could use a rheostat, but then it would have to be in series with your
fan and then the voltage across your fan will change if the resistance of
your changes such as you using a different type of fan with a different
internal resistance. You could use a pot, I'm sure that by now you have
figured out how to. The voltage will be always be more or less what you set
it to be.
An even better alternative is to use a voltage regulator, because when you
place a load on the output of a pot this causes the current two divide
between the load and half of the pot. A voltage regualtor contains electronc
circuitry to overcome this "loading effect"
Well I hope that answers your questions, Have fun
Peter
.
TattooGeek <te...@destroybabylon.com> wrote in message
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cpemma
news:<a8qgao$9av$1...@newsg4.svr.pol.co.uk>...
>
not
> > > really recommended, can occasionally cause problems.
> >
> > The only *real-world* danger with the 7-volt trick is that fan speed
> > sensors won't work (the hardware monitor chip wants a 0v "low" level,
not
> > +5v) and a +5v signal could damage the mobo chip. If you've no speed
> > signal connected, no worries.
>
> I thought the problem was that in some situations the computer would
> not draw enough 5v load to keep the PSU going, and either the 5v rail
> could rise, or the PSU shut down.
That's what I meant by "real-world" - unless your total fan current is
*extremely* high, the minimum 5v load will always swamp their effect.
Especially with an AMD cpu ;)