Continue to Site

# HELP THE NEWBIE<------

Status
Not open for further replies.

#### Super Mario

##### New Member
Hi ,how can I use a potentiometer as a voltage dividor "RHEOSTAT" ??
Where will the voltate be divided and how can I measure that by the voltameter...................

Oh well,

re

Here use this: The voltage at that point is equal to
Vs * (R2)
------------
(R1+R2)

since you want adjustable do the calculation for the lowest and highest values of R1 for the range

#### Attachments

• VD.gif
2.2 KB · Views: 731
and ...
measure voltage 1 from battery + to the center tap of the variable,
measure voltage 2 from the center tap of the variable to the battery -

Thanks,but if I put any resistor instead of the potentiometer in ur circuit it would divide the voltage too in the same way...so what's so special about the rheostat other than the adjustable value?

oops forgot that part, im just a newb here
here ya go:

#### Attachments

• Divider.gif
4.3 KB · Views: 647
Resistors are available in standard values such as 100 ohm and 220 ohm plus they have some variablity unless you buy precision. To get the voltage you want you may need some oddball resistance such as 275 or something. You may have to put a few resistors in parallel or series to come close to the needed value or, you find a potentiometer with an appropriate range and you can adjust it to get the voltage you need

and there is one more thing. resistors, in general, unless you use ceramic, support small amount of power dissipated on them, so they are not good for large currents.
if i remember well, a rheostat is usually a large resistor, wich can support currents from 1-2A up to 10-15A or even more. plus they are large and heavy.
a potentiometer is basicly a smaller rheostat.

bogdanfirst said:
and there is one more thing. resistors, in general, unless you use ceramic, support small amount of power dissipated on them, so they are not good for large currents.
if i remember well, a rheostat is usually a large resistor, wich can support currents from 1-2A up to 10-15A or even more. plus they are large and heavy.
a potentiometer is basicly a smaller rheostat.
You can get fixed resistors of any wattage. Smaller wattage types are usually metal film on ceramic or carbon composition types, and high wattage types are wire wound on a ceramic former - 10 or 20watts being typical.

The basic difference between a rheostat and a potentiometer is the rheostsat is a two-terminal device as drawn in the postings above, ie. it is a variable resistor. The potentiometer is a 3 terminal device where a potential is applied to the 2 outer terminals and a selected voltage is taken from the variable tap.

That said, potentiometers are often used as rheostats.

Note that for the the formulae [Vs*(R2)]*[(R1+R2)^-1] the pot has to be wired up as a potential divider rather than a varible resistor (ie the remaing leg must be connected to -ve V) other wise you are not dividing the voltage just controlling current.

sam_h said:
Note that for the the formulae [Vs*(R2)]*[(R1+R2)^-1] the pot has to be wired up as a potential divider rather than a varible resistor (ie the remaing leg must be connected to -ve V) other wise you are not dividing the voltage just controlling current.
No that is not true. In the above configuration also, when the value of potentiometer is changed, ratio of resistances changes and thus it changes the output voltage. The forumla is correct for this confuguration.

But surely if the pot is not being used as a potenitial divider but is being used as a variable resistor then the voltage is not being divided but instead is just being limited by the variable resistor so you could just use v=ir

No, sorry my mistake, they ARE the same formulae when you work it out. Sorry for any inconvinience, after all I am new to this.

sam_h said:
No, sorry my mistake, they ARE the same formulae when you work it out. Sorry for any inconvinience, after all I am new to this.

So a potentiometer and a rheostat r the same thing,they both have adjustable resistance and they both can divide voltage just like any nomral resistor ?

I think a potentiometer can be the same as a rheostat, so long as you use the inner pin and one of the outer pins so that it acts like a variable resistor.

"rheostat , device whose resistance to electric current depends on the position of some mechanical element or control in the device. Typically a rheostat consists of a resistance element equipped with two contacts, or terminals, by which it is attached to a circuit: a fixed contact at one end and a sliding contact that can be moved along the resistance element. Electric current enters and leaves the resistance element through the contacts. By moving the sliding contact toward or away from the fixed contact, the length of the resistance element through which the current travels can be decreased or increased. In this way the current through the circuit can be increased or decreased."

"potentiometer. Manually adjustable, variable, electrical resistor. It has a resistance element that is attached to the circuit by three contacts, or terminals. The ends of the resistance element are attached to two input voltage conductors of the circuit, and the third contact, attached to the output of the circuit, is usually a movable terminal that slides across the resistance element, effectively dividing it into two resistors. Since the position of the movable terminal determines what percentage of the input voltage will actually be applied to the circuit, the potentiometer can be used to vary the magnitude of the voltage; for this reason it is sometimes called a voltage divider. Typical uses of potentiometers are in radio volume controls and television brightness controls."

I hope this some help for you.

Pebe, i got a rheostat and it has got 3 terminals.
well actually i has got 4, the 4th is connected to the case. it is a 15A one. i is about 3Kg in waight.
i also had a 5A one, wich has resitance up to 20R or 200, i don't remember well, and you have to choose from 20 values between 0 and max with a switch. and this one indeed had 2 terminals.
hm....maybe it is a confusion related to how the term is translated in the dictionary.

I have a potentiometer with three terminals,can someone explain to me what every terminal is used for?

bogdanfirst said:
Pebe, i got a rheostat and it has got 3 terminals.
well actually i has got 4, the 4th is connected to the case. it is a 15A one. i is about 3Kg in waight.
i also had a 5A one, wich has resitance up to 20R or 200, i don't remember well, and you have to choose from 20 values between 0 and max with a switch. and this one indeed had 2 terminals.
hm....maybe it is a confusion related to how the term is translated in the dictionary.
A 5A one at 20R would need a power rating of 500W. Are you sure the devices you describe are not Variacs, which are toroidal adjustable transformers?

My understanding of a rheostat goes back a long way. I remember them used as series resistors to control the current through the field windings of large (5HP) DC electric motors to vary their speed.

Just to be sure, I've looked up the definition in dictionary.com and it says:

"A contrivance for adjusting or regulating the strength of electrical currents, operating usually by the intercalation of resistance which can be varied at will."

To me, that implies a variable resistor.

well, i use them in scool at the physichs lab. it is rated 1000W.
basicly is a wire turned around a metal tube, and insulated from it. it is about half meter long. i has a cursor that mooves around the wire to adjust the resistance.
and there is another kind wich is for lower current, i don't remember exactly how much, but it is a 50 cm wire, connected at both ends to some contacts, and cursor wich you can place somewhere on the wire.
but both types have 3 connections, 2 ends of the wire, wich forms a fixed resistor, and one connections, wich is the cursor.
but, i repeat again, i might have a problem with the dictionary......

bogdanfirst said:
well, i use them in scool at the physichs lab. it is rated 1000W.
basicly is a wire turned around a metal tube, and insulated from it. it is about half meter long. i has a cursor that mooves around the wire to adjust the resistance.
and there is another kind wich is for lower current, i don't remember exactly how much, but it is a 50 cm wire, connected at both ends to some contacts, and cursor wich you can place somewhere on the wire.
but both types have 3 connections, 2 ends of the wire, wich forms a fixed resistor, and one connections, wich is the cursor.
but, i repeat again, i might have a problem with the dictionary......
Judging from its 1000W and size of half a metre long, I would think that item was for laboratory experiments, rather than being a practical potentiometer.