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.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.
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.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.
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 ?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.
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?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.
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.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......