Rheostat / Variable resistor

I have never understood why both ends of the track are connected in a circuit. Can anyone enlighten me?

Reply to
DerbyBorn
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in a rheostat they are not. You are thinking of a potentiometer.

Reply to
The Natural Philosopher

See, I don't understand!

Reply to
DerbyBorn

They're not always. But if say a volume control, it wouldn't go to zero if one side wasn't connected to ground.

Reply to
Dave Plowman (News)

A rheostat generally just adds (variable) resistance into a circuit and the surplus energy is absorbed via that.

So say you put one in series with a battery charger you could reduce the charge current by using the resistance in the rheostat to 'waste' (in heat) what you didn't want going to the battery (voltage dropped across the resistor), ranging from max current (no resistance) to minimum current (maximum resistance).

A potentiometer generally has both ends connected to a circuit so that you get a variable potential divider (so works more on voltage / potential than current). So typically, one end of the pot might be connected to the supply rail (say 5V) and the other end to ground (OV) and therefore the voltage seen at the wiper will range from around 0V to around 5V, as you turn the 'pot' from one end to the other.

Cheers, T i m

Reply to
T i m

Basically because that ensures that even if the wiper isnt always in contact with the track all the time, the worst you get is the same result as with the wiper at one end of the track.

Reply to
Hankat

cuit. Can anyone enlighten me?

It can be the differnce between varaible resistor and a potentiometer, but sometimes a componet will be used as a varible resistor but have all 3 'ter minals' used this can help reduce 'noise' and in some cases I've heard to reduce reflections in some circuits.

Rheostats seems to be the term applied to high power variable resistors th at most people would refer to as pots nowadays. I think the reason why they are called potentiomenters is becausxe they ca n divide the potential applied to the end terminals using the 'wiper'

Reply to
whisky-dave

The main reason is to avoid leaving the unused end of the potentiometer floating and so potentially acting like an aerial for RF noise.

It also maintains a worst case impedance of Rmax instead of open circuit if the wiper happens to have a dead spot somewhere.

Reply to
Martin Brown

They're called potentiometers because they were originally used to measure voltages. And their zigzag symbol apes their original physical construction, a long zigzag of wire on a board.

NT

Reply to
tabbypurr

More or less utter bollocks

Reply to
The Natural Philosopher

all resistors, fixed and variable use a zigzag symbol. Variable resistors simply had an arrow added.

Reply to
charles

No, that's quite right. A potentiometer was originally used to measure potential, or voltage, and would include what we now call a potentiometer with a galvanometer and a reference cell. Later, the term became applied to the resistive part alone. They were often made with resistance wire zig-zagging up and down a wooden board in order to make sufficient length.

Cheers

Reply to
Clive Arthur

ey can divide the potential applied to the end terminals using the 'wiper'

ure voltages. And their zigzag symbol apes their original physical construc tion, a long zigzag of wire on a board.

Hardly. See

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p17 fig 1.5.

The slide wire resistance was connected across a battery, and a galvanomete r connected from its tapping to the voltage under test. Move the connecting point on the zigzag slide wire until the galv nulls out, at which point yo u have a voltage match. The slide wire is marked 0-100 so you can multiply the %age by the reference battery voltage to get your voltage reading. (You 'd also use it on a reference cell before making your measurement to get it set precisely.) This is how voltage was measured before voltmeters came in to existence. It's painfully slow but capable of impressive accuracy.

A zigzag shape wire was used to get as much length as practical on a given space of board, and today's zigzag R symbol comes from that. Much later tes t sets put the wire into a spiral to save more space.

NT

Reply to
tabbypurr

The zigzag symbol is no longer the standard though is it? A boring rectangular box is used nowadays.

Reply to
Chris Green

No, the IEC standard is an oblong box these days

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Reply to
The Natural Philosopher

No, it wasnt that nor was it used that way. It is to 'meter' potential - i.e. reduce it proportionat;y.

NOTHING to do with measuring at all.

Later, the term

NO, they never ever were.

Because yo cant slide anyhing up a zigzag. They were originally wound wire on e.g. ceramic formers with sliding contacts.

zigzags are just easy to draw. But boxes are easier, and boxes have been IEC standard ever since I've been in electronics in the late 60s.

Reply to
The Natural Philosopher

That isn't why its called a potentiometer though.

That's just a standard half bridge.

Reply to
The Natural Philosopher

voltage divider

Word Origin

noun, Electricity.

  1. a resistor or series of resistors connected to a voltage source and used to provide voltages that are fractions of that of the source. Also called potential divider, *potentiometer*.
Reply to
The Natural Philosopher

More or less true, actually.

I'm sure you understand how adding the word "actually" considerably enhances the trueness of a statement.

Reply to
Paul Herber

They were never zig-zaged wire the were wound, today you can still buy wirewound resistors. What some might be getting confused with is the wheathstone bridge which was used to measure potentail or voltage, this may have used a zig zag to a more precise measurement could be taken.

Next you'll be saying that transformers use curly wire in them. Or wire that looks like a series of Us

Reply to
whisky-dave

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