Potensiomedrau
Potentiometers
Dyfais dair terfynell yw potensiomedr. Mae’r terfynellau allanol wedi’u cysylltu â’i gilydd â thrac hanner cylch wedi’i wneud o garbon neu wifren gwrthiant. Mae’r derfynell ganol wedi’i chysylltu â chyswllt llithro a all gael ei gylchdroi dros y trac.
Mae’r lluniau’n dangos detholiad o botensiomedrau y gellir eu cael yn rhwydd oddi wrth gyflenwyr. Math (c), sydd wedi’i osod ar banel, yw’r math mwyaf cyffredin. Gellir addasu ei osodiad drwy gylchdroi’r werthyd. Caiff y mathau eraill eu gosod ar fyrddau cylched fel arfer a chânt eu haddasu drwy ddefnyddio sgriwdreifer bach. Ym math (b), caiff y llithrydd ei yrru ar hyd trac syth gan edau sgriw. Math rhagosod yw math (a), ac fel gyda math (d) mae’r llithrydd yn cylchdroi oddeutu 270o. Gellir gosod gwrthiant y rhain yn dra chywir.
Defnyddir potensiomedrau ar gyfer dau gymhwysiad mewn cylchedau electronig:
A. Cyflenwad foltedd newidiol Yn yr achos hwn, cysylltir y foltedd cyflenwad sefydlog ar draws terfynellau allanol y potensiomedr. Cymerir y foltedd allbwn newidiol o gyswllt y llithrydd ac un o’r terfynellau pen.
Pan fydd y llithrydd yn safle 1, mae foltedd llawn y cyflenwad ar gael yn yr allbwn. Mae’r foltedd allbwn yn gostwng wrth i’r llithrydd gael ei symud tuag at safle 2. Yn safle 2 bydd y foltedd allbwn yn sero.
B. Fel Gwrthydd Newidiol Gellir gosod potensiomedrau i weithio fel gwrthyddion newidiol i reoli llif cerrynt mewn cylched. Yn yr achos hwn, cysylltir terfynell y llithrydd ag un o’r terfynellau pen trac, a gosod yr uned fel y dangosir isod:
Pan fo’r llithrydd yn safle 1 caiff trac y gwrthydd ei ‘siortio’ a bydd cerrynt y cylched ar ei werth uchaf. Wrth i’r llithrydd gael ei symud tuag at safle 2, caiff mwy a mwy o wrthiant ei gynnwys yn y cylched.
Gellir defnyddio’r gwrthydd newidiol i reoli disgleirdeb bylb.
Bydd y bylb ar ei fwyaf disglair pan fydd y llithrydd yn safle 2.
Potentiometer are three terminal devices. The outside terminals are linked together by a semi-circular track made from carbon or resistance wire. The centre terminal is connected to a sliding contact which can be rotated over the track.
The pictures show a selection of potentiometers that are readily available from suppliers. The most commonly used is type (c) which is mounted on a panel. Its setting can be adjusted by rotating the spindle. The other types are usually mounted on circuit boards and are adjusted using a small screwdriver. In type (b), the slider is driven along a straight track by a screw thread.Type (a) called a preset and as with type (d) the slider rotates approximately 270o. Their resistance can be set very accurately.
Potentiometers are used for two applications in electronic circuits:
A. Variable voltage supply In this case, the fixed supply voltage is connected across the outer terminals of the potentiometer. The variable output voltage is taken from the slider contact and one of the end terminals.
When the slider is at position 1, the full supply voltage is available at the output. The output voltage decreases as the slider is moved towards position 2. At position 2 the output voltage will be zero.
B. As a Variable Resistor Potentiometers can be set up to act as variable resistors to control current flow in a circuit. In this case, the slider terminal is connected to one of the end of track terminals and the unit set up as shown below:
When the slider is at position 1 the resistance track is shorted out and circuit current will be at its maximum value. As the slider is moved towards position 2, more and more resistance is included in the circuit.
The variable resistor can be used to control the brightness of a bulb.
The bulb will be at its brightest when the slider is in position 2.