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20th Aug 2002, 10:56
OK People

In a carbon pile regulator, as the strength of the voltage coil reduces the resistance in the pile

increases or decreases

From what I remeber the resistance increases if the pressure reduces.

Would the question infer that the pressure has reduced, or does the voltage coil release pressure if its voltage reduces therefor increasing voltage?

Help now very confused, and no notes near by

Canada Goose
20th Aug 2002, 13:54
Not having studied JAR ATPL Instruments, but knowing a little bit of electronics to be dangerous, I'd put my money on the resistance decreasing as the voltage (pressure) decreases !!

........ I guess, I'm gonna be proved wrong now !! :o


20th Aug 2002, 14:04
What I dont know is if the voltage coil is connected to the carbon pile via a leaver type arm arangment

If it is then as its voltage decreases it will release the pressure on the carbon balls therefore increasing the resistance.

20th Aug 2002, 14:19
Did Instruments a little while ago so apologies if the exact details are incorrect but the basic principle is something along these lines.

Voltage regulation is acheived by balancing spring pressures against an electromagnet. The Carbon pile is pulled together by a set of springs. The constrictive force of the springs is offset by electromagnets which are powered directly by the voltage from the circuit and situated above and below the stack.

As circuit voltage increases the magnets pull with increased force separating the carbon piles and increasing resistance. Eventually a equilibrium is reached between the pull of the magnets (apart) and the springs (together).

As voltage drops off the magnets pulls with less force, causing the springs to pull the stack back together and decreasing resistance. This continues until a new equilibrium is established.

To answer the question resistance increases as voltage does.

Hope this is helpful.

20th Aug 2002, 14:31
Thanks for that, I forgot about the spring in the system.

20th Aug 2002, 15:28
Best way to think of it and also a way that gets your mind going for the rest of the systems style questions is that the circuit which the carbon pile regulator is a part of is a sensing circuit. It runs off in parallel from the main circuit giving the carbon pile regulator a "sample" of the circuit voltage. Therefore naturally, if the circuit voltage is low, the resistance thru the carbon pile needs to be decreased to allow the circuit voltage to return to the level required. Voltage down - resistance decreases (carbon discs pulled closer together) and vice versa.

You say this is for instruments but I don't think that it comes into the instruments syllabus. If you mean the systems exam then I seem to remember from my days at Oxford that it is in the syllabus but has never had a question that goes into any depth.

However, it doesn't hurt to understand a concept as it is relevant for all aspects of voltage regulators. I'd concentrate more on the solid state stuff like diodes.

If you need any more help email me.


malaysian eaglet
22nd Aug 2002, 12:57
First it is a very classical question in DC ELECTRICS. Voltage regulator of a DC generator.
The fundamental of the system is in the state of the surface of a washer of carbon: made of compressed powder, quite soft, irregular, contact depending upon the crushing of the grains. With no current, a spring maintains the stack of carbon washers compressed, as a consequence, the contact between the washers is quite good and the total resistance of the stack is low.
When the current is established, an electromagnet (a coil) is working against the spring and tends to extends the stack, so doing, the contact of the carbon washers become worse, and the resistance of the carbon pile increases.
Best regards
Malaysian eaglet
Malaysian Flying Academy