RC Circuit Voltage: Why So Low?

[Cyrus]

I gota quick question for my lab report. We did an RC circuit for my electronics class. The capacitor charged to about 4.51 volts. The source was 5 volts. In theory, it should reach 5 volts as t goes to infinity. What should I write as to why it was under the source voltage by a large amount, 0.49 volts? Is it just because I did not wait long enough, or is there something to the construction of the capacitor causing it?

Thanks,

 

[gulsen]

How did you measure it to be 4.51? Was it connected to the source when you made the measurement?

 

[nrqed]

I gota quick question for my lab report. We did an RC circuit for my electronics class. The capacitor charged to about 4.51 volts. The source was 5 volts. In theory, it should reach 5 volts as t goes to infinity. What should I write as to why it was under the source voltage by a large amount, 0.49 volts? Is it just because I did not wait long enough, or is there something to the construction of the capacitor causing it?

Thanks,

It depends. How do you know the source was 5 volts? Is it a battery or a variable voltage source that you adjusted to 5 volts? If it is a battery, did you measure it with a voltmeter or did you just read the value indicated on the battery? Often a battery will have a voltage quite below the value indicated (a 10% difference is not rare). It could be because the battery is drained out a bit. Even if you wait for an infinite amount of time (or, in practical terms, for many many time constants), the voltage across the capacitor will never reach the value indicated on the battery.

 

[Cyrus]

It was a power supply at 5.01V, all the time. Not a battery. But I am saying that it should be close, and half a volt off is not all that close.

 

[Bystander]

Ceramic? Mica? Electrolytic? Film? Give you any ideas?

 

[Cyrus]

I have no clue. It was just blue. I am just going to write the time was not long enough. It was 17 time constants out however, its probably something to do with the capacitor. Its due tomorrow, so that's what I am putting down.

This lab was a major disaster last monday. Nothing went right the first time.

 

[Bystander]

Electrolytics are "leaky," even when operated at correct polarity. Getting a long enough time constant in an RC circuit for labs demonstrating time constants with just reading a voltmeter requires a large value of capacitance, usually an electrolytic. There should have been something in the lab procedure about the various types of capacitor construction. If not, it's not a real surprise this day and age.

 

[Cyrus]

Nope. Just use a 10uF capacitor and a 1M-OHM resistor. They mentioned laminated capacitors in chapter 3? but it was a joke really. This is a class for non-EE's, so don't expect too much.

 

[berkeman]

Something else was going on. When you see strange things like that which don't seem right at the time in the lab, start poking around to try to figure out what is going on. The leakage current idea is not bad -- calculate how much leakage current it would take to give you the 0.5V drop across the 1MOhm resistor, and then compare that to typical leakage currents for 10uF electrolytic caps at room temperature...

BTW, this leakage current of electrolytic caps is one reason that you don't use them in long-life, battery-powered applications...

 

[Cyrus]

I literally had, no time to fiddle around with anything. It was get it done to almost finish on time. That's a good idea; however, I don't know the typcial leakage currents for 10uF capacitors at room temperature. I might do it when I have some time, but right now it's really not that high up on my priority list. :shy: It's nice to know why though.

 

[Astronuc]

It was a power supply at 5.01V, all the time. Not a battery. But I am saying that it should be close, and half a volt off is not all that close.

Besides a leaky capacitor,

how does one know that the 5 V power supply was actually 5 V? Is it digital or mechanical, and has it been calibrated recently? In nuclear and aerospace industries, every instrument I've seen is on a calibration cycle. Key instruments may be checked for calibration every three months, and if there are inherent drifts, even more frequently based on the history of the instrument.

It looks like the time constant was long enough to get close to 5 V - 5RC should get one about 99% of max charge. Maybe recheck time constant.

 

[Cyrus]

I used a DMM to read the power supply source voltage, it was ~5.01Volts DC.

It's a wall unit voltage supply, I know 100% that it was 5 volts steady.

The time constant was 10s, and it took 170seconds, or 17 time constants. That's bad, as it should take ~10 time constant's to get to steady state.