Simple Capacitor and Resistor Circuit

Discussion in 'Homework Help' started by exidez, Sep 12, 2008.

1. exidez Thread Starter Member

Aug 22, 2008
26
0
I did a lab last week to investigate the affect a capacitor has on a circuit.
The circuit was simple. 15V(pk-pk) supply in series with a 1kΩ resistor and a 0.18 μF Capacitor. The frequency was at 500 and 2000 Hz. I want to calculate the RMS voltage across the circuit & and then just the capacitor

I calculate the total impedance to be
500Hz:
Z = 1000 - j1768.39 => 2031.55 Ω < -60.57°

I(RMS) = (7.5/sqrt(2) < 0°) / 2031.55 Ω < -60.57°
=2.61mA < 60.57°

then Ohms law V= IR
2.61mA * 1000Ω = 2.61 V (RMS)

2000Hz
Z = 1000 - j442.10 => 1093.37 Ω < -23.85°

I(RMS) = (7.5/sqrt(2) < 0°) / 1093.37 Ω < -23.85°
=4.85mA < 23.85°

Ohms law V= IR
4.85mA * 1000Ω = 4.85 V (RMS)

These calculated voltages are different to what the DSO gave me when i tested it
500Hz
Circuit
pk-pk: 14.8V
freq: 500 Hz
Period: 2000ms
RMS: 5.05V

Capacitor
pk-pk: 12.8V
freq: 500 Hz
Period: 2000ms
RMS: 4.39V

2000Hz
Circuit
pk-pk: 14.4V
freq: 2000 Hz
Period: 500μs
RMS: 4.9V

Capacitor
pk-pk: 5.68V
freq: 2000 Hz
Period: 500ms
RMS: 1.98V

Have i done something wrong????
I have simulated this circuit on the computer and the values seem to be right, so am i calculating this wrong???

2. hgmjr Retired Moderator

Jan 28, 2005
9,029
219
Most likely what you are seeing is the fact that the real components you used have a tolerance associated with them.

hgmjr

3. exidez Thread Starter Member

Aug 22, 2008
26
0
that would explain the the difference in small voltages but when i calculated the RMS Voltage at 2kHz my RMS Voltage was higher than the 500Hz
Where the DSO gave me a lower RMS voltage for the 2kHz and a higher voltage for the 500Hz
im sure tolerance is not to blame for that.... ?

4. exidez Thread Starter Member

Aug 22, 2008
26
0
dounf out what i was doing wrong. With ohms law i was multiplying by the resistance and not the reactance! duh

Anyway, here is my next questions. Why is is that when i was inputting 5.3V RMS, the circuit only displayed 5.05V RMS for 500Hz and 4.9V for 2000Hz ?
i never changed the voltage at all. There was this .25V RMS drop somewhere from thevery beginning and i dont know where it went. Is this normal?

5. Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
Do you mean that the open circuit generator voltage was 5.3V, and it dropped when you loaded it? Signal generators have non-zero output resistance, often 50 ohms. Check the specs on your generator.

6. exidez Thread Starter Member

Aug 22, 2008
26
0
thats probably it. Thanks