Capacitive reactance affecting voltage divider output

[Harald Kapp]

So please could anyone give me a few suggestions on how i could overcome this.

Again: Please read the article linked in post #3.

 

[Rajinder]

Depending on your goal, and why you placed the capacitor in series with the voltage divider, if your desire is to minimize the capacitive reactance effect of the capacitor on the resistive voltage divider, then you will require as large a capacitance as possible. Allowing |X_c| = 1 / (2 π f C) to approach zero by arbitrarily increasing C without bound would accomplish that goal, but you must also consider the effect of whatever source impedance is driving the series combination of the capacitor and the two resistors, as well as whatever load impedance is in parallel with the lower divider resistor. All this will involve the arithmetic of complex numbers, which clearly you do not seem to understand yet because of your incorrect statement:


Reactance is not the same as resistance, and you cannot simply add 20 kΩ capacitive reactance to 10 kΩ ohmic resistance to obtain 30 kΩ to use in the DC voltage divider equation. The AC voltage divider equation is V_out = V_in [ Z_lower / ( Z_lower + Z_upper ) ]. Impedances are complex numbers and their arithmetic is different than the arithmetic used with real numbers.

A capacitor in series with a resistor has an impedance, |Z_effective| = [X_c² + R²]^1/2. The voltage divider output is NOT V_out = V_in [ R_lower / (R_lower + R_upper + X_c) ]. Instead V_out = V_in [ R_lower / ( R_lower + Z_effective ) ]. The addition of R_lower and Z_effective in the denominator is a complex number addition, and the fraction R_lower / (R_upper + Z_effective) is a complex number.

I will ask again one final time: WTF are you trying to DO?

Hi
Thanks for this.
Basically my friend has a piezo buzzer with three wires. When the piezo has been driven (he monitors the output) via the voltage divider into a PIC. Trying to differentiate if any wires may have broken off, or not connected.
He says that he gets a different waveform when the piezo is connected to when the wire (s) have been removed.
So he is trying to check if the piezo is connected or not or if any wire has broken offf.

 

[Audioguru]

A piezo transducer is a speaker or microphone. It is fed AC or produces AC. It has two wires.

A piezo beeper has a built-in transistor oscillator. It is fed DC. Its piezo crystal has three connections: output, ground and feedback.

We do not understand what you say your friend is doing. If he is simply checking if the piezo is connected or not or has a wire broken off then why doesn't he look at it to see.

 

[Rajinder]

A piezo transducer is a speaker or microphone. It is fed AC or produces AC. It has two wires.

A piezo beeper has a built-in transistor oscillator. It is fed DC. Its piezo crystal has three connections: output, ground and feedback.

We do not understand what you say your friend is doing. If he is simply checking if the piezo is connected or not or has a wire broken off then why doesn't he look at it to see.

Hi
Thanks for the message.
The unit is enclosed, so he cant look at it. He must periodically test it (once a week) to ensure it is connected.

 

[(*steve*)]

Let me guess.

The device is a piezoelectric alarm module (like a sonalert).

It has three wires. Two are connected to power and the third is a logic input used to turn the alarm on and off.

I love these guessing games.