Polarized and non polarized capacitors ?

[curry87]

Couple of questions from a newbie about capacitors.


Are polarized capacitors mainly reserved for use in dc and not ac circuits as incorrect polarity causes damage in one ?


Should all ac coupling be done using non polarized capacitors ? but what do you do if theres a requirement for a cap is in the 10-100uf range etc ?

 

[audioguru]

Non-polarized electrolytic capacitors are available Moderated off Topic.
You can make your own by connecting two polarized electrolytic capacitors in series back-to-back.

 

[William I. Behen]

A polarized electrolytic will conduct DC current in the opposite direction, possibly damaging the part. Non polarized caps are often used for AC coupling, but film caps etc. are not usually available in larger values, as the size grows. Back to back e'lytics work, but remember that the net capacitance will only be half that of either part.

What all this means is that you won't have trouble if the (AC plus DC) voltage on the cap never reverses! Decoupling between stages in a linear amplifier might be do-able with a polarized cap, if the signal at the collector (Drain if FET) is biased at a higher DC voltage than the base (Gate if a FET) of the next stage. The voltage on the part would then be DC, with a superimposed AC; if you can keep this voltage always positive, you're in good shape!

P.S. Curried food is not so bad, in my experience....

 

[curry87]

When you see on a circuit diagram with a polarized cap in series with the input of a opamp this is not a good idea right ?

 

[William I. Behen]

Depends on the circuit. What circuit do you have? Where is input from, what is its DC level?

 

[William I. Behen]

Yes, if there is nothing but a polarized cap in series with an opamp input, you would probably have problems, as the node voltage would be not very well defined. As to cap value, it depends on the impedance you're working with. Remember that the RC time constant of you blocking cap translates to the cutoff frequency of the circuit, via 2*pi* freq = 1/RC. You must choose your cap so that this freq is below any frequency you are trying to pass through the circuit!

 

[curry87]

On this circuit the cap before the non invert terminal of the opamp and the cap on the output do they have to be non polarized in this particular case or does it not matter because of the low voltages involved ?

 

[audioguru]

On this circuit the cap before the non invert terminal of the opamp and the cap on the output do they have to be non polarized in this particular case or does it not matter because of the low voltages involved ?

If the mic is a dynamic type then the input coupling capacitor has 6VDC across it and it should be a polarized electrolytic type as shown.
The capacitor to ground also has 6VDC across it and it should also be a polarized electrolytic type as shown.
If the output feeds a load connected to ground then the output coupling capacitor also has 6VDC across it and should be a polarized electrolytic type as shown.

 

[KeepItSimpleStupid]

Note the (+) sign. This denotes a POLARIZED cap. Relative to ground, that + terminal would be at aproximate 6 Volts. The cap say sees somthing like 1 to +11 volts ( did not calculate the actual gain), so it never reverses polarity.

Capacitors in a loudspeaker that are part of the crossover have to be non-polarized, because they see the reversal.

 

[curry87]

If the mic on the diagram is replaced by a tiny electret one does this the polarized cap on the input still hold true ?

 

[audioguru]

If the mic on the diagram is replaced by a tiny electret one does this the polarized cap on the input still hold true ?

An electret mic will not work in that circuit because it will not be powered.

The 47k bias resistors for the opamp cause the input impedance of the preamp to be 23.5k ohms so it will not attenuate the signal from an electret mic and a 0.33uF non-polarized film coupling capacitor can be used for a low frequency cutoff of 20.6Hz.

A non-polarized capacitor is used because you don't know the voltage at the mic, it might be 4V or might be 7V (the input of the opamp is 6V) so you don't know which way the polarity of the capacitor should be.

 

[curry87]

If the output feeds a load connected to ground then the output coupling capacitor also has 6VDC across it and should be a polarized electrolytic type as shown.

A little confused surely if the output of the opamp outputs a amplified ac signal to drive the loudspeaker doesn't that mean an alternating voltage is across the cap and thus it should be non polarized or am i barking down the street ?

 

[crutschow]

An alternating voltage can be across the cap if the AC negative peak is not greater than the DC bias, in this case minus 6V. As long as the net voltage across the cap doesn't reverse bias the cap then you are okay.

 

[audioguru]

A little confused surely if the output of the opamp outputs a amplified ac signal to drive the loudspeaker doesn't that mean an alternating voltage is across the cap and thus it should be non polarized or am i barking down the street ?

Your opamp in the preamp circuit cannot drive a speaker, instead it drives a power amplifier that drives the speaker.
The alternating voltage is not across the output coupling capacitor, it is across the speaker. If the supply is +12V then the output coupling capacitor always has 6VDC across it. The voltage at both ends equally fluctuate up and down with the signal. The positive wire of the output coupling capacitor goes as low as +1V and as high as +11V and the negative end goes as low as -5V to as high as +5V.

 

[curry87]

An electret mic will not work in that circuit because it will not be powered.

The 47k bias resistors for the opamp cause the input impedance of the preamp to be 23.5k ohms so it will not attenuate the signal from an electret mic and a 0.33uF non-polarized film coupling capacitor can be used for a low frequency cutoff of 20.6Hz.

A non-polarized capacitor is used because you don't know the voltage at the mic, it might be 4V or might be 7V (the input of the opamp is 6V) so you don't know which way the polarity of the capacitor should be.

How did your calculate the input impedance of the preamp if the input frequency from the mic is unknown ?

Referring back to the schematic i posted, the 10u cap between 22k and 1k i presume helps ac gain is there a equation where you input the feedback resistor value,R1 and capacitor i mentioned to get a rough ac gain value ?


How do you calculate what capacitor value to use on the output if you were to put a 8ohm speaker across the output and you wanted to allow frequencies between 100hz to 2000hz just as an example ?

 

[audioguru]

How did your calculate the input impedance of the preamp if the input frequency from the mic is unknown?

The input resistance of a half-decent audio opamp like a Jfet-input TL071 is Giga-ohms. So the input impedance is simply the two biasing resistors in parallel. Two 47k resistors in parallel are 23.5k ohms at DC or at any frequency.

Referring back to the schematic i posted, the 10u cap between 22k and 1k i presume helps ac gain is there a equation where you input the feedback resistor value,R1 and capacitor i mentioned to get a rough ac gain value?

The 10uF capacitor is a dead short at high audio frequencies. Then the gain of the circuit is 1+ (22k/1k)= 23 times.
The 10uF capacitor has a reactance of 1k ohms at 16Hz.
The frequency that the reactance of the 10uF capacitor to ground equals the resistor in series with it is the frequency that the gain is reduced -3dB which is 0.707 times. The calculation for the frequency is 1 divided by 2 pi RC. So 10uF and 1k ohms equals a -3dB frequency of 16Hz where the gain is reduced to 23 x 0.707= 16.26 times.

How do you calculate what capacitor value to use on the output if you were to put a 8ohm speaker across the output and you wanted to allow frequencies between 100hz to 2000hz just as an example ?

If you use a POWER AMPLIFIER (that is driven by this preamplifier) to drive the 8 ohm speaker (because your preamp can drive a load that is only about 2k ohms) then the reactance of the output coupling capacitor is 8 ohms at 100Hz (no bass)= 1 divided by 2 pi x (8 x 100)= 200uF. The high audio frequencies need a lowpass filter to reduce frequencies above 2kHz (no important syllibant speech frequencies that reach 14kHz).