Dear All Hello,

I constructed a simple common emitter amplifier circuit, for educational reasons, and since the coupling and decoupling capacitors were at the μfarad range and I had only electrolytics, I used them. The strange thing is that it worked (the circuit) when the coupling one was connected with the positive lead towards the signal source and the decoupling when connected with the positive lead towards the collector.

Can I use them (electrolytic) or after a while they will be destroyed? The circuit was active for 20 minutes only

Many thanks for your support

 

They will die after a while if a reverse voltage is applied. How long it takes depends on how big the reverse voltage is compared to the capacitor voltage rating.

 

Many thanks for your reply,

my signal was a sinusoidal 2 V ptp. The capacitor was a 50V one. That's why it worked for 20 minutes. So we must use capacitors in the μfarad
range.

Why also, the way the leads were connected, affected the circuit?

Many thanks again

 

If the base voltage of your transistor is lower than 2V then the capacitor will be reverse biased for part of the input cycle even with the positive terminal connected to the base.

 

Use two electrolytics connected neg to neg to create a nonpolarized cap. Two 100 uf, connected this way will give you a 50 uf nonpolarized cap.

 

The general rule is - make sure the DC bias on the capacitor matches the polarity marking on the body of the capacitor.

To check, remove any AC input signal to the circuit and measure the DC voltage across the capacitor using a DC voltmeter and align the capacitor accordingly.

 

Thank you all,
But let me clear some things out. I am still at the beginning of my self training in electronics, and AAC is very useful.

Either how I will connect the electrolytic cap, in a common emitter amplifier, the cap will be destroyed after a while.

In a DC circuit, if I connect an electrolytic cap the wrong way, it will not work? Or it will, and then destroyed after a while? I will experiment with that during the weekend.

What kermit2 mentioned can be applied in any application? We can create non polarized caps, by using two polarized? I never thought of it

Many thanks for your support

 

You can back-to-back polarized electrolytics with diodes across both caps to keep the reverse voltage under 0.6 V.
This works for some applications, but mind the non-linearities, it's probably a terrible idea for anything audio.

I think without the diodes you will still see early death from exposure to reverse polarity.

 

Where you need to use electrolytics then you must arrange the circuit so that the voltage is always the same polarity.

In a DC circuit, if I connect an electrolytic cap the wrong way, it will not work? Or it will, and then destroyed after a while? I will experiment with that during the weekend.

TAKE GREAT CARE doing this. If the current is not very restricted they can explode violently.

 

Many thanks for the input.

Just to summarize: we shouldn’t use polarized capacitors, when the voltage moves from negative to positive. Only for a very short period of time just to check if our circuit is working.


many thanks again

 

A schematic would help here.

Assuming you have a traditional CE amp with 2 input bias resistors where the junction is the input, as long as the positive peak of the AC is equal or less that the divider voltage the + of the cap should go to the resistors and not the source. This is because the junction has a DC level, the source can vary but this way the cap always see a positive voltage.

If the AC can be greater you should not use a polarized cap.

I can only guess where the other cap is going so I have no comment there.

 

I will post it tomorrow. Mostly I am interested when and how I can use a polarized cap due to the higher capacitance you can easily find.

many thanks again

 

Here is a typical common emitter amplifier circuit.

Note the polarity of C3. The positive end of C3 is connected to the emitter.

The positive end of C2 is connected to the collector since the collector is more likely to be more positive than the next stage to which it is connected.

The polarity of C1 is questionable. If the input signal is a low level signal, then the orientation is as shown with the positive lead on the base.
If the input is from a similar BJT common emitter amplifier, then it is likely that the input is from the output of a similar collector signal. Hence you would want to turn C1 around.

Again, measure the voltage polarity across the capacitor in-circuit and wire accordingly.

If the capacitor is rated for 50V or 63V, I wouldn't lose sleep over it.

BTW, I would use aluminum capacitors and not tantalum capacitors for this situation.

 

That's the schematic. The capacitors are 50V rated

thank's for your reply