# Basic question about capacitor and zener Diodes

Hello and sorry to bother you with these 2 silly questions. But they are on my mind for a while now and I cannot seems to find an answer because the answer must be so obvious no one asks this question.

If you have a 60v capacitor with an 2 micro farrad capacitance when does it reach that capacitance? When it is fully charged at 60 volt? So if you put this capacitor in a 30 volt circuit can it only reach a max capacitance of 1 micro farrad?

How can I know what that max voltage is that a zener diode can handle? I think that all the examples can find explain the break down voltage. ie the point at which the zener starts to conduct. But can put a 5V1 zener in a 30 volt cicuit? or 40v? or 100v? I have a set of 1 watt zeners from 3v3 to 24v.

Thats al.

Regards

Hans

macron0:
If you have a 60v capacitor with an 2 micro farrad capacitance when does it reach that capacitance? When it is fully charged at 60 volt? So if you put this capacitor in a 30 volt circuit can it only reach a max capacitance of 1 micro farrad?

The capacitance does not change depending on the applied voltage. The 60V is the maximum potential you can put across the capacitor terminals without shorting internally through the plates.

And BTW, there's only one 'r' in farad.

How can I know what that max voltage is that a zener diode can handle? I think that all the examples can find explain the break down voltage. ie the point at which the zener starts to conduct. But can put a 5V1 zener in a 30 volt cicuit? or 40v? or 100v? I have a set of 1 watt zeners from 3v3 to 24v.

Your question does not make sense. Zener diodes break down, that is start conducting, at their specified voltage. You use them reverse biased in with a series resistor. If you connect a Zener diode and series resistor across some voltage higher than the Zener diode's voltage then the Zener diode's breakdown voltage will appear across the Zener diode and the rest of the voltage, the difference between the Zener voltage and the supply voltage, will appear across the resistor.

PerryBebbington:
Your question does not make sense. Zener diodes break down, that is start conducting, at their specified voltage. You use them reverse biased in with a series resistor. If you connect a Zener diode and series resistor across some voltage higher than the Zener diode's voltage then the Zener diode's breakdown voltage will appear across the Zener diode and the rest of the voltage, the difference between the Zener voltage and the supply voltage, will appear across the resistor.

Hi,

Thank you for your answer. So the zener works with any voltage as long als you do not exceed its watt rating? So I could just use one to get 3.3 volt reference from 100 volt? (with the proper resistor to limit the current?) Thats rather cool.

dougp:
The capacitance does not change depending on the applied voltage. The 60V is the maximum potential you can put across the capacitor terminals without shorting internally through the plates.

And BTW, there's only one 'r' in farad.

One R. I try to remember :). Thank you for the info. So the "storing" of energie has nothing to do with the voltage. I did not know that.

Thank you for your answer. So the Zener works with any voltage as long as you do not exceed its watt rating? So I could just use one to get 3.3 volt reference from 100 volt? (with the proper resistor to limit the current?) That's rather cool.

The Zener diode sees current, which is controlled by the resistor. The Zener diode does not see the 100V or whatever it is, it just sees the current according to Ohm's Law. Once that current is flowing the Zener diode drops voltage according to its rated voltage. I think you probably need to ask some more questions, but at the moment I don't know what those questions will be.

macron0:
One R. I try to remember :). Thank you for the info. So the “storing” of energie has nothing to do with the voltage. I did not know that.

The amount of energy stored has a lot to do with the voltage. It’s the CAPACITANCE that doesn’t change.

For a fixed capacitance energy is proportional to voltage squared.

Steve

Thanks I am learning slowly.

The zener only drops it rated voltage. So the voltage left must always be dropped by the resistor? Now I think about it. I never seen a voltage rating on resistors either and that's completely normal.

I guess it is just the years old idea that if you apply to much voltage to things they explode. I have to unlearn some old habits.

So the voltage left must always be dropped by the resistor?

Correct.
You also need to learn Kirchhoff's circuit laws, which deal with how voltages and currents behave.

Now I think about it. I never seen a voltage rating on resistors either and that's completely normal.

Resistors have a maximum voltage rating, but that is their safe working voltage, if you exceed it then the resistor could well break down due to the insulation not being able to withstand the voltage. For low voltage circuits it is not important.

I guess it is just the years old idea that if you apply to much voltage to things they explode.

That's the film industry for you!

To be precise the energy stored is 0.5 * C * V^2

The capacitance is just the charge per unit voltage - charge on a capacitor is proportional to applied voltage,

Q = C*V

Which leads directly to the differential equation for a capacitor

dQ/dt = C x dV/dt

Where dQ/dt is simply current, the rate of change of charge.

So I = C x dV/dt

For instance 1µF capacitor charging up at 100V/second will have a current of 100µA

And of course if no current is flowing in the capacitor its voltage cannot be changing (but does depend on
the past history of charge/current - unlike pure resistance)

PerryBebbington:
That's the film industry for you!

But things do explode with too much voltage, like lightning amounts of voltage! Semiconductors that are
over voltaged normally fail silently unless they fail to a short, then pull huge currents and vaporize. Over-power
usually bursts/cracks the package and lets out the smoke.

A serious safety point working with high power semiconductors is to wear eye-protection as packages can
definitely explode, seen it happen. Basically any very high current power supply should be treated as dangerous
this way as even low voltage sparks with lots of power can spray incandescent molten metal in all directions.

At low voltage and low power though things are thankfully much less 'exciting'...

MarkT:
But things do explode with too much voltage, like lightning amounts of voltage!

They do indeed! But they explode a lot more violently (or should that be 'excitingly') in films than in real life!

I refer you to numerous youtube videos of arcing substations and so forth...

I refer you to numerous youtube videos of arcing substations and so forth...

You don't need to convince me that real life electrical explosions are big and dangerous, I know they are. I just think they are bigger and more dramatic when a film director gets hold of them.

My bother recently retired from working in power stations, he's told me a few stories, unfortunately not accompanied by video. I was once called out to a mobile phone site and saw the damage lightning can do, and it's not pretty.

dougp:
The capacitance does not change depending on the applied voltage.

Unfortunately not true for most ceramic capacitors. Capacitance drops drastically with increased voltage - the smaller the package (physical package; not capacitance value) the stronger that effect, making this a major issue when working with SMD components. Electrolytic caps (what OP's 2 µF probably is) don't have this problem.

PerryBebbington:
That's the film industry for you!

Very often film industry is based on reality. Just exaggerated for theatrical reasons. As as result the mythbusters usually followed the pattern of "replicate the circumstances, then replicate the result".

Though I do remember very well a capacitor exploding on me, luckily the case missed me or it'd have hurt big time. That was a paper/aluminium cap connected the wrong way around. The inside pretty much turned into dust instantly upon connecting the power... that was a seriously big bang.

Modern caps are a lot smaller in size, won't explode with that much force. On the other hand, a tantalum cap in a 5V project bursting into flames (totally unexpected - turned out the PCBA fab placed them the wrong way around!) can also be pretty shocking.

Many people specifically avoid tantalums due to the incendiary failure mode.

I reversed a tantalum cap once. It did not flame but, it smelled REALLY bad!
Modern electrolytics have built in vents or weak spots in the can so they
will not explode when they boil. I think the biggest fault with tantalum caps