ВЭФ 12 (Astrad VEF 12 Project) - including a transistor tutorial

[pwdrive]

Maximum current pertaining to what?

Lawrence.

[Nowhere-Man]

My replacement capacitor I think has a 100 volt margin but nothing like that volltage floats around the Astrad. Even so, low voltage circuits can have higher currents. It was juat a passing thought.

[Terry]

At DC, which is its normal operating environment, no current can pass.

Current will only flow if there is AC present across the capacitor (with or without the presence of a DC voltage).

Capacitive reactance (think of this as AC resistance if you like) decreases with increasing frequency so current through the capacitor increases with frequency. This is why you often find capacitors such as yours in parallel with electrolytic decoupling capacitors which perform well at low frequencies but are not so good as the frequency rises. This ensures that the decoupling action is maintained at both low and high frequencies.

Under normal conditions, the AC voltage across the capacitors will be zero or very close to it - that is, after all, why these components are fitted in the first place - so the current will be equally low. That is very much a simplification - electrolytic capacitors, for example, can overheat and even explode if the internal series resistance rises too high, which is why you find that main smoothing capacitors, for example, will have maximum ripple currents specified in the data but it won't  be a problem with a 33n foil capacitor!

[Nowhere-Man]

I haven't tried it but my guess is a 0.01 microfarad across a marine battery would probably melt. Even though it's only 12 volts.

[Pat Pending]

I haven't tried it but my guess is a 0.01 microfarad across a marine battery would probably melt. Even though it's only 12 volts.

Why would it do that?  

[Terry]

I haven't tried it but my guess is a 0.01 microfarad across a marine battery would probably melt. Even though it's only 12 volts.

Oh! Please try it! Please do!

Then you could explain to us why it didn't melt ....

[Crackle]

I haven't tried it but my guess is a 0.01 microfarad across a marine battery would probably melt. Even though it's only 12 volts.

Give it a try and see what happens, it is the best way to learn.

Mike

[Nowhere-Man]

The theory is electrons would exit the battery cathode and enter the first foil layer. The dielectric would prevent any actual passage of electrons but would repel the ones in the other foil. At that point the charge should peak. Even so, bear in mind up to 300 amps can flow. You can also get 230 VAC from a 120 amp hour battery.
In the very old days they used to use an grid bias battery like this and put a capacitor and neon lamp in circuit. The flickers could be used to measure capacity.
However, this is straying off topic.

[Mark Hennessy]

Any notion of staying on-topic was lost ages ago. Just go for it!

[Crackle]

The theory is electrons would exit the battery cathode and enter the first foil layer. The dielectric would prevent any actual passage of electrons but would repel the ones in the other foil. At that point the charge should peak.

Yes, your knowledge of theory is good, but your understanding of it is not so good at all.
I feel your statements need some clarification in case a child or inexperienced person reads your post and gets the wrong idea.

You said "Even so, bear in mind up to 300 amps can flow."
Considerably more than 300 amps can be supplied by a large lead acid starter battery, but you do need a resistive load (very low resistance) to get these sorts of currents.
In the case of the capacitor example you mentioned this with , NO you will not get large currents flowing.

You said "You can also get 230 VAC from a 120 amp hour battery."
This is also a misleading statement, it needs qualification, yes a 12 volt battery can be used to supply an apparatus to generate very high voltages, e.g. ignition coil in a petrol engine, an electronic or rotary inverter to generate 230 volts AC.
But as far as a 12v battery giving 230 volts on its own, NO.

In the very old days they used to use an grid bias battery like this and put a capacitor and neon lamp in circuit. The flickers could be used to measure capacity.

Not heard of this, maybe true, but I thought a neon needed a higher than 12 volt charge to get it to strike.
Most grid bias batteries were made from "dry cells" similar to our current zinc carbon types still in use today.

Mike