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

[pwdrive]

I think NM might be referring to the maximum current that flows when a voltage is initially applied to the capacitor, ESR, source resistance, C*R etc etc.

Lawrence.

[Terry]

The charge current that flows into a capacitor follows an exponential law so, even assuming that the 300A figure is achievable, it would only flow for, say, a nanosecond or so before rapidly tailing off. So heating effect: virtually zero.

You will be able to prove that when you conduct your experiment by connecting a low value fuse in series with the capacitor to see whether or not it blows ...

[Mark Hennessy]

A similar question was asked on the EEVBlog forum: http://www.eevblog.com/forum/beginners/s...-12-volts/

I haven't read the whole thread by any means, but I'd suggest that NM takes a look.

NM, for some of the questions about basic electronics, it might be a good idea to try them on that forum as well as here. Simply because that forum doesn't deal with vintage radio, so that'll help you stay focussed on the particular question without drifting back towards the radios. It'll also be good to hear a wide range of views from practical electronics engineers, so he doesn't have to just take our word about stuff. The more time you spend with engineers, the more you'll start to think like them - that will really help, I promise. As Mike says - and I've said similar things myself - there's a world of difference between knowledge and understanding. The books only get you so far - even the good ones.

Be aware that that forum - like most others - has quite a different "dynamic" compared to this one.

[Nowhere-Man]

Most people on boats underestimate 12 volt battery banks. The currents that flow in a valve radio are really minute. Likewise the wattages are low. Currents in an engine circuit are huge. You need 300 amp cable and 100 amp cable. Wattages are up to 3 killowatts. Soon I have to finish my alternator charging system and may use a relay. Automotive relays aren't normally adequate. I've heard cases of under-rated relays catching fire. For example, a heavy load on an inverter on engine start with the domestic batteries low. So the risk of shock is low but the currents are high, more so with large banks.

[ppppenguin]

Even the humble car battery is a significant hazard due to high currents. I knew somebody who got his metal watch strap caught between battery positive and the car's metalwork. Nasty burned wrist.

[Crackle]

I think something which catches out a lot more people with boats, is the size of the cable that is needed when it is a long run.
For instance, say one has a 24 watt bulb at the top of a 12m mast, hmm, 24 watts 12 volts that's just 2 amps, and then there is a further 8 meters from the mast around the boat to the battery.
Some would think 3 or 6 amp cable would be sufficient, but if you used that you would be putting yourself in danger by severely reducing your visibility in bad weather.
The cable could have reduced the voltage by 2.5 volts or more giving just  9.5 volts at the light, it will be noticeably dimmer and well down on what is specified in the safety at sea regulations.
Add to that any slight corrosion in any joints in the cable due to the salty and damp atmosphere and you have a real problem.

In the case if the example above the correct cable size to use is about 2.5mm ^2 or BIGGER, or in other words 26 amp cable, and even this would only give you about 11.5 volts at the mast head, surprised?
Have a look and scroll down this page and try for yourself. http://www.12voltplanet.co.uk/cable-sizi...ction.html


Mike

[Nowhere-Man]

Back to radios. Here is something more relevant to that capacitor we discussed: AC signal current and capacitors but at resonant frequencies. Example: AC into circuit that has resistor, capacitor and winding in parallel across the lines. Now if the signal is low frequency, capacitor reactance will be higher than XL and AC current will react to the reactances and resistance of the circuit. At higher frequencies, XL will be higher reactance than XC. Yet at resonance both reactances will equalise but the phases cancel. So only the pure resistaance allows current. This is in part similar to what Terry referred to earlier with AC and capacitors. And to add something I just worked out mathematically: series and parallel resistance circuits like this have the same Q if XL and XC (in both circuits are equal and RP (parallel) times RS (series) equals the square of the resonant reactance. See page 43 ARRL

[Nowhere-Man]

That ARRL handbook is the mother of all books. Could well be I may drift into HAM in the future.
As to boats this year I plan to sell my steel boat and move onto the fibreglass boat I've been working on. Times are hard economically so I plan to "go solar" and live simply. The trannys should work better too (steel tends to block signals).

[Nowhere-Man]

Just soldered the capacitor in. Really need magnification as my eye-sight is awful. Really took my time. The old capacitor seemed OK but very flimsy. The tracks are very slender. I tend to carefully stick a bit of sellotape between to prevent any accidental solder run. It saves any headaches.

[Nowhere-Man]

Fancy that! If you put your meter probe on the contact points of the plastic drum, and the springy lower contacts, some turn out to be infinity. They look to be in contact but the tension has weakened a bit. Perhaps a bit of wear and tear. The same with the scale lamps - have to sometimes tap them.