LED problem

[clyst]

One of my LED indicators on one of my switch panels remains glowing ,albeit dimly, for about half a minute AFTER the main iso switch is turned off /forums/images/graemlins/frown.gif . can anyone offer an explaination for this ???

Cheers
Terry

 

[William_H]

I can only presume that it is a LED indicator that was glowing normally ie had power connected to it before the main iso switch is turned off.
Presumably therefor any LED indicator that is operating before the power is turned off also glows for a short while. More LEDs quicker dissipation.

My guess is that you have a capacitor in the system across the power from +ve to -ve perhaps in the input circuit of electronics or a interference filter of some sort.

A capacitor of large value is like a battery, it charges up with power on then when power is disconnected it discharges in your case the only place to discharge is through the LED indicator. Fortunately unlike a battery a capacitor is happy to be completely discharged any number of times without damage.

Because I am in the mood to waffle..... Capacitance?

Electricity flow is actually a movement of electrons from the orbit of one atom to that of another. There is huge force trying to maintain correct number of electrons in an atom's orbit. So one can't move out until another moves in. The force trying to shove electrons down the line is called the voltage while the actual number of electrons moving represents the current.
If you have a battery with a lamp and 2 wires you get current flow. Electrons move from negative to positive pushed by the chemical energy of the battery. ( Initially it was thought +ve to -ve but later the truth was discovered) (hence the contradiction of name)
If there is a break in the wire the current can't flow. You get a pressure on the negative side of the break with electrons trying to get across the gap and on the +ve side you get an atom that is pulling electrons across the gap so it can release elecrons to move em down the line.
What happens at the actual gap is interesting. There is influence across the gap. Electrons can congregate (atoms with excess electrons) at the negative side of the gap and there can be a shortage of electrons at the positive side of the gap. There is this tension across the gap which is more noticeable the closer the gap is and the larger the area of the 2 edges of the gap. Some insulating materials in the gap can have a more noticeable effect than air.

So if you had a really large area close spaced gap when power is applied to the circuit a measurable current will flow into this stress (tension) area. If you disconnect the circuit the tension remains and a voltage can be measured representing the tension that was. In fact a current can be made to flow from one side to the other until the tension is dissipated.

If a gap we call it a capacitor is connected in parallel with a power source going to say a radio. and if the voltage of the power source rises momentarily then the capacitor will soak up some of that voltage rise and if the supply voltage drops momentarily the capacitor can work like a battery to supply the shortfall. The supply is smoothed.

So my guess is that a a capacitor is supplying voltage to the electrical system once the batteries are disconnected. If I remember correctly a one farrad capacitor can provide one amp for 1 second. A rough way to describe a current which starts high and decreases with time as does the voltage which pushes the current
Your equipment may well have a 50 microfarrad capacitor which might provide 10 milliamps (and decreasing) of LED current. for some seconds. A capacitor might have two plates in the form of rolls of foil seperated by thin paper so the area is quite large with very small gap. A capicitor usually has a voltage rating because the gap is so small an excess of voltage can jump across the gap destroying the capacitor.

So all is normal. If you disconnect your electronics or radios you may be able to identify which one has the capacitor. olewill

 

[VicS]

William is right, something on the sytem somewhere with a bit of capacitance.

 

[clyst]

Thanks chaps its the LED on the VHF trip/ iso switch . Presumably there is a capacator there somewhere .

Terry

 

[boatmike]

Er yeah... thats what I thought he said! /forums/images/graemlins/smile.gif