12v Electrics Advice - light lingers on after switching off

[Blue Drifter]

I have several Blue Sea Switch panels at the chart-table. They all seem to work fine except for the switch for the VHF radio. The switches are illuminated and all extinguish immediately after switching off except the VHF which takes about 8 seconds to slowly dim. Any advice/diagnostic process will be gratefully received.

 

[yoda]

Mine does the same, probably capacitance in the circuit somewhere.

Yoda

 

[padge]

possibly just the radio psu capacitors slowly discharging through the switch lamp.

Does the light go out immediately if the VHF radio is still on when the panel switch is turned off ?
If so nothing to worry about.

 

[Blue Drifter]

2 responses in 8 minutes - fantastic. Thanks for the replies - I will check using the switching off at panel with radio still on. Had a quick look at capacitance on Wiki - far too many equations in the explanation. Thank you both.

 

[Gwylan]

Basically some of the myriad bits in the black box that is your VHF store up energy. When the electricity [technical term for electricity] is switched off it is in their nature to hang onto this energy and let it dribble away slowly. This is sufficient to keep the lamp illuminated but reduces over time. Some smart person will be able to tell us amazing things about the circuit from the time that the lamp remains illuminated and the particular nature of the decay in output. But I really do not care and you do not need to worry, unless you can smell burning.

Bit like the sink, fill with water, pull out the plug and the water takes a finite time to run away.

 

[William_H]

Capacitance looks a bit like a battery of small capacity. It is used in the 12v input of the radio to smooth out any iregularities in the 12v supply. (they would show up as static interference). So this stored energy is enough to light the led indicator on your panel for some time.
Capacitance. Consider that electricity is electrons moving along a wire by jumping from one orbit of an atom to another orbit of an adjacent atom. Flow can only occur if there is a vacancy nearby and another electron ready to jump into the vacancy to be made. So electric flow is a shuffling of electrons like pass the parcel game except every must have a parcel. Now consider that if you connect 2 wires to a battery with not connection between the two. The electrons on the negative line push and the electrons on the pos line pull but nothing gets through. The last electron on the negative end peers out and says nope no where to go. So nothing moves.
Actually if the 2 ends of the wire are close but not touching there is an electrostatic ternsion or force betweeen the 2 ends. This ethereal force allows a pile up of electrons on the negative wire and a pile up of vacancies on the pos wire. The proximity of the 2 ends of wire and the area of the 2 ends influence proportionally the amount of pile up possible. Now if we remove our 2 wires from the battery that electrostatic tension remains. If we short the 2 ends of the wire that were connected to the battery together that tension discharges and a minute current flows back to relieve the tension. If you reconnect the wires to the battery you get a tiny momentary flow of current as the tension builds up again.
Now a capacitor has large area plates that are insulated frrom one another but very close. The excess electrons spread out over the plate on the negative side and the vacancies spread out over the pos side. On a large capacitor like 1000 microfarrads the current flowing initially can be amps depending on the resistance of the circuit (so no resistance infinite current) for nearly a second but reducing with time. If you reverse the application of the battery the capacitor must discharge (relax tension) then charge resume the tension in the opposite way. Capacitors can be made in capacitance of 1 farrad down to 1 picofarrad. (A huge range 12 zeros) One picofarrad might be the capacitance between the heads of 2 drawing pins at 1cm spacing. Really big capacitance is acheived by using 2 layers of foil with a conductive fluid to a chemically created insulator. (some insulations make more capacitance than air however for high voltages vacuum is used).
You cna see that a capacitor across a battery line will charge up more or discharge a little to smooth out any sharp variations in voltage. Like a hydraulic or water supply accumulator.
Capacitance also tends to conduct AC but not DC so capacitance is vital in any radio circuit. Hence at VHF frequencies 1picofarrad can have a real effect on a circuit but 10 to 12 power ie 1 farrad is used to smooth battery supplies.
Op radio probably has 100 microfarrad capacitance to smooth battery supply and this is enough to light the tiny LED in the indicator panel.
olewill

 

[Leighb]

Very clear explanation olewill and I could actually understand it! I knew that capacitors stored electricity and in things like TVs it could be large amounts, hence advice not to fiddle around inside until some time after switching off, but didn't really understand how they worked.