VHF power supply advice sought

[Gin]

I posted this a couple of hours ago but it seems to have disappeared into a parallel universe. So apologies if you have found it, cos I can't. Strange things are going on inside my computer tonight- found a Worm last night, wonder if that is to blame?

Anyway here is what I would appreciate advice upon:-



[ QUOTE ]
Am I missing something I wonder?

My boat has been well and carefully wired using a distribution board and bus bars, with the exception of the vhf radio.

This unit is wired directly to the engine start battery and thus whether the main Off/1/2/Both is off, the vhf can be used regardless.

I am anxious not to leave the boat with this live circuit, if only in case I forget to switch off the radio when I leave the boat. So I intend to rewire it, but it bugs me as to why it has been set up this way in the first place(it uses 14 awg cable by the way)

Can anyone else see any good reason to stick with the current(sorry) layout?


[/ QUOTE ] /forums/images/graemlins/confused.gif

 

[wragges]

From a design and safety (electrical) point of view this is a very bad thing to do and I don't recommend it !

However, from a safety (preservation of life) point of view it is probably the one obvious scheme that would ensure that the VHF remains operational when all other electrical systems could have failed. By bypassing all other circuits, switches, breaker etc. you significantly reduce the probabilty of critical failure of this life-saving aid at the point you might just need it the most, i.e. sinking / fire etc.

However, I don't like it. If you needed this level of sureity of VHF operation I would instead install a small GEL float-battery (available from Maplin etc. for Alarm boxes) close to the VHF radio and issolote it from the rest of the boats power cicuits using a diode to ensure it gets changed, but does not supply any other devices.

 

[VicMallows]

No. Not really. And I fully understand your concern about accidentally leaving it on.

However, the VHF does draw a lot of current on transmit and does need a really good electrical feed. If your installation is anything less than perfect, it is arguably better the way it is.

I have my GPS directly connected to the battery....but that is really a hang-over from DECCA days when it greatly helped to get ride of alternator interference.

Vic

 

[LadyInBed]

I can't see any reason for it.
Rewire it through the distribution board or go straight off the 1/2/both switch, as it has its own on/off sw and probably an in line fuse.

 

[pvb]

Just add a fuse...

There's no problem having the VHF permanently connected to a battery as long as there's a fuse at the battery end of the cable. However, bear in mind that the VHF could drain a battery if it's inadvertently left switched on when you leave the boat.

 

[Soong]

Re: Just add a fuse...

G1's advice is solid, A seperate battery will give you all the plusses and none of the problems.

-S

 

[bdsweeting]

Re: Just add a fuse...

Idiots input here....

Had a call from the harbour office one day to say that my boat alarm was going off.

That's clever I thought as I don't have an alarm...

Got to the boat to find that I had left the house battery master switch ON as well as the DSC radio. Yep, you've guessed it; someone sent a DSC alert and my radio was waiting for someone to acknowledge it.

 

[VicS]

It the past I seem to remember it being recommended that the power for the VHF should come directly from the battery. In a dual battery situation I think I would opt for the domestic battery rather than the engine start battery if they are separately dedicated although the engine battery is the one most likely to be fully charged so it may be the better choice.

The reason to do this is because the VHf does draw quite a current when transmitting (nearly 6amps in my case) so any resistance due to poor connections in distribution panels, fuse boards etc could lead to a significant voltage drop which will impair the operation of the radio.

It may also help to reduce interferance caused by other electrical equipment. I have never understood how a diesel engine can cause interference though.

A fuse shold be included in the wiring as close to the battery as possible and rated to protect the wiring. To some extent that negates the reason for the direct connection though.

Edit:
In view of the "idiots" input above perhaps after the isolation switch might be a good idea.

 

[trouville]

Re: Just add a fuse...

I have mine connected directly and am very careful to turn it off.All VHFs ive seen have a fuse in the powerline.

On my other boats (and soon this one) i fitted a house plug socket and plug on the VHF The sockets handy for 12v TV DVD CD MP3 and any other 12v needs better contact than marine plugs and more economic and easier to buy extra plugs

 

[Ships_Cat]

Many manufacturers recommend connecting directly to the battery and is probably the best solution for a small yacht. Apart from the increased security of supply and often less voltage drop due to cable shorter overall cable length, the capacitance of the battery will likely limit any hum or other noise problems on the DC line to the radio caused by any invertor or whatever. That can feed into the transmited signal but is less likely to be a problem if the cranking battery is used and it is isolated from the services supply.

However, on my own boat all the radios are sourced off the services distribution panel with adequate cable size. One must keep in mind though that the voltage at the board may be considerably less than at the battery if there are other loads being drawn from the board at the same time, that dependant on the voltage drop between the battery and the board. The larger vessels I have dealt with all have the supply from the services board.

John

 

[marcusgillies]

INTEFERENCE!!!
The reason your VHF is probably wired direct to a battery is to stop inteference from other electrical items on board however its better from the output side of the batery master switch not directly on the battery as stated above and with its own fuse just after said switch, to protect the cable.
Often on boats interference from fridge compressors or motors ie: water pump bilge pump can cause 'static' on the line etc
As for the size of wire for a VHF i would perfer to use nothing smaller that 28 typicaly units consume 5-6amps when transmiting and 0.1-0.7amps when recieving although 14awg cable has been used consider voltage drop over distance.

 

[pvb]

[ QUOTE ]
A fuse shold be included in the wiring as close to the battery as possible and rated to protect the wiring. To some extent that negates the reason for the direct connection though.


[/ QUOTE ]
Why would a fuse negate the reason for direct connection? Fuses are perfectly reliable unless there's a wiring fault - in which case they prevent major damage and possibly fire. Direct connections to batteries should always be fused for safety.

 

[Boathook]

My VHF is wired directly to the master switch through a suitable fuse. Becuse of the wire size between battery and switch it is like being directly on to the battery. This gets over any voltage drop and interference and ensures that everything is turned off by the one switch.

 

[Gin]

Well, very interesting reading and all good thoughtful comments on which I shall ponder.

Seems to me there are good arguments both ways and to get the best of both alternatives I like the sound of keeping direct supply but introducing a robust switch/socket and plug in-line which I can easily disconnect upon departure.

I am very much obliged to all of you who took the trouble to give me your advice-

Thanks very much

 

[VicS]

[ QUOTE ]
Why would a fuse negate the reason for direct connection

[/ QUOTE ] Sorry I should have expained my reasoning.

Introducing a fuse into the battery connection means several more potential bad connections. Two at the fuse holder terminals and two between the wire and what ever terminations are used on the wire. Then there are the contacts between the fuse and the holder (the most likely to cuase trouble). A likely choice of fuse carrier is the inline type and then there will be two more ie the connections between its wire ends and the terminations. That makes a possible total of up to 8.
In an in an ideal world none of these will be a problem but in the marine environment slight corrosion at any of thes points could lead to a slight resistance and therefore volts drop.

I only said to some extent negates the reason for a direct connection., meaning that in an attempt to eliminate the effects of poor connections in a dist. board or fuse board one is merely introducing other potential places for bad connections. The anti intereference argument is still valid as is the anti volts drop on the system caused by other electrical equipment, which has been mentioned by others.

Sorry that is still not all that well explained. I agree, though, that a fuse must be fitted if a direct connection to the battery or isolator is made.