What type of transformer?

[Clyde_Wanderer]

Rather than have 230v ac lighting units in the boat, I was thinking of wiring all my 12v cabin lights through a double pole change over switch, something like the Kraus and Naimer type switch.
In this set up I could select power from batts while away from a marina etc, or select alternative power from presumably a 230v -12v transformer run from shore power.
The question is What type of transformer would/should I use, or is there such a unit capable of outputting enough amps?
Cheers, C_W.

 

[misterg]

'Scuse my ignorance, but wouldn't a mains powered battery charger achieve the same ends without having to switch anything?

Andy

 

[lenseman]

A Car battery charger answers your second question and as far as the first part of your thread (switch for 12 volts), you have to take into consideration the current drawn through the switch and the arcing moment every time you turn off the lights.

Heavy current DC switching can be quite problematical but switching small current loads is OK as long as you are aware of problems that might arise.

 

[lenseman]

I think you will find that the current ratings of the Kraus and Naimer type switches are for AC switching not DC switching!

If you require heavy current DC switches you have to start thinking along the lines of a knife switch, like the types you see in the cinema in those old black and white films when someone being topped in the electric chair in the USA.

Check before you buy as the switches you mention are NOT cheap at all!!!

 

[Norman_E]

I cannot see why you need to do this. A good marine battery charger will also act as a 12 volt power supply, so why go to extra complication.

I do not recommend that you do what you are suggesting because of the risk of feeding mains voltage into the 12 volt system if anything goes wrong. In addition without a rectifier a transformer will give you 12 volts AC, that is OK for lights, but would be fatal to most of your boats electronics if it got connected into anything other than the lighting circuit. Boat wiring is complex, and you could easily have a connection somewhere that you have not noticed.

If you do what you suggest it is vital that any changeover switch is "break before make", that is to say that during switching it cannot connect both circuits.

As for transformers, domestic 12 volt halogen lights comes in sets complete with the transformer, or you can buy the lighting transformers separately.

Having said that, Don't do it, just use a combined battery charger and power supply like the rest of us.

 

[William_H]

The concept sounds OK however to run lights on AC or DC would limit you to incandescent or halogen lights. No LEDs or Flouro cos they require DC.
It would really be far simpler to run everything off the battery then recharge the battery while on mains. This has the advantage that you can keep your batteries in top charge state all the time which will increase their life.

The capacity of either a transformer or charger depends on the power you need. An advantage of using the batteries is that peak loads can be taken from the battery so the charger only needs to supply the average load over 24 hrs.
So 100 watts of lighting will require about 9 or 10 amps of AC from a transformer and likewise from a battery charger except that if 100 watts is only used for 5 hours then you only need battery charger capacity for 5/24 of 10 amps. Take the excess out of the battery at night and replace during the day. Of course that is simplistic and you will probably have or will get a charger that is rated well over minimum requirements. Especially as you will want to run other services apart from just lights.

You can use a battery charger of the stepped charging type or just a regulated power supply which provides 14 volts accurately regulated so that current drawn will come from the power supply until the charger starts to fall in voltage when batteries will kick in or when load falls the battery gets a charge.
Yes a little more power wastage heat etc than running lights on AC direct but much simpler. olewill

 

[fireball]

You can purchase simple 12v rectifiers - we have a 5amp one (60w) with a Cig lighter in - cost bout 20 quid .. got it because we use an electric coldbox and we were in a marina on hols with shore power ... so rather than run the batteries down we used this ... we don't have built in battery charger as we spend most of our time on a swinging mooring so it would be redundant ...

I have made up a cig lighter to cig lighter lead and can plug the rectifier straight in to the domestic circuit using the cig lighter socket .. it won't power much - a few lights and perhaps the radio - all I have to do is make sure I've turned the domestic battery off before connection ... but I assume this is exactly what you need ...

Alternative transformers - you can use an old PC power supply - the yellow lead that normally powers a hard drive or CD Rom is 12v DC, the red is 5v - use a multimeter just to check first though! I've got one of these on my workbench so I can run up 12v kit without need for a battery - again, the transformers will be rated, so best to check - but it will run a couple of lights with no problem.

Incidentally, whilst looking at our power drain away from shorepower, I found I had less drain but more light by running the 300w inverter with a energysaver lamp plugged into it rather than running the internal lights ... I really must look to get some LEDs!!

 

[fireball]

Whilst it is good to mention about the AC transformers ... I believe CW was actually after a 12v DC supply through a transformer on the mains ...

A "marine" battery charger is upwards of £100! Where as this: http://www.kwikbreaks.co.uk/shop/view_product.php?product=136 is just £15 ...ok it won't charge your batteries but if you don't generally have a battery charge problem then why spend £100 on a rectifier!!

 

[Clyde_Wanderer]

Thanks everyone for your advice, but Lenseman, would it not be simply a matter of switching all loads off before changing the co switch to avoid any arching or burning of contacts.
Anyway I guess it will have to be a decent batt charger then, as I dont have one worth its weight in salt at the moment, but should really have one.
which brings me to 2 more questions.
I have a driftgate split charge unit in the alt charge line, how will I wire up a charger without effecting this system?
Can someone recommend a good safe, reliable charger which can be left perminantly connect to the domestic batts, if thats the way it is normally done?
Thanks again all.

 

[Anonymous]

[ QUOTE ]
Can someone recommend a good safe, reliable charger which can be left perminantly connect to the domestic batts, if thats the way it is normally done?
Thanks again all.

[/ QUOTE ]First, yes, the battery charger is the way to go and a decent charger will keep your batteries in good condition. A good investment. I haven't researched the market recently so will let others recommend a charger though you'll need to tell them how many amps you want.

The main reason for this post is to remind you that there are dual-purpose charger-inverters on the market now that automatically switch 230V on when the mains is turned off (if you want it to). If I was buying a new charger I would look into those very carefully as a good sine wave inverter makes life a little more comfortable on board. I know that Sterling badge one made in the Far East; I know one chap who had problems but he was trying to do something rather too cunning - they are probably OK.

 

[Strathglass]

Just a few thoughts

The cost of a suitable switch would be quite high.

A mistake resulting in AC going into instruments, alternator and other boat systems could be catastrophic and terminal to them.

I imagine a transformer could be purchased from Maplin.

It could be quite a heavy transformer if you count up all the possible current.

The lack of regulation in a simple transformer could mean that there would be quite a variation in voltage berween one and all the lights on. This could lead to shorted bulb life.

I use a Halfords four stage charger just to keep the batteries topped up.

Cheers

Iain

 

[john_morris_uk]

[ QUOTE ]
Can someone recommend a good safe, reliable charger which can be left perminantly connect to the domestic batts, if thats the way it is normally done?
Thanks again all.


[/ QUOTE ] There are lots about. We happen to have a Stirling which is connected to the batteries permamently. When the alternator with its split diode system is working, it takes not notice of the Stirling charger and vice versa. My Stirling Charger has up to three (isolated) outputs. I have coupled two together and connected this to the domestic bank and the other single one is connected to the engine battery.

 

[Graham_Wright]

[ QUOTE ]
I think you will find that the current ratings of the Kraus and Naimer type switches are for AC switching not DC switching!

If you require heavy current DC switches you have to start thinking along the lines of a knife switch, like the types you see in the cinema in those old black and white films when someone being topped in the electric chair in the USA.

Check before you buy as the switches you mention are NOT cheap at all!!!

[/ QUOTE ]

That implies that 1-both-2 battery switches and isolator switches (at £4) should not be operated while carrying current. I hope that isn't true!

 

[Gordonmc]

I use an adjustable DC power supply for bench testing. If you want to Google its a SkyTronic 650.676, 0-30amp, 0-30V.
I suspect it is over the top for your application, Eamon and would not stand up to conditions on a boat. As others have said a car battery charger would be more belt 'n braces.

 

[William_H]

Hello Fireball and others. Yes our language is difficult. As an old time electronics person to me a transformer is a transformer with AC out wheras now days a transformer might mean a transformer with rectifier etc that provides DC.
The power supply in the link is not well described. It may well be just a transformer and rectifier with filter capacitors. One of the problems as already pointed out is that the nominal 12 volts out can be more like 16 volts with light load down to 11 volts at heavy load.
That may be OK if it is used as a battery charger where the battery will charge if the load is light (possibly even too much although unlikely) and will kind of help if the load is high and the voltage falls from excess load.
However a regulated supply is far better in that it will attempt to provide 14 volts precisely which will ensure an accurate charge and also supply a load until the load exceeds the rating of the regulated supply. Or you could power the 12 v system with battery off. But only up to the rating of the transformer/power supply
So any transformer rectifier set may suffice but you would need to monitor the current from the transformer and of course have it charging the battery at the same time.

The idea of using a computer power supply has problems. It would have to be the old type with power switch which is not turned off and on by the computer. It will only provide 12 volts (not 14) which will not charge the battery so would have to supply the lights etc with battery turned off.
So it all gets complicated to discuss. olewill

 

[fireball]

[ QUOTE ]
It would have to be the old type with power switch which is not turned off and on by the computer. It will only provide 12 volts (not 14) which will not charge the battery so would have to supply the lights etc with battery turned off.

[/ QUOTE ]
Ah - yes, something I forgot to mention ... I've got a small supply of power supplies from old computers with the physical on/off switch ... /forums/images/graemlins/smile.gif I don't intend it to charge my batteries because it is in the garage !! The rectifier - which I provided a link to above is on the boat, again - this isn't there to charge the batteries - other than over winter, the batteries generally get sufficient charge from the engine .... and the shorepower - rectifier means we don't need the battery power whilst in a marina (not that often).

As for the transformer - yup, just laziness ... the vast majority of consumer transformers are for electronic kit and have DC output (ie rectified!), only recently have I noticed AC transformers for low voltage lighting circuits .... so I appologise with the reservation that the OP probably meant a 12v DC output ... /forums/images/graemlins/smile.gif

 

[Graham_Wright]

[ QUOTE ]
As an old time electronics person to me a transformer is a transformer with AC out wheras now days a transformer might mean a transformer with rectifier etc that provides DC.


[/ QUOTE ] or a vehicle that disassembles itself and reconstructs as an alien! /forums/images/graemlins/wink.gif

 

[William_H]

Yes Graham it took me a while to figure what you were talking about until I remembered my boys transformers. Horrible ugly toys I never did like em. We hav a collectors show on tele and I remember a while back some fully (?) grown adult who collected the blighters.
I think I will stick with my own handicap of being a yachty. olewill