24v to a 12v via resistor???

[ukbaz]

Hi I have a spare 12v jabsco searchlight I have put on the boat aft. Main boat electrics are all 24v and routing back to only 12 volt available is a very awkward job.

Question is what value resistor do I need to put in line to make this work from 24v? I know this is possible as one of the heads on the boat has a 12v toilet running this way from 24 volt supply. The light draws 3.6A.

Any 'bright sparks' out there?

I figured out I'd need a 3.5 ohm 45 watt... but not sure if this is right??

Barry

 

[alan006]

Hi Barry,
May I suggest you get a 24v to 12v dropper. They are not expensive and then you can feed your 12volt items from it.
Hope this helps.

 

[CaptainBob]

That won't work unfortunately, unless the 12v lamp was drawing nearly zero current. You need a voltage regulator for a really cheap and simple solution, but it's wasteful of electricity and will get mighty warm.

 

[CaptainBob]

You could buy TWO Jabsco searchlights and always use them both at once!

 

[VicS]

I make it 3.33 ohms (12 divided by 3.6) so yes 3.5 if thats the nearest. But you are going to waste as much power as the light is using and that power is ging to be in the form of heat. Youll get as much heat from the resisor as you do from the bulb.

The sensible solution is to get a 24volt bulb for the lamp. It'll then only take half the current for the same wattage.

 

[narooma]

Would it take a 24v bulb? (sorry vic, just noticed your post)

 

[rogerdog]

you can - but very wasteful of power + as already stated the resistor will get very hot so will need to dissipate this heat somehow.

 

[rogerdog]

sorry chaps should learn to type faster -24v bulb best method

 

[VicS]

[ QUOTE ]
Would it take a 24v bulb?

[/ QUOTE ] I'd be very surprised if it wouldn't but I dont know the lamp. I thought Jabsco only made pumps.

I dont understand why Captainbob says a resistor wont work. It'll work, but the snags have been explained.

Checked with the Jabsco website and found that they list their search lights both as 12 volt and 24 volt I'd expct just the bulbs to be different

http://www.jabscoshop.com/category.asp?ls=&v=0CBE7F97FDD64B3B9AE0D856FCBA3EDA&id=10

 

[wooslehunter]

So 3.6A at 12V implies that the bulb resistance is 12/3.6 = 3.3 ohm as rightly worked out above.

The power dissipated is 12 * 3.6 = 43W. So, you could use a 3.3 ohm, 50W resistor.

But here's the rub. You need to bolt the resistor to a heatsink and have it properly ventilated. Even a heatsink rated at 1C/Watt would rise 43C above the ambient temperature and these are quite big. So on a day when it's 20C, the heatsink would get to 63C and you wouldn't be able to touch it. You'd need something of the order of 0.5C/W - expensive, big & heavy.

The alternative would be to use a smaller heatsink and only use the light for a few seconds - not practical. It would work fine for the bog as unless you had a really serious curry you wouldn't be flushing continuously. /forums/images/graemlins/ooo.gif /forums/images/graemlins/ooo.gif /forums/images/graemlins/ooo.gif

So, get a 24v bulb, a proper 24v-12v converter, or a new lamp.

 

[captainslarty]

I have a Jabsco searchlite and the unit is sealed beam unfortunately, I dont know if they all are though.

 

[VicS]

[ QUOTE ]
the unit is sealed beam

[/ QUOTE ] Yes you are right. Interestingly they list the same replacement sealed beam units for both 12 volt and 12/24 volt units (Prices ranging from £32 to £68 depending on the lamp model.)

Presumably some form of electronic control allowing the same unit to be used on 24v as on 12 v

Changing bulb therefore is not the answer after all!

Lesson to be learnt: research the problem fully before suggesting solutions!

 

[mormargh]

Not sure I follow this. Wouldn't a resistor have to be the size of marrow to get a voltage drop of 50%?

 

[VicS]

[ QUOTE ]
Wouldn't a resistor have to be the size of marrow to get a voltage drop of 50%?

[/ QUOTE ] It nothing to do with 50% drop as such. For the current quoted (we can only assume that's correct) the value in ohms is the value calculated, from volts drop divided by the current (Ohms law).

The physical size will be determined by the power dissipated In watts that's
volts drop X current,
volts drop squared divided by resistance
or current squared X resistance

which ever you prefer. It'll be the same as the lamp in this instance anyway.

Whether a 3.5 ohm 45 watt resistor will be the sze of a marrow I don't know. I would not have thought so. (The largest listed by Maplin are only 10watts)

 

[saltwater_gypsy]

Best option is a voltage convertor. Maplin are doing 24 to12V at 10A for £15 which looks a bit "domestic" or Victron do the business at £50 for 8Amps. Your'e bound to have other things that run off 12V.
The guys a Raymarine have most of their stuff running at 10 to 32 V except for the ST60 kit which is 12V. So I've been down this road before.

 

[lw395]

Although the running current is about 3.5A, the switch on current can be a lot higher, so a converter may have a problem unless it is generously rated. Resistor solution will be ok, if you can get the heat away, maybe use the body of the light as a heatsink, as it will be outside? Maybe the whole thing would over heat tho?
Tempting to suggest sell it on ebay and get something that takes readily available H1 or H3 bulb? Or maybe run it from a small 12V battery charged 24-12 converter (actually need a 24to14 ish converter in this case!)
Resistors are nice and simple, but watty enough ones will not be cheap. 3 off 10ohm in parallel (=3 1/3ohm) would do, and be easier to heatsink. Aluminium body type may be better than ceramic, designed to put heat into a heatsink, rather than getting v hot and putting heat straight into air, thus needing airflow and/or being a hazard. 10ohm 25W resistor RS157-550, £1.55 each is the type of thing. You need something metal pref outdoors to lose the heat into.
Hope that helps?