one for serious electronic type people

[mowgli]

hi.

i am trying to sort out a cheap 24v-12v lighting converter for a new (used) iveco wagon.

boring theory bit:
http://learning.codasign.com/images/9/97/Voltage_divider_schematic.png

so Vin is 24v(nominally), and Vout is 12v(nominally) R2 is the 12v light bulb.
R1 is a resistor...

if a 12v 21w bulb is used, and if my O level electronics is anything to go by, Watts=Volts x Amps, so current (A) =W/V or 21w/12v = 1.75a. and resistance is V/A or 12/1.75=6.86ohms. is this correct???

so if i use a 6.8(ish) ohms resistor as r1, would this make the 12v light bulb work properly with a 24v supply?

answers & input please

thankyou

mike

ps. i really don't like the idea of a 12v feed run from the batteries & a set of relays in a box, or an off the shelf one, we had one years ago & it was shyte and kept blowing seperate circuits & eventually was utterly useless

[Stuart]

Www.electronics2000.com look at the potential divider calculator.

Don't forget that you will need a well heat sunk 21w resistor which will get sodding hot. For reference I'm running 8w through a 25w Ali cased resistor which is riveted to a 75x75mm Ali plate and it hits 80deg c in free air.

youd be better to run a pair of bulbs in series IMHO

[mowgli]

the trouble is that we have 4 pickups, and 6 vans that use the trailers already, and only one 24v vehicle.. so the double bulb thing is a bit tricky to fit somewhere, because of the heat sink issues.

i was planning to go for metal cased resistors & bond them to an ally sink, possibly sealing them in a resin too, as i can't see how a little connector box will get close to coping with it without melting, the worse case scenario is 126w on at the same time, albeit for very short periods of time, and with lights, brake lights & fog light, it would be more like 84w, which is still considerable to keep cool

[meritlover]

i agree with Sturat. The maths is simple in the sense that your after half the supply voltage, so you need to dissipate twice the amount of energy to drop the voltage...so you need a 21w 'sink' which is rather a lot.

2 lamps in series is great until one blows. Something like a heater motor resistor would be ideal for the job, but i have no idea what value they are.

a linear voltage regulator would be more sensible, here is a quick search result. the last circuit uses a transistor that will allow up to 5A, but this might be too small for all your lights. would be about 60w but you could heatsink it to hell and probably get a good bit more out of it.

Edit, i just read your reply and realised you are looking to power a whole trailer board, thats a pretty substantial load...

http://www.reuk.co.uk/24V-12V-DC-DC-Converter.htm


(http://www.reuk.co.uk/24V-12V-DC-DC-Converter.htm)

[meritlover]

http://www.ebay.co.uk/itm/QZO-24V-to-12V-DC-DC-Car-Power-Supply-Inverter-Converter-Conversion-Device-30A-/141138534326?pt=UK_BOI_Electrical_Test_Measurement _Equipment_ET&hash=item20dc831fb6

[Stuart]

A hard ass zener diode might do the trick, I'll chuck the numbers into a calculator I have at work

[mowgli]

ML, i appreciate your answers, but i am trying to avoid these types of circuits, as they have a really annoying habit of discharging batteries as they have to have a permanent feed and they get very hot.. quality control is also a bit sketchy imho...

this is more like what i'm thinking

http://www.ebay.co.uk/itm/10Pcs-6-8-Ohm-6-8R-25W-Watt-Power-Metal-Shell-Case-Wirewound-Resistor-/161101601199?pt=UK_BOI_Electrical_Components_Suppl ies_ET&hash=item25826759af

[mowgli]

A hard ass zener diode might do the trick, I'll chuck the numbers into a calculator I have at work

ta

[Stuart]

Do not, I repeat DO NOT, AND Again for luck DO NOT!!!!!! Buy cheap resistors on ebay. They are crap and are about a quarter of the rating they claim. A chap used a 50w rated resistor for the 8w job, it caught fire.

look on cpc Farnell (free postage) at tyco, welwyn or arcol branded ones, you will also need to over rate the resistor to about 100w IMHO, and do not bond it down, use rivets to something big and metallic (chassis)

[mowgli]

Stu,
the pic was more of an example of what i was after than an intention to purchase something dodgy...(even though the 5 slide switches i got for £2.50 incl p&p off the bay from hong kong to fix my argos camping fridge are doing great, as it really was the only place i could get them from)
i intend to fix them properly, as even rivets corrode with road salt. i was looking at a resin to seal the connections to keep corrosion out then a liberal coating of paint, as it will be on the inside of a wagon chassis, and will be steam cleaned weekly just for good measure

[Stuart]

Zener diodes are off the cards... Too big and juicy.
Looked at a Buck convertor but you are looking at about £10 per light if you factor in connectors/sealing etc.

http://www.rapidonline.com/electronic-components/6r8-hs-100w-arcol-aluminium-clad-resistor-62-8186
http://cpc.farnell.com/te-connectivity-cgs/hsc1006r8j/resistor-ww-100w-5-6r8/dp/RE03533
the 100W resistors will cope with 30W when NOT mounted to a heatsink, so when mounted to a chassis they will be more than adequate

CPC are free postage no matter what, and Rapid are free after £30

[meritlover]

seeing as you are pulling twice as much power as designed through the trailer circuit, consider the thickness of the loom and also whether or not the indicator flasher will be ok and flash at the right rate when pulling 42w.

resistors are the way to go.

[Stuart]

Would probably be easier to add another alternator to the engine to make a 12V lighting circuit completely

[mowgli]

seeing as you are pulling twice as much power as designed through the trailer circuit, consider the thickness of the loom and also whether or not the indicator flasher will be ok and flash at the right rate when pulling 42w.

resistors are the way to go.

the truck circuit is more than man enough for it. all the major mfrs build in seriously heavy duty lighting harnesses & then the commercial body builders use cable that would make any car wiring loom maker cry... just look at some of the custom stuff going up the road with masses of extra lights.. they don't need to change flasher units or relays. the mindset with a truck is that the maker has to ensure that it is more than up to whatever anyone else will add to it before it hits the road.

the trailer circuit will actually be running twice the current than the truck circuit, simply cos 21w at 24v is 1/2 the current than 21w at 12v. the trailer is not the issue..

I am still planning on resistors to fix this..

[mowgli]

Would probably be easier to add another alternator to the engine to make a 12V lighting circuit completely

that is what the yanks do already... 24v to run it & 12v to light it.. thus twice the wiring/fuses/relays etc it was done out of necessity because of the need to find a light bulb in some remote desert part of the states.. not really necessary in leicestershire