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Dave's Bus Conversion12V on 24V converters - 24V to 12V conversionAll material © 2006, David Ljung Madison |
Lots of buses have 24V systems, but many appliances and lights come in 12V. A common problem is a 24V bus with a 12V towed vehicle ("toad") and no way to hookup the toad 12V lights.
This is a common question and there are many solutions, I've tried to collect many of them here (please feel free to send more!). Some of the solutions (splitting the battery, equalizers, power converters) are only used for creating a 12V supply, not for converting a 24V signal to 12V (such as a brake lights) which might be what you need.
Executive summary: (for those who don't want to read the whole thing) If you need 12V to power something, look at the equalizer or converter solutions. If you need to convert 12V lights, look at replacing the 12V device, resistors, or use one of the 12V power solutions with relays to control the lights.
Some people will tap off the middle post to get a 12V supply,
(though this is generally a bad idea - explained below):
You can even tap off the "top" battery as well for another 12V
circuit, but please note that the ground (-) for this circuit will
not be the same as your bus ground:
Because the grounds are different for this second circuit, this means that you can't power anything with it that is grounded through it's installation to your bus. Furthermore (since there is a 12V potential between the bus ground and the 12V ground), you should probably fuse both sides (+/-) of the circuit for safety.
The 12V circuit that this creates will always be on, you can make it be switched (such as by the ignition switch) by simply wiring it through a 24V relay.
This is generally a bad idea because it means your draw on your batteries is out of balance (unless you match the two 12V circuits) One battery will continuously lose more charge than the other and then be at a lower voltage. The charger on your bus does not charge the two batteries separately, it simply places 28V or so across the battery terminals. This means that eventually one of your batteries will die, or the other will be overcharged or both.
Owners of some 24V buses (such as MCI) may note that the manufacturer has a 12V tap coming off the center posts and assume that this is a safe place to get 12V. Unfortunately this is not the case, this is usually used to power 12V headlights - and the center tap is to ensure that if one headlight goes out, the other headlight is still running. The side note here is that if you lose a headlight, be sure to replace it, otherwise you'll eventually kill the batteries as well.
Consequently, if you want to tap off the middle, you should probably use a..
Essentially it holds the middle wire at half the voltage of the outer wires. If one of the batteries has a higher charge, current will pass through the equalizer from the higher battery to the lower battery.
Most equalizers can also do plain 24->12V conversion and often 12V->24V as well. If it's hooked up as an equalizer, though, then it will keep your batteries balanced even if you aren't using the 12V circuit, which is good for battery life.
Equalizers have a max current rating, but that's just the balancing max current. To use an equalizer you would still use the splitting techniques as above. This means that you can actually use more than the maximum equalizer current for short periods of time, as long as the equalizer has a chance to balance the batteries out afterwards. Furthermore, if you do a double split like above, then the equalizer only has to deal with the maximum current difference, so if you can mostly balance two circuits, the equalizer can make up the difference.
Side note: Most equalizers need to be connected/disconnected with ground wire last, or you can fry the equalizer! See your manual.
This also means you don't need a relay if you want it to be switched (such as by the ignition) or to match a signal on the bus (such as a brake light). This does mean, however, that you would need a separate converter or relays for each switch/signal.
The part number (according to KDS Controls?) is 2004502
The 24V signal will turn on a magnet that pulls the switch from off to on and allows the 12V signal to go through. Easy! Most relays have an off signal as well (called "double throw" or "DT"), so if your 12V signal is doing the opposite of the 24V signal (i.e., brake lights on when you are not braking), then you've hooked up to the wrong contact.
You can pick up a 7812 3pin regulator at Radio Shack for 1A or less. This is 12W, so it might be enough for indicator lights (one per light), and they should only be a buck or two. If you use 1A on average, but need bursts of more than 1A (for things like motors that have startup power burst needs) then you can put a capacitor across the output to supply these bursts - sizing the capacitor is a problem left to the reader.
One prominent use of small regulators is to use one per light for towing. You can usually find regulator diodes that can handle small current, then hook them up to all your 24V signals to drop them to 12V.
Linear voltage regulators are inefficient, they basically waste the "unused voltage" as heat and may need heatsinks, but are fairly simple to hook up, sometimes just two wires to the 24V gives you a third wire as a 12V source. Look at the LM338 for a 5A adjustable regulator, you can set the voltage to 12V with some extra parts.
Switching voltage regulators are a little more complicated but more efficient (85% or higher). They generally need a few capacitors attached very close to the regulator. As an example, TI has part 78SR112HC and 78SR112SC for 12V at 1.5A. For more power, National Semiconductor has LM2679 for 12V at 5A and it can be found online for a few dollars (though other parts are required!). Natoinal has a neat "Webench" tool that can create a design for you given your power requirements. It created a 93% efficient design with 5 caps, and a resistor, diode and inductor (example BOM). Also see the similar LM2678 (12V at 5A) which has this simple design with similar stats (might be easier to build?)
To calculate, remember that V=IR (voltage = current * resistance).
If a light is .1A at 12V then: 12V = .1A * R, hence resistance is 120 ohms. Find a 120 ohm resistor (Radio Shack has plenty) that can handle .5A or more, and put it in series (in-line) with the light. Use a voltmeter to make sure you're reading ~12V across each half of the circuit.
Realize that the resistor dissipates energy mostly as heat, so if you have lots of current flowing through it you may need to protect it from your wires and such.
Another option is to actually use twice the load instead of adding a resistor, for example if it's a 12V bulb, then just use two of them in series, though if one burns out then they both will be off.
Again, this will be inefficient, because you'll be using twice the power that you need, whether through a resistor or a second bulb.
An incandescent bulb should only be used as a "ballast" (resistor) for another incandescent bulb. Otherwise your second device might receive too much current since incandescents start with lower resistance when cold.
Some three post 24V bulbs have the wires switched from 12V bulbs, be sure to watch for this as you may need to switch the wires (or else you'll get "bright" when you want "dim.":
Because alternators have a low voltage cutoff you'll need to make sure that the alternator you choose will supply 12V at idle, or else add a battery. Same goes if it has a high voltage cutoff.
Pick a zener that can handle power:
Pmax = ( (Vin-Vout)/R - Iout ) * Vzener
Use worst case values of highest Vin (~28V) with lowest Iout.
A more complicated emitter circuit can be made to be more efficient:
Rc is optionally the voltage drop across it should be less than
the drop across R for highest normal current. To calc R, use:
Vin-Vout > Iout(max) * R
Use the lowest value of Vin (say 22V) and Vout of 1
For more info, do searches for "emitter follower" circuits.
A basic speed controller circuit for 24V->12V conversion could consist of a power transistor running at about 50% duty cycle hooked up to a capacitor to smooth the signal to DC. Let me know if you build one, I'd be curious to hear about it.
irf.com digikey.com
westmarine.com newenglandsolar.com secamerica.com
westmarine.com Extra bulb sockets
Relay circuit Tail Light Converter (gumpydog)