Dave's Bus Conversion

12V on 24V converters - 24V to 12V conversion

All material © 2006, David Ljung Madison


I do NOT sell converters, nor can I help you build one, nor do I know where you can buy one. All the info I have is on this page. Good luck!


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.


Solutions: Splitting the battery | Equalizer | Converter | RTS Equalizers | Relays |
Regulator | Resistors | Replacing 12V device | MCI Converter | 120V to 12V | Charge Controller | Two Alternators | Forward Diode circuit | Emitter Follower circuit | Zener Diode circuit | Speed Controller


"Splitting" the battery

Most 24V buses are built from a pair of 12V batteries in series:

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..

Battery Equalizer / Battery Balancer

A Battery Equalizer (a.k.a. "Battery Balancer") is a simple three-wire device that keeps your batteries balanced:

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.

Voltage Converter

Voltage converters simply take an input voltage (such as 24V) and convert it to an output voltage (such as 12V). The big advantage of these is that they don't need access to the middle battery posts, so they can be installed anywhere on the bus electrical system that you have 24V:

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.

RTS Equalizers

If you have access to a bus junkyard or bus parts salvage, then you might be able to pull a working equalizer off of an RTS bus, they have a high current equalizer because they have 12v lighting on a 24v bus.

The part number (according to KDS Controls?) is 2004502

Relays

A relay does not convert voltage by itself, but it's useful for converting a signal to a different voltage (for example, a 24V brake light signal to 12V). You can think of a relay as a switch that is turned on/off using voltage instead of a finger. Hooking one up is almost as simple as hooking up a switch:

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.

Voltage Regulators

A voltage regulator basically clips the voltage at a maximum value (such as 12V). Many of the solid-state regulators are not very high current (nor are they usually efficient, since they often waste the excess energy).

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?)

Resistors

If you know what your 12V load is and it is static (such as for most lights at most temperatures) then you can put a resistor in series with the light to approximate 12V across the light.

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.

Replacing 12V Device

Sometimes it makes more sense to replace the 12V device with a 24V unit. Examples would be replacing lights with 24V lights/LEDS, or using a 24V magnetic light bar on their Toad vehicle. Some people have even put a second set of bulbs (24V) inside the light compartments of their Toad and use those for the towing hookup, an entirely separate circuit from the Toad's 12V system.

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.":

MCI Converter

Evidently MCI has a cheap converter meant for using 12V lamps on 24V systems - I'm not sure if this is the same thing as just using the center tap on the batteries in case one headlight blows or not, but if it's an actual converter then it should be able to cover lots of situations (headlights draw some decent wattage). The module kit is T07-2297. If anyone has more info on what these do, please let me know! (Thanks to Taconic Tours for the tip)

120V to 12V

Another option for creating 12V power is to use a 120V converter. Many of our buses have 120V power either from inverters and/or generators, and 120VAC->12VDC are plentiful and often inexpensive. This is due to the fact that it's generally easier to convert AC voltages (with a transformer) than DC voltages.

Forward Diode Circuit

A diode drops (wastes) about .7V, so you can put 20 of these in series for a brute force low current method to use up 14V (most 24V systems can hit upwards of 28V). Some diodes have a larger drops so you can use less of them.

Emitter Follower Circuit

If you have a 12V supply that's not powerful enough, you can build an emitter-follower circuit to get almost 12V at higher current. The output will be .7V lower than the 12V input, but most 12V circuits can handle that (and your 12V input will likely be higher if you're using something like an equalizer). The current comes from the 24V source, not the 12V source, so it can be much higher.
Resistor sizing will be a tradeoff between efficiency and maximum power available.

Charge Controller

If you want to charge a 12V battery bank off of a 24V system, you could repurpose a solar charge controller and just feed in the 24V power where you would hookup the solar panels. (Thanks to Dave Hartley for the idea)

Two Alternators

If you absolutely must have high power 12V and you only need it when your engine is running, you could consider mounting a second, 12V alternator to provide power (conceivably with a 12V battery as well to smooth out the voltage and hold charge when the engine is off). Obviously this is a big modification and is uncommon (though it's more common in the marine world, it would seem). If you need more than 10-20A, then this might be a useful, cheaper solution.

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.

Zener Diodes Circuit

If you know the range of currents that your 12V circuit will use, you can build your own regulator using Zener diodes:

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.

Speed Controller

This is like pounding in a tack with a sledgehammer, but if you're feeling rather "project-y" or if you need high-power, then you can use a speed controller to adjust voltage. Sometimes you can find 24V speed controllers from old scooters or golf carts, or you could build one from scratch. One problem is that the output of the speed controller is "throttle" adjustable, so you'd have to find the correct throttle setting to get the voltage you want.

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.


Any other ideas?

Please let me know!


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