How does a 24 volt aircraft battery power a 28 volt bus?

Question

Can someone explain to me how a 24 volt battery can power components that advertise requiring 28 volts to operate?

The aircraft I fly has a 28 VDC generator that powers both battery and generator buses under normal ops, but in the event of generator failure, the bus tie opens and isolates the battery bus. The 24 VDC battery then gives you 30 minutes of power to the battery bus.

But today I found that the solenoid dump valve, which needs 28 volts to remain closed and keep the aircraft pressurized, is on the battery bus and (supposedly) still operates normally when the 28 volt generator fails and the 24 volt battery takes over. What am I missing?

Answer 1

Most critical electrical equipment is on the unswitched (battery) bus and the standard operating range is (for DO-160 certified kit) is 18V to 32V.

All safety critical equipment is on the battery capable bus and almost universally have internal hold-up capacitors to ride through short power drop out events (which are quite common).

Aircraft power is the power source from hell, rivalled only by vehicles.

Update: It is not unusual for each box to have a couple of isolated DC-DC converters on the front end. Typical part linked.

For those applications that need a 28V input, this is a more likely solution.

Answer 2

That 24v is sufficient for the valve - it has a wider working range than just 28v.

Edit : The bus is nominally named a 24v bus as the two 12v batteries sum to 24v.

Note however that a 12v battery usually is about 12.7 volts as each of the 6 cells are 2.1v, this means that the 24v batteries will give 25.4v ie 2*12.7v.

The charging of the batteries will be above the 25.4v - and can possibly go above 29v but this depends on the alternator specifications as well.

Answer 3

Your 12 volt car battery voltage is about 14 volts when the engine is running. Your 24 volt aircraft battery and various busses,voltage will be about 28 volts when on charge, engines running, external power in etc. All equipment is designed to operate down to about 20 volts, on battery power the 24 volt battery will be around 24 volts and slowly discharge with use down to around 20 volts when it is deemed to be discharged and equipment will begin to fail, non essential busses normally automatically shed to extend this time.

Answer 4

A solenoid is a device made of an electromagnetic coil, which has a typical characteristic of having 1, a nominal voltage (here that's 28V), a minimum trigger voltage (typically 80% of the nominal) and a minimum hold voltage (sometimes as low as 40%). These three voltages define how it works. The nominal voltage is what the solenoid is designed for in normal use. So the solenoid in question has a wide voltage range to turn on and operate. There are also maximum voltages.

Add to this that the battery also has a nominal voltage, in your case 24V, while the active vehicle system has a generator or alternator designed to charge the battery, running at a nominal 29V. This increased voltage is needed to charge the 24V nominal battery. The battery can run down to 20V if not actively charging.

So in short, the solenoid is picked to run through the full range of a vehicle system, while charging or not From 20V to 29V.

Just pray you don't need to rely on the reserve capacity of your battery while flying.

Answer 5

Enough people have answered the question, but as an interesting side note:

I was once told that 28V was decided upon as a standard voltage because the battery voltage is 24V, this means the battery is always taking a charge (4V differential) while the generator or external power is connected. If both the generator and battery were 24V, the battery could be partially discharged in normal operation depending on generator performance, which isn't ideal if you need to rely on it upon generator failure.

I have no evidence to hand to back this up, but it sounded plausible and logical at the time. I used to volunteer looking after EE Lightning aircraft XS452 and XS458, and it was an ex-RAF guy who told me.

Answer 6

Although you are making several questions, they are all related, so a single answer might suffice. What you are "missing," is knowledge/experience of real electronic circuits. There are no real circuits that operate at one exact parameter. Because of manufacturing tolerances and operating requirements, a given circuit/component needs to be able to operate within a parameter range. In your particular example, the "28V dump valve" must have an operating range of at least 24V to 28V (most likely 20V to 32V). So once you know this, it should be clear how it is possible for a solenoid, labeled 28V to be able to work/operate from a 24V buss.