Teardown – Random Projects

Aircraft Alternator Teardown

The aviation industry might make you think that aircraft alternators are made out of special materials, have been tested to impeccable standards and are manufactured on special production lines sprinkled with pixie dust. A normal engineer couldn’t hope to service one. The reality is that they are 1950’s or 1960’s American car (like Ford) alternators that wouldn’t stand up to the quality, power output or or rigours of a modern car alternator. Modern car alternators can produce a smooth output from 750 to 6000 rpm with little or no noise in the output because the tolerances and power electronics / frequency are far more advanced.

Mid overhaul, rotor shaft removed. The red ring is the laminated stator ring. The top casting has a simple sealed ball bearing and the rear casting contains a roller bearing. The through bolts contain 2mm holes cross-drilled at the end of the thread for lock wire. (actually the same alternator as first figure)

Basic Operation

The principle of operation is simple, a rotating magnet induces an alternating current in a stator winding (hence the name alternator) that is then rectified and delivered to the battery.

The figure above shows the schematic layout. The stator would work with a single coil but by arranging 3 at 120 degrees (equally around the circle) the sine waves from each phase sums to a smooth output. In theory it shouldn’t whine or produce large amounts of ripple or pulse. This 3-phase approach also doesn’t require a return path at the centre of all 3 connected coils because each phase works against the other two. The aux output (at the centre of the windings) isn’t required and can be left unconnected.

Stator windings. The bearing at the bottom. The stator is laminated steel which helps control eddy currents increasing efficiency. The round things are diodes of which there are 6 because its a 3-phase rectifier.

rectifier plate mounted at the end. The plate is the output of the positive end of the diodes which leave via the output terminal (left) the other 3 diodes go to ground which is the alternator casing (top two and bottom right). The white wire is tied to the centre of the 3 coils which leaves to the aux terminal which is isolated from the rectifier plate. One or two needle bearings are visible in the bearing (centre) and the black ring is an oil grease seal.

Regulation

We said that all we do is swing a magnet around in a coil to produce our output but obviously, there is more to it because that would overcharge the battery. If an alternator became detached from the battery, the output voltage may reach hundreds of volts.

Aircraft alternator regulators tend to be very simple on or off affairs and the electric regulators are in some cases mechanical. They are unreliable to the point that its typical to include an over voltage relay (OVR) which cuts off excitation completely if a preset output voltage is exceeded. For a 14V system this would typically be set to 16V. If the system reached 16V the relay will latch on turning the alternator off.

To regulate the alternator output we want a variable strength magnet in the rotor instead of a permanent magnet. In an alternator the rotor is an electromagnet and when the battery is low the strength of this magnet is high and when the battery is fully charged the magnet is its weakest and its the job of the regulator to control this. In a car this will be built into the alternator and it will use fast modulation of the rotor current in a closed control loop to produce an output that is always spot on. In an aircraft regulators tend to be separate units like they used to be in cars. In cars they stopped doing that because regulation control is poor, being affected by voltage drops, stray magnetic fields and radio interference as well as emitting radio interference all over the place. What normally happens when there is parasitic resistance is that the voltage will fluctuate, often oscillating, but even under the best situation its poor.

The rotor. The slip rings (copper coloured) connect a single winding axially wound around the centre former. The windings are visible.

An electromagnet is just a winding, and winding around an iron core concentrates the field. As such, the rotor is a single winding around an iron former. The two interlocking sets of fingers are opposing poles (each end) of the electromagnet which means that the rotor actually has alternating N-S-N-S… arrangement. As we can see in the picture, one finger is part of one end and the next finger is part of the other end. Just like the stator where there are multiple windings, there are multiple poles rotating in the stator which improves output power.

The problem of how to connect the windings is solved using slip rings where graphite brushes are pressed against the rings by springs to maintain contact. The graphite has good conduction, is hard wearing and also lubricating.

The external regulator is a sealed unit that essentially varies the current flowing through the rotor from terminal F to ground via the output transistor T3. If T3 is fully on, then F would be driven to ground providing full current, typically 2 – 4 Amps.

The heart of the regulator is a reference voltage. A zener diode has a fixed voltage drop across it which can be used to work out if our measured voltage is above or below a threshold.

When the system voltage connected to ‘I’ terminal reaches a value at which the Zener diode connected to the divider network conducts, current will flow from the ‘I’ terminal through Rl through Z1 causing Tl to conduct which diverts the base current of T2 flowing from ‘I’ terminal through R4 to ground, turning off T2 which turns off T3, de-energizing the rotor Winding. When the alternator output voltage falls to a value which permits Z1 to cease conduction, Tl will turn off which turns on T2 and T3, re-energizing the rotor winding. Filtering is provided by C1 which usually dries out and can’t be fixed.

This type of regulator is on or off but happens so quickly that the average current flowing through the rotor should be proportional to the charge current required and thus we can loosely refer to it as “regulating”.

Its fairly easy to bench test the regulator by using a variable bench supply, and a bulb in place of the field coil, if the voltage is below about 14 the bulb should be on and should go out as the voltage rises. You can vary the point slightly using a variable resistor (potentiometer) P1.

Self Preservation

Of course, the rotor windings are unlikely to always have perfect connection to the regulator due to the slip ring arrangement and because the rotor is a big inductor, the resulting field collapse when it becomes interrupted will induce large voltages which would rapidly result in the destruction of T3. To quench these spikes the designer has used a neon bulb L1. This is quite neat, a neon bulb will strike an arc at about 95V and will then clamp the voltage to about 60V. I would expect the bulb to have a hard time if the slip rings were dirty and I’m not sure how often they fail in practice.

In contrast a modern automotive alternator such as Bosch has solid state protection, diodes with larger breakdown breakdown voltages and much more robust transistors. This means regulator failure isn’t really a thing even if the output is shorted or reversed. The most common failure mode is bearing failure or salt water corrosion.

For aviation alternators regulator, wiring faults, poor earths, marginal batteries, diode and brush failure are all so common that if it were in a car it would fail every 50,000 miles or so – which is about what you’d expect from a 1950’s alternator.

Finally, although the output ought to be smooth because of the use of 3-phase the diodes do take some forward voltage to conduct so the current will suddenly kick-in when they become forward-biased, and this can caused a whine on the electrical system that may be heard on radios and amplifiers. So an output capacitor is typically fitted to the output terminal to ground usually around 2.2uF. These also dry out and should be replaced if a whine is heard and they can be doubled up to get 4.4uf if its still an issue. The suppressors supplied with Prestolite alternators are between 1 and 2.2uf but they have such a high ESR (effective series resistance) of 15+ Ohms that they might as well not have bothered.

Zaon Flight PCAS MRX Teardown

Zaon MRX – Approx 1.9 miles – 500ft above descending.

The Zaon PCAS portable collision avoidance system is a very simple box which is considered as carry on – therefore requires no certification. Aircraft carry transponders which are interrogated by ground based radar and respond with a signal which may include height information. This is used to plot the aircraft on an ATC screen.

The Zaon device listens to the responses from nearby aircraft and estimates their distance from you based on signal strength. If the aircraft is transmitting height it will also show you that and if it is climbing or descending.

It prioritises the received aircraft and gives you a warning. It does not tell you in which direction it is – but its enough to add a level of safety.

The main PCB is split into 3 distinct parts. On the left there is the power supply, its a buck-boost circuit which provides 3.3 from batteries that may below 3v. An external connector uses the same power supply and can operate up to 28V.

On the right there is a radio front end which operates on 1090Mhz, with basic decoding of mode C signals.

In the middle there is a microcontroller that does all of the processing and drives the display. My guess from the label P16F877 is that its probably a Microchip Pic 16F177 controller. I’m familiar with these from my past, they are a 40 pin controller with USB connectivity – you can see it does have 40 pins. These are still in production in 2022.

The only other point worthy of note is the display which is insanely bright. A broadcom device found here.

Sadly, Zaon no longer seems to exist but Garmin make an XRX version which gives quadrant directional information. But to be honest you can achieve the same today with a Raspberry PI and a software defined radio – which also allows you to decode ADSB.

Charging 2S lithium from 5V

I recently bought a toy car for my Son which has a 2S lithium battery and comes with a USB charger FTX Tracer Truggy. I couldn’t get the toy off him, so I bought my own monster truck version – but my charger had a flashing red light whereas his has green when charged with red when charging. So I thought mine was broken. (it wasn’t it was just a slight variation)

When I removed the case I was puzzled for a while how this works. The battery is a 2s battery which is nominally 7.4V. Normally if you want more than 5V you’re going to need an inductor or switched capacitor boost circuit. But there are none on here. Only a couple of tiny supply decoupling capacitors; so how does it work?

Why is it interesting?

Above is what happened in my living room a few years ago when charging a toy helicopter. This was a 7.4v/2s battery of 800mAh capacity. Even for such a small battery the fireball was huge and very hot. This is why I never leave them unattended. You can see that the battery charger has only 2 wires and so there cannot be any balancing. I now charge lithium polymer batteries in the kitchen – on the stainless steel stove top. That way, fire cannot spread, there’s a cooker hood to extract any fumes and in the worst case, the stove can be easily replaced.

Unlike older rechargeable batteries, lithium cells do not tolerate over-charging with the risk of spectacular explosions. So battery packs with a series (2s, 3s….) of cells can be charged in series but if one cell has a lower voltage than the others then serious problems result. Its not as easy as limiting the charge current with a resister like in the good old days of NiCads.

You need to switch from constant current to constant voltage at around 60% state of charge and each cell may have a different state of charge. Cheap chargers monitor each cell and terminate charging when any cell reaches 4.2V, so a cheap drill battery may seem to get less and less capacity until its useless, all you actually need to do is charge them all independently to restore its capacity.

A proper balance charger monitors each cell independently and prevents that an individual cell charging whilst continuing to charge the rest. This is not trivial because they are all still electrically connected together.

A proper balancer will provide a voltage that is at least 4.2V x <cell count> and whist charging will shunt each cell as it becomes full. This means that current bypasses the cell through the transistor Q1 – Q3 and continues to charge the remainers.

Shown above is a discrete (made from bits not a chip) self-balancing system. It uses a simple zener and transistor that starts to turn on as the cell approaches 4.2V, its adjustable per cell because components are never exact. Its great because it can’t really go wrong.

No Shunt, No voltage boosting

Taking a closer look at the charger, we see USB plug on the right with 5V available. On the left we see the 3 wires going to the cells, the centre wire being a tap between the cells. To the right of those wires we see 5 transistors. And the whole thing is controlled using an anonymous (no label) micro-controller. This left me puzzled for a bit but an hour later I worked out what it must be doing. Its charging one cell, then the other and the transistors are used to connect each cell to the charging “bus”.

You could do this one cell until full then the other – but if you were to then interrupt the charge, say in a RC car, then the flat cell would end up reverse charging and that would ruin it. A quick connection to a multimeter shows that the charger charges each cell for 1 second then switches to the other and back again continually until charged. This swapping batteries coincides with the flashing charge light. I think the charge current is limited by the large 100 Ohm resistor, and the micro controller will also be able to measure that. Since USB is 5V and a fully charged battery is 4.2V that leaves 0.8V to be dropped by the transistors and the current limiting resistor, which you can just about do with normal transistors which have a voltage drop Vce of about 0.3V. So that would be 0.3+0.3 leaving 0.2V drop on the resistor.

Overall, its a clever design and may have been obvious, might even be common with cheap toys that require more than a single cell – but I hadn’t seen it before.

My concern is referring to the charge vs. capacity graph – when charging and relaxing as opposed to continual charging, I think its much more difficult to detect when the battery is at 100%. So it probably doesn’t optimally get the last 10%. Having said that, pulsed charging and charging to 85% is usually good for the battery. In an EV or Hybrid Vehicle for example, the battery takes charge and discharge intermittently at any state of charge.

Another concern is that the micro-controller runs a software which can easily glitch or get stuck. You’d hope the software had a watchdog reset that resets it if it does but if it doesn’t then you could find that the circuit dumps 1.2A into one cell until it explodes. That cannot happen with the discrete design, for safety you probably want to see that the cell voltage can never exceed 4.2V but doesn’t look like here is any stuck protection, unless those big blobs that look like diodes on the output are 4.2V Zener diodes, such a thing does exist – but they appear to be marked ss34 which is basically a 3A shottkey diode to stop the charger being `driven` from the batteries.

Finally, if a lithium cell is over-discharged (below 2.5V per cell) you cannot whack a full charge current on to it to start charging, you need to charge at a low rate until the voltage recovers, then resume charging. Similarly, you must also take care charging hot batteries, its better to reduce the current or wait for it to cool before charging. A proper BMS (battery management system) will typically have temperature monitoring for that reason.

For all its cleverness – probably best to never leave it unattended.