Generators / Alternators Off for Start?

[Slick Goodlin]

Again, I worry more about transient voltage spikes
damaging electrical equipment which is powered on during cranking.

Not just cranking but I assume the spike of just turning on your charging system must be potentially damaging too.

A few years ago we had a medic at work who was in the habit of charging his laptop off the medical inverter in the back. The thing is, he’d leave it plugged in all day so it would get power as soon as the battery master was on, then more power when the generators were switched on. He couldn’t figure out why he had to buy a new battery for his laptop every 3-4 months but I figure that’s got to be the culprit. Come to think of it, when the voltage dips on start his laptop battery was probably trying to power the plane too.

[Colonel]

when the voltage dips on start his laptop battery was probably trying to power the plane too.

Too funny. They make diodes for that, but it was nice of his laptop battery to take one for the team.

This stuff is tricky ... electrical, and also chemical with the batteries ... I wonder if the voltage spikes
were aging his batteries? Generally their BMS will kick off when they overvoltage, to avoid damage and
fires from thermal runaway.

Anyone here old enough to remember the original ELT's we had to install, back in the day? They would
explode. Great safety equipment. This is complicated stuff, even if Air Canada pilots like Rockie say
that embedded software like this

https://en.wikipedia.org/wiki/Battery_management_system

is a stupid waste of time. Maybe someday I will be that smart.

https://en.wikipedia.org/wiki/2013_Boei ... _grounding

I mean, who gives a fuck if airplanes catch on fire?

[Slick Goodlin]

when the voltage dips on start his laptop battery was probably trying to power the plane too.

Too funny. They make diodes for that, but it was nice of his laptop battery to take one for the team.

On further thought (or any thought, really) I suppose for the laptop battery to power the airplane its DC voltage would have to be converted to AC (which the rectifier at the charger can’t do) then rectified back to DC at the inverter (which it probably can’t do either but everything inside that box is PFM to me. Seriously how TF does a DC-DC converter step up voltage?).

[Colonel]

Seriously how TF does a DC-DC converter step up voltage

By switching:

https://en.wikipedia.org/wiki/Boost_converter

A boost converter (step-up converter) is a DC-to-DC power converter that steps up voltage (while stepping down current) from its input (supply) to its output (load).

It is a class of switched-mode power supply (SMPS) containing at least two semiconductors (a diode and a transistor) and at least one energy storage element: a capacitor, inductor, or the two in combination.

To reduce voltage ripple, filters made of capacitors (sometimes in combination with inductors) are normally added to such a converter's output (load-side filter) and input (supply-side filter).

Pay careful attention to that last sentence. I had a comm fail once. The squelch was permanently open, despite
all the fiddling I did with the knobs. So, I bought a new comm. And it did the SAME GODDAMNED THING.

Turned out the portable GPS had a failing DC-DC switched power supply, which was injecting enough ripples
into the aircraft 12V system, to cause havoc with the auto-squelch circuitry on the fucking VHF comm which
clearly didn't have enough filtering on the DC input. I'd like to kick that EE's @ss.

Who knew?

Switched DC-DC voltage converters (both up and own voltage) can introduce nasty noise. LDO's (think of a voltage
divider, on the way down) do not, albeit at the cost of decreased efficiency.

https://en.wikipedia.org/wiki/Low-dropout_regulator



I'm sure ya'll roll your eyes at my technical gibberish, but it is a result of many painful lessons over many decades.

You need mechanical, electrical, hydraulic, pneumatic and chemical engineering knowledge to be a good mechanic.

[Slick Goodlin]

I figured a DC-DC converter must walk it through AC so that a step-up transformer would work before rectifying it back to DC. By switching it I guess you just get a sort of lazy, squared, half-wave AC?

[Colonel]

Right - you use switching, and some components (capacitor, inductor) which
are capable of storing electricity.

To get rid of the ripples you use inline inductors and capacitors to ground to
form a variation on a RLC (tank) circuit. Using a 2nd order ODE IIRC you choose
the values of L and C to give the desired damping of the resultant frequencies
of the switching. And in the lab, vary the L and C values and look at the scope.

Pardon if I got some details wrong. I took this in school, in the early '80's.

If you think about it, an old-fashioned points/condensor used in cars before
1975 and in aircraft magnetos, is a kind of switch mode power supply. It
pumps the voltage up from 12V to 20,000V using points to open the circuit
charging the coil at strategic times, and then the EMF collapses around the
coil and pumps all kinda voltage out to the spark plug. The condensor (capacitor)
across the points, is just there to reduce arcing across the points when they open.

We all know about noise from ignition systems!

Fucking switched mode power supplies. Give me an LDO any day.

This analog stuff is really complicated - far more than a bunch of ones and
zeroes. I haven't even explicitly mentioned harmonics.

https://en.wikipedia.org/wiki/Harmonics ... cal_power)

[vanNostrum]

[quote=Colonel post_
]If you think about it, an old-fashioned points/condensor used in cars before
1975 and in aircraft magnetos, is a kind of switch mode power supply. It
pumps the voltage up from 12V to 20,000V using points to open the circuit
charging the coil at strategic times, and then the EMF collapses around the
coil and pumps all kinda voltage out to the spark plug. The condensor (capacitor)
across the points, is just there to reduce arcing across the points when they open.
We all know about noise from ignition systems!


Old radio equipment with vacuum tubes that were powered by batteries, had a simple electromechanical
vibrator to switch the battery DC on and off, connected to the primary of an step up transformer , the 80-120 V
in the secondary was rectified and filtered to supply the DC plate voltage
An old door bell could easily modified for that purpose
In the late 20s when radios first appear in cars, they all had this vibrator that had to be replaced often
as the contact points didn't last too long
At a the time when a Ford A was around $ 650 , a Motorola or Philco radio $ 150

[Colonel]

the contact points didn't last too long

I'm pretty sure the ancient voltage regulator for the generator in my Maule has points:



Sometimes when it sits, it doesn't charge at first, and I think the points are stuck closed.


My Lycomings have a combined solid-state voltage regulator/over-voltage cutout
with their alternators. Oddly they do not have impulse couplings on the left mags,
like most Lycomings - they use this ancient technology Bendix vibrator to create the
spark for start, with a second set of retarded points. It looks a bit like this, bolted to
the firewall behind the left mag:



Right mag is normal, and is grounded out while cranking by the Bendix switch.

Honestly, I'd prefer a couple of hefty toggle switches for the magnetos, like the
Citabria and Decathlon. That and a momentary for the starter is all you really
need.

[vanNostrum]

Problem with electro mechanical Voltage regulators is any variation in the contact point's gap or
spring tension will affect the system voltage
The points live a hard life, the sparks between them oxidizes the metal
creating high resistance areas compounding the problems, but by and large
they do the job

[Slick Goodlin]

an old-fashioned points/condensor used in cars before
1975 and in aircraft magnetos, is a kind of switch mode power supply. It
pumps the voltage up from 12V to 20,000V using points to open the circuit
charging the coil at strategic times, and then the EMF collapses around the
coil and pumps all kinda voltage out to the spark plug. The condensor (capacitor)
across the points, is just there to reduce arcing across the points when they open.

You’d dig some of the electromechanical stuff that makes my Model T work. The trembler coils to spark the plugs are wild (and also work to energize an electric fence) and it used to have the mechanical equivalent of a diode on the generator output before I swapped that for a solid state voltage regulator.