USB adapters for airborne use

[India Four Two]

Yesterday, while flying my club's C182 towplane, I experienced very bad audio noise, which I thought was due to failed auto-squelch.

I called over another tow pilot, who after some trouble-shooting, pointed out the culprit - one of these:


I pulled it out of the socket and the noise went away. My colleague explained that these adapters are notorious for generating RF noise. Interestingly, the noise was only apparent on 123.4, our gliding frequency. It went away completely when I switched to 126.7.

So an interesting lesson, which leads to a question: Is there an adapter I can buy, which will not generate RF noise?

[airpolice]

Car Charger USB Socket for charging iPads, iPhones, GPS's and any other USB device | Tallon Systems Accessory Store

This might need an engineer to sign it off.




NEW: Tallon's Smart Double USB Charger

Takes 12-24V input & puts out 5V 1.0A & 2.1A simultaneously. Delivers enough juice to power Amp-Hungry devices like iPads, tablets and all other USB powered devices- all whilst blending seamlessly with your vehicle, marine craft or aircraft:

Converts 12/24V input to 5.5V/3.0A output
Will rapidly charge tablets, smartphones, GPS' etc.
When paired with our versatile Mini Socket Mounts, provides a full mount, power, data solution
CE Rated & designed to be retro-fitted quickly & easily
Requires no specialist tools or professional installation
See Mini Socket Installation pack for quick, clean & easy install
Available also in both Light Vehicle and Truck/Carling switch-blank USBs
£ 24.95

[rjtjrt]

Watch out for possible problems with USB chargers.
There are reports of MyGoFlight DualMicro USB charger issue, as in this customer review:

Amazon

Amazon

"On September 20, 2015 on a flight from w95 to KAGC we loss the ipad,
iphone, and the stratus. All three will have to be replaced.
During the flight my wife observed the ipad smoking at the charger
connection. She disconnected the ipad and we started using the iphone.
Not thinking that the problem might be the cigarette lighter charger
we plugged the iphone into the charger. It very shortly it stopped working.
Still not realizing the problem was the charger we did not unplug the Stratus
from the charger. Fortunately the flight was in VFR conditions and
nothing in the panel was effected and we were able to complete the flight.

I took the apple produces to the apple store and they could not get either of
them to respond and suggested I replace them. Kal at Appareo said
about the Stratus "the USB protect chip was blown which resulted in
damage to the circuit board". The failure seems to be the charger,
a mygoflight, it is outputting 24.4 volts when it should be only
outputting 5 volts. Ignoring the financial cost this a very
dangerous. We often fly long distances and find it convenient to just leave
the ipad and Status plugged into the charger."

[rjtjrt]

These are said to be US FCC approved re lack of interference.

https://www.pilotlights.net/dual-usb...st-charger-48a

[this is my username]

Safe Universal USB device charging in the air - Charge2 / Charge4

[GipsyMagpie]

The great flying ripoff

Sorry, but £100 for a USB adapter? A quality 4 way high power USB charger should be more like £10! I have the Anker onrs off Amazon for flying and have no issues at all. Or you could buy yourself a quality charge block several times over for definite interference free operation.

[RJC]

I designed and make the Charge2/4 units mentioned in this post, so if people have any questions I will answer as best I can.

The OP was the reason for starting the whole project, although it wasn't just radio interference but other odd instrumentation issues too in a friends aircraft. He had spent some time with engineers looking into issues that the root cause of was a plug in USB charger. Whilst his plug in charger was an effective short range radio jammer, it also had very bad conducted emissions so things were compounded as the aircraft wiring behind the panel was also acting as an aerial and path to other equipment for these emissions. When I shorted the USB output it also catastrophically failed after a few seconds.

I'll go for the most asked question in advance, the cost.
Well the basis of the design wasn't to start from 'as cheap as possible for 100,000 units' but to work backwards from compliancy with the ETSO-C71 regulations, and also providing the universal device support. They have been through testing at an avionics shop and the reports accepted by EASA, currently on a per type basis with the aim to have a wide ranging minor mod approval available for GA aircraft in the EU. In terms of USB chargers designed for aviation use, they are probably the cheapest out there.

[Jan Olieslagers]

Regarding the overvoltage: it is a danger few people seem to realise, until of course it is too late.

I am setting up my own (as per usual), basically using a cheapo LM2596 module from the far East. It can handle a wide range of input voltages, will produce 3 amps which should be ample, and can be adjusted. This last point is apparently relevant because certain devices take up so much current that one needs to offer 5,1 or even 5,2 V= at the other end of the charge cable.

But most importantly, I will definitely add an over-voltage protection, in the form of a crowbar circuit. There will be a fuse in the 5V= supply line, and downstream of that a thyristor to shortcircuit the output in case of overvoltage, thus blowing the fuse. That requires fast detection and a fast thyristor, I intend to use a little schematic by dear old Mr. Jim Weir.

PS back to the original issue of radio interference: yes, one should take care, inserting some inductors and (ceramic) bypass-capacitors in any application of a switching power supply. But I can't help having my doubts about the quality of the radio that - on one particular frequency! - was so upset with the switcher's noise. What make/model was it?

[The Ancient Geek]

This is the result of air radio being AM for historical reasons..
If there is interference on a channel an AM radio will open its squelch, an FM radio will usually be desensitised until a stronger signal arrives.
The safety implications of annoyance versus desensitisation are debateable, either way the channel is blocked.

Any switchmode power supply is a "usual suspect" for generating interference on the harmonics of its switching frequency. Consumer gadgets are very price sensitive so the makers tend to leave out suppression to save money.

[horizon flyer]

The same RF problem can occur with LED lighting with built in switch mode voltage regulators, wipes out FM radios. These control the output by switch at very high frequency the input voltage off and on dependant on the output voltage required. So connect the input voltage 14volts, in an aircraft to the output that requires 5 volts. As the output goes through 5 volts the switch is opened and the voltage falls, a capacitor across the output smooths this out. The faster the switch time and higher the switch frequency the smaller the parts can be. ( cheaper ) With these high frequencies in the MegHz range and harmonics the interference can stretch up in to the 100s of MegHz range if no radiation precations are taken, which costs money and space. Most of the cheap regulators only have an IC and capacitor plus few of parts with no overvoltage input or output safety circuits which in most cases in cars is ok but in a plane with a master & alternator switch there is an opportunity for Alternator runaway. This can happen if the battery is disconnected from the alternator when the engine is running, an expensive fault that can wipe out all the electronics if they don't have overvoltage protection built in. I would suggest the power socket had an over protection and RFI filter on the back of it plus the USB 5 volt had a pass through adapter with an overvoltage sensing circuit that blows a fuse if the max input voltage to your device is exceeded. Note if you ever need to change or update the regulator in your aircraft a US company called Zeftonics make a good after market unit with a lot of this built in protection.

[India Four Two]

Thanks for the discussion everyone. Very interesting.

These are said to be US FCC approved re lack of interference.

https://www.pilotlights.net/dual-usb...st-charger-48a

rjtjrt,

$14.95 - that's cheap enough (even after converting to Canadian dollars) that it is worth experimenting with. I'll get one and report back.

Jan,
I don't know the radio name and model! I normally never touch it and only look at the display to see if 123.4 is selected. I'll check next time I'm at the club.

I'll also try other frequencies using the USB adapter that caused the problem.

[India Four Two]

but in a plane with a master & alternator switch there is an opportunity for Alternator runaway. This can happen if the battery is disconnected from the alternator when the engine is running, an expensive fault that can wipe out all the electronics if they don't have overvoltage protection built in.

hf,
Can you elaborate? This is an issue I had not heard about before.

[RJC]

Some good points raised in this thread;

You do need protection on the connections, the ETSO standard on the input has the following tests (note they state 13.75 VDC & 27.5 VDC as the nominal voltages);

2.4 Over Voltage
There shall be no permanent damage to any of the solid state devices (transistors) or the components when the power converter is delivering full rated output power and is subjected to the following over voltage conditions:

(a) The input DC voltage is increased to 50% above the standard operating voltage for a duration of not less than five minutes.

2.5 Short Circuit Conditions
There shall be no degradation of the power converter or its components as a result of a sustained short circuit applied separately to each output of a multiple output power converter, or simultaneously to all DC outputs for a period of not less than one minute. Within five minutes after removal of the short circuit condition, the equipment shall be capable of continuous operation at the manufacturer’s rated output load for a period of eight hours without, degradation of performance.

This kind of implies that an input event should not carry through to the output, although having output protection is no bad thing.

Disconnecting a battery when the charging circuit is active can cause what is known as a Load Dump in the automotive world. This can cause voltage spikes into the DC bus of circa 120v peak, but only for ~400-500 milliseconds (0.4 - 0.5 seconds) but a fast response is needed to handle these events or they will cause issues. A brief description of what causes this can be found here https://en.wikipedia.org/wiki/Load_dump

Also please remember that interference may not be limited to what you can hear on the radio, the aviation emissions testing is very different to the normal CE / FCC testing in terms of ranges and power levels. They have lower limits for the radio NAV/Comms bands and GPS frequencies.

I have seen lots of people recommend various plug in supplies, some work for some people and not others. Some won't charge certain types of products, some can. There is a sweet spot of the charger, aircraft electrics, installation, and equipment fit that can make this hard to find in any given situation.

One of the worst I've seen was a scratch built unit based on one of these (pic below) from eBay (5 for £4.99 in shipping from China). The voltage regulator IC (LT2596) must be a fake, as 10k quantity direct from TI costs more than they sell the whole product for. When pulling just 1A out (they are nominally rated at 3A) the output had a 800mv (0.8v) ripple. With it at 12v and set to 5.2v output pulling 3A the output at the board fell to 4.8V, it failed after a couple of hours I suspect due to overheating.
Basically if you want to build your own, please don't base it on one of these.

[India Four Two]

Update

I went out to the field today to have a look at and to test the radio. It was cold with a howling cross-wind. I expected to be alone, but in true gliding-club spirit, there were four other members there, doing odd jobs - fixing tractor hydraulic hoses, water-well maintenance, etc.!

The radio is an Icom A210 COM radio - 25 KHz spacing - 118.0 - 136.975 MHz.


A210 VHF Air Band Transceiver - Specifications - Icom America

I tested which channels were noisy when my USB adapter was plugged in, by listening to each integer MHz frequency from 118.0 to 136.0.

The auto-squelch function was defeated from 121.0 to 125.0, with the most noise on 122.0 and 123.0. All the other COM frequencies were not affected, except for 118.0, so I expect that the interference might extend down into the NAV frequencies (no NAV radio in our 182).

hf and RJC,

Following on from your posts, I dug out the manual for our 1970 C182N, which has the typical Cessna red split master-switch. The manual is not very informative about the master-switch operation (which is typical for manuals of that era). In the Emergency section, it states that in the case of non-charging or over-charging, the ALT side of the switch should be turned off.

There is no discussion about turning the BAT side off and leaving the ALT on. Is there any risk of "Load Dump" in that situation?

[RJC]

Originally Posted by India Four Two

I tested which channels were noisy when my USB adapter was plugged in, by listening to each integer MHz frequency from 118.0 to 136.0.

The auto-squelch function was defeated from 121.0 to 125.0, with the most noise on 122.0 and 123.0. All the other COM frequencies were not affected, except for 118.0, so I expect that the interference might extend down into the NAV frequencies (no NAV radio in our 182).

As expected really, some of these chargers are worse than others, and shows why it isn't just the radio comm that can be affected.

Originally Posted by India Four Two

Following on from your posts, I dug out the manual for our 1970 C182N, which has the typical Cessna red split master-switch. The manual is not very informative about the master-switch operation (which is typical for manuals of that era). In the Emergency section, it states that in the case of non-charging or over-charging, the ALT side of the switch should be turned off.

There is no discussion about turning the BAT side off and leaving the ALT on. Is there any risk of "Load Dump" in that situation?

The ALT switch is used to energise the field coils in the alternator, which in effect turn it on. The voltage regulator (or one built into the Alternator) will regulate the current into the coils depending on load, its constant adjustment keeps the voltage stable. If you turn off the ALT switch the field coils will take a second or so to dissipate but that is fine, the output will be decaying all the time. So if you get an indication of no alternator output then cycling he ALT may bring it back if there is some kind of tripping protection in the circuit.

If you turn off the battery, and there is a heavy load on the bus such as charging the battery following a start (30A for example), then it takes the regulator a small amount of time to turn down/off the field coils, then there is the time for the field in the coils to decay. During that time the current produced has nowhere to go and is seen as a high voltage on the bus, this is the load dump.

i suspect the only place in the manual where it mentions turning off the BAT side will be in emergency procedures for inflight fire somewhere. I'm a rotary pilot and that is the only place I have found details in a POH of turning the BAT off with the engine running, can't really make it worse at that point!

Note you can get mini load dumps during normal use, say you have landing lights that draw 10a+, when you turn them off it takes a finite time for the regulator to adjust for the loss of the load, this is seen as a small voltage spike on the bus, but equipment is designed to ride this out without causing any problems. This is why the specs say equipment should be able to take a higher voltage without issue, they are just a fact and need to be handled.

The perfect storm is to start up, have the RPM increase so the alternator is at speed trying to charge the battery following the start, and you turn the battery switch off. That would be nasty event power wise.

In normal circumstances a load dump is pretty rare, at least in he automotive world, as people don't have a battery switch to play with. When shutting down post flight I always leave a couple of seconds between turning the alternator off and master power to ensure the field coils have time to de-energise.

[Fly4Business]

USB adapter are a science ... I ended up in looking for small adapters which are also sold by aviation shops. Buying cheap in the next china store is the worst I can think of, after encountering two burning adapter. There is quite some electronics in that little beasties and obviously it can be done very different.

[horizon flyer]

Yes the probability of alternator run away is very low and it is connected to the input side of the master switch and always connected to the battery, but a good starting technic is always switch off the avionics master and alternator master only battery master on for start. This means the battery is not supplying the alternator field, 60amp ones draw about 4amps and it removes the drag of turning it, kinder to the battery. Once engine is stabilised, then alternator on check voltage and charging then avionics on. Just good practise.

Remember as said by railway engineers for the last 150 years if it can go wrong it will, even if millions to one.

I even made the mistake on my boat by switching over batteries with a none make before brake master switch took out the 12 volt regulator in my GPS but still runs on internal batteries but smokes on 12 volts external power and I should know better as an electron engineer.

So I think some form of fast over voltage protection is needed built into the plane with the amount electronic kit used and relied on even VFR these days and these cheap USB chargers are a bit of gamble even Apple states this about after market chargers.

[RJC]

Originally Posted by Fly4Business

USB adapter are a science ... I ended up in looking for small adapters which are also sold by aviation shops. Buying cheap in the next china store is the worst I can think of, after encountering two burning adapter. There is quite some electronics in that little beasties and obviously it can be done very different.

I tested quite a few plug in chargers, the result was an eye opener to just how bad some of them were. Often they are the same inside just branded differently, easily spotted by very similar shapes in different colours. Some I would not even put in a car, let alone in a cockpit, at least in a car I could pull over and put them out.

There are just so many of them out there it isn't really possible to enforce standards, people buy them, so people will sell them. I got an e-mail recently from a supplier in China offering mains USB chargers, a 2 port 2.4A charger for $3.99 each for 2000. That was their RRP before discount and included shipping. All the blurb says how they fast charge devices, being able to charge 4x times faster than the original charger. Which would be great, but it is the device that decides how much power to draw not what the charger is rated to supply, utter

Originally Posted by horizon flyer

So I think some form of fast over voltage protection is needed built into the plane with the amount electronic kit used and relied on even VFR these days and these cheap USB chargers are a bit of gamble even Apple states this about after market chargers.

If you are installing things into a plane, they should be designed to meet the specification of the environment IMHO. Looking to retrofit or design in an additional aircraft wide OV protection device really doesn't address the problem that if the electronics being fitted doesn't have suitable protection for OV, then what else doesn't it do correctly...

[India Four Two]

but a good starting technic is always switch off the avionics master and alternator master only battery master on for start.

hf,
Yes, I use this technique for starting. I then watch the ammeter when turning on the alternator, to confirm that it starts charging the battery.

i suspect the only place in the manual where it mentions turning off the BAT side will be in emergency procedures for inflight fire somewhere

RJC,
No, it's just "Master OFF".

[aox]

With Nokia phone car chargers, I found that their own are very good about not causing RF noise on the car radio, whereas a couple of generic ones were terrible.

There is a Nokia car charger with two USB sockets. This might be a candidate for testing for the use here.

Beware if buying on eBay or similar. Some chancers seem to count the word genuine as an adopted brand name rather than an honest description.