Originally Posted by
Steph10025
Good! You're welcome.
Because this message has become so long I've switched to composing it in Notepad and pasted it into the window. Let's hope that does not result in a lot of orphan lines. If it does we'll understand why they're there. The message may also get truncated when I paste it due to length. We shall see!
You didn't mention that you intended to power the phone from the aircraft. There are a number of potential major problems with that which the technician may not be aware of.
In the first place if you look at my diagram you will notice that the microphone circuit is marked with polarity symbols. That's because electret mikes such as used in light weight headsets require a tiny DC power supply from the input circuit. Even the MIC jacks on the very first Sound Blaster boards for PCs provided this. You cannot isolate the mike circuit with a transformer without blocking this DC supply to the mike, in which case you would need a few batteries such as LR44s in the adapter to power the mike. That would be a nuisance. You would have to be sure to unplug the headset when not in use to prevent running them down. So you might be better to isolate the power supply circuit instead even though it might be more complex. Obviously this reflects my own prejudice against using batteries where avoidable even if it means more complexity.
I suggest you first determine whether there's a problem and how severe it is by connecting the phone to the aircraft headset jack using the adapter you built and checking to see whether you read DC voltage between any of the Nokia power jack connections and the aircraft power supply. If so that means there is no isolation of the audio in the aircraft.
It seems prudent to me for the aircraft audio to be isolated from the rest of the aircraft's power and ground for noise reasons. But it might not be and connecting to both power and the headset circuit could not merely introduce a ground loop but actually put a relatively unlimited amount of current through the phone, destroying it.
Isolation of the power supply, if necessary, can be done using a small DC-DC converter if you can determine the proper specs. But that may not be so easy. It's not certain whether the charging control circuit for any given cellphone is in the cell phone itself or in the external charger, be that an AC power line charger or a vehicle cigarette lighter adapter. If the charging control circuit is in the adapter and you don't reproduce its function accurately you may destroy the phone and/or the battery. I've seen phones that used both methods. So while it's likely that any modern charging adapter contains a DC-DC converter, it's not a certainty whether the charger or the phone controls the charging and whether the DC-DC converter in the vehicle charging adapter is isolated from input to output.
The Nokia AC line chargers I've seen do use isolated switching converters which have tightly regulated outputs but I don't know for sure where the charge regulation is done. AC adapters which use switching converters are easily recognizable by their very small size and light weight relative to ones that use a 60Hz transformer. And AC chargers must be isolated to prevent a shock hazard. Vehicle chargers don't necessarily need to be.
Since the location and requirements of the charging circuit are probably going to be difficult to determine and the Nokia vehicle chargers itself might contain an isolated DC-DC converter, that would be the safest thing to use if the aircraft has a lighter socket. But first you need to determine whether the Nokia vehicle charger is actually isolated by performing resistance tests between the output jack conductors and the cigarette lighter plug side of the charger or by plugging it into a vehicle and testing for voltage between the output plug of the Nokia charger and the vehicle chassis.
You should measure zero volts to both elements of the Nokia charger output plug relative to the vehicle chassis. An automobile is a perfectly valid environment for this test. If the aircraft does not have a cigarette lighter socket but 12V is in fact available and tests demonstrate that the Nokia charger is in fact isolated, using the Nokia adapter would by far be the most prudent solution. Buy a cigarette lighter socket at Radio Shack, such as a cord mounted one that does not require drilling a hole for mounting and which is fully insulated. Connect it to a fused 12V source in the aircraft.
If the aircraft operates at 24V and there is no 12V source available, a surplus 24V/12V DC-DC converter can be easily found to convert the aircraft voltage to what the Nokia needs. Since the charging control would still be performed by the Nokia adapter, the issues that would arise using a DC-DC converter directly without the Nokia adapter would not arise in this case with the two in tandem. It's also possible for the small amount of power the Nokia phone needs to be obtained using a simple resistor and shunt zener regulator to drop the aircraft power to 12V. We'll cross that bridge if we arrive there.
Let me know which of these possibilities plays out and makes sense.
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