Voltage regulator in AV-60007 912iS ignition module oscillates due to missing capacitor

[unladenswallow]

Some time ago while bench testing a faulty AV-60000 unit I noted that it included a KA7808ERTM voltage regulator but there was no capacitor between its input and ground. Such a capacitor is mandatory (specified in the datasheet) unless there is already a suitable capacitor nearby at that point in the circuit. In that unit, when powered from a bench power supply the regulator does not oscillate because the adjacent Pololu motor driver module includes a suitable capacitor. I found that with the motor driver module removed, the regulator oscillated.

Recently I had cause to look at the schematic of the AV-60007 which Vans, to their credit, include in the wiring harness schematic. I noticed that again there was a KA7808ERTM voltage regulator without a capacitor between the input pin and ground. This time there is no other nearby device that provides a suitable capacitor, and I suspected that the regulator might oscillate. So I took an oscilloscope to the airfield to check. It was indeed oscillating as can be seen by this oscilloscope trace of the output voltage.



The peak-to-peak oscillation voltage appears to be about half a volt, however I was using a long ground lead between the oscilloscope and the aircraft so it is probably somewhat higher.

Oscillation of a voltage regulator is undesirable. Its consequences might include the following:
- Disturbance to the Arduino. The Arduino has a further voltage regulator on board, however the oscillation frequency of the KA7808ERTM is quite high (over 2MHz) and the regulator on the Arduino is unlikely to be very good at ensuring that it is not in part passed on to the microcontroller.
- Interference with avionics. The oscillating regulator will draw current modulated at 2MHz from the aircraft bus, which may affect avionics. In particular it might cause interference with radio reception.
- Reduced life of the voltage regulator because of stresses induced by oscillation.

I fixed the oscillation by adding a capacitor between the input and ground terminals of the voltage regulator. The datasheet for the voltage regulator specifies a minimum of 0.33uF. I used a 2.2uF 50V ceramic capacitor because that was what I had available.



I confirmed that with the capacitor fitted, there is no more oscillation.

 

[KEG]

I'd like to see Van's respond to this!

 

[WJaviation]

Im interested in performing this "fix" as well. Is there any difference in installing the minimum capacitor size vs a higher uf capacitor?

 

[unladenswallow]

Im interested in performing this "fix" as well. Is there any difference in installing the minimum capacitor size vs a higher uf capacitor?

I wouldn't expect any significant difference. Bear in mind that applying voltage to ceramic capacitors reduced their capacitance. If I was ordering capacitors for this purpose I would choose something like 1uF 50V or preferably 100V X7R.

I didn't add an output capacitor to the 7808 regulator because the datasheet for the regulator says that the output capacitor is optional (although recommended) and I assumed that the Arduino Nano had an input capacitor that would serve this purpose. However, I've just checked the Arduino Nano schematic and it appears that the Arduino Nano does not include an input capacitor. So if I were doing this fix again, I would add an output capacitor to the 7808 regulator too.