Rotax and the 22,000uF smoothing capacitor

[Kyle Communications]

Much larger heatsink and a proper heatsink not a cast bit of scrap alu is used for the unit.

[RFguy]

9 hours ago, aro said:

How likely is that to be a real problem? I have never heard of it in practice.

 

That switch needs to be rated to open a 20A+ DC circuit - is the capacitor going to draw enough energy to damage the switch when it closes?

@ARO, I was looking around on the net for somdthing that might be a good treatment on switches- 

here is a useful article you might get something from : 

http://www.aeroelectric.com/articles/Switch_Ratings.pdf

remember- he is talking toggle switches- not relays. Relays do not have the same mechanical advantage as a toggle switch to open a stuck contact. 

 

Edited December 28, 2020 by RFguy

[skippydiesel]

I learn something new every day - you can write a paper on toggle switches - who would of thunk?

Edited December 28, 2020 by skippydiesel

[skippydiesel]

So Kyle & Glen - For us less talented types - do we have concise advice/conclusion that we can act on??

[RFguy]

it depends if you have the capacitor or not.

and a hard and fast answer is hard because the wiring for different rotax installs varies. not sure how much there is common. If the charging system is taken to the battery direct and the master bus is taken from the battery, direct (via contactors, fuses etc) there should be little noise. The more the system departs from direct conections, the more tha capacitor is required becausethe master bus begins to see the ripple currents that are common between battery connections.

 

 

[skippydiesel]

Okay - will get back to you when I have had time to thoroughly familiarise myself with new aircraft (with CV-19 that may be some time off)

[Kyle Communications]

The cap is there to absorb the spikes and crap coming from the reg. The rotax engine manual actually states that it an "optional component". It appears its main function to Rotax is noise filtering not so much ripple filtering

 

Glenn is correct it does depend a lot on how the wiring is setup. Everyone has a different idea about how it should be done. It depends on how you look at it..the perspective I mean. What Glenn and I would do is not necessarily the same as stated in the manuals. We look at it more from a electronics point of view and protecting those expensive bits of silicon

 

For example every single power supply requires both ripple filtering for the rectifier and also noise filtering for interference and also there needs to be voltage spike protection.

Powersupplies most often have a zener diode across the output. For example a DC supply that is used for say your base station aviation radio may have a zener diode across the output. That is to put a direct short on that output if the reg craps itself and puts more than 13.8V on the output. The 15 or 16V or even 18v zenner will conduct and cause a massive current to flow that then blows either a DC or the AC fuse in the supply. This should protect the equipment attached to the supply.

 

This is no different to the DC being generated in your aircraft. There needs to be ripple conditioning for the actual DC coming out of the reg then there should be some sort of over voltage protection which there is not in any Rotax install that I have seen and most installs have a fuse somewhere that blows when your radio or EFIS becomes the zener diode

 

[Moneybox]

Today I was back trying to sort out my charging system on the Zenith. When I left for a 3hr flight on Tuesday afternoon I looked at the plug-in volt meter and it was showing 13.4v so I was quite happy with that but a little while later it dropped to 12.4v and by the end of my flight it was on 12.1v. This is with my Ducati copy $44.77 rectifier.

 

 

On the trip home it started out at 12.1v and ended on 11.4v. I was using a USB power pack to look after the iPad.

 

 

I started out with a stator check again. No problems there with the AC voltage rising nicely at higher revs so then I went looking for high resistance wiring or connections. All looked good until I took the wires off the capacitor and checked that. I never got a resistance reading but each of the terminals sounded the beeper on the ohm meter. There's a leak to earth from both terminals.

 

 

Now with the capacitor removed from the circuit I have good battery charge. It was 22000uF 40v so now I'll go looking for a replacement.

 

 

[Kyle Communications]

Capacitors do "wear out". age and the constant pounding of the pulsed DC they are trying to smooth takes its toll.

usually replace the capacitor every 5 to 8 years or so depends on usage but age also plays a part in the capacitor ability to filter the DC

 

[Kyle Communications]

You can get good quality caps from Element14 or RS components make sure though you get a 50V or 63V rated one to be on the safe side. Do not fit a 25V one and it needs to be at least the 22,000 uF capacity