Wings Forum

gme9261236 · Joined: 27 Jan 2007 Posts: 95 Location: Hampton, NH 7B3

Is everone using the 25v 22000 mf capacitor for their regulators? My master switch is at the battery in the back, In case of an electric fire and I turn off the master will the engine run right without it.If not where can I get one,radio shack only has small ones with bare leads nothing lihe the one in install vidios .Thanks Glenn

KevinC · Posted: Sat Apr 26, 2008 3:47 pm Post subject: B & C

Here's where I got most of my electrical stuff. http://www.bandc.biz/
and I got a Rotax wiring diagram here (figure z17): http://www.aeroelectric.com/articles/Rev11/AppZ_R11M.pdf

K

alan · Joined: 04 Feb 2005 Posts: 116 Location: Baytown, TX

Yes, the engine will run just fine with the master switch off. The capacitor is strictly to smooth the voltage for your accessories.

Alan

gme9261236 · Joined: 27 Jan 2007 Posts: 95 Location: Hampton, NH 7B3

Alan, Did you not use one, is it nessasary thanks Glenn

scubarider2 · Joined: 28 Jan 2008 Posts: 224 Location: Cleveland, GA

I did not use one and so far no spikes etc from the volts gauge. It stays right where it needs to be except when I put on the landing lights....then down it goes :lol:
Dennis

alan · Joined: 04 Feb 2005 Posts: 116 Location: Baytown, TX

I am using an XCom radio and the manufacturer not only requires a capicator, they supplied one with the radio. The volt meter is not going to indicate a spike that could damage your equipment because spikes are really quick. The capicator smooths the voltage and should be considered manandatory for the Rotax installation.

I'm sure there is someone, Wes maybe, that can explain this better than I.

Alan N1010Z, 178 hrs and counting.

Wes · Posted: Mon Apr 28, 2008 4:35 pm Post subject: Capacitor

I agree with Alan, the filter cap is mandatory with Rotax engines, especially if you have any sort of sensitive electronics installed that are fed off that bus.
The way Rotax generates DC current for the electrics, is with a permanant magnet 'alternator' inside the rear case. The 'alternator winding supplies an AC voltage whose amplitude and freq. are directly related to the RPM of the engine. This AC voltage off the alternator winding is fed out on the 2 yellow wires to the supplied rectifier/regulator. The output of the Rect./Reg. (pin B+) is where the DC voltage appears except with a fair amount of 'ripple' AC riding on top of the 14VDC and this is also where the filter cap should be connected with some fairly heavy wire, #10 ga. If we had the battery (which acts like a BIG capacitor) right there next to the rect/reg. to connect to, we wouldn't need the cap. But our Highlanders are not configured that way.
You could maybe get away without a cap. if only running lights or pumps or something that doesn't care about noise on the DC bus.
The filter cap recommended by Rotax (22K ufd at 25VDC) should be considered a minimum rating and more filtering may be necessary depending on how heavily the bus is loaded (more load = more ripple) and how sensitive the devices are.
A couple of examples might illustrate.
- On Lynn's Highlander, her Dynon D-180 was giving all sorts of erroneous amps and fuel flow indications until a filter was installed.
- any audio system with a 'whine' that follows the engine RPM would be a dead giveaway of a filter problem.
I bought a cap. from Lockwood at Sun-N-Fun to fit on Lynn's airplane and thought $34 was highway robbery for a 22K ufd. at 40 VDC cap., but then I started pricing the 'Computer Grade' caps at Digikey and Mouser and the price is not far off.
Stick with a quality brand (Mallory or Cornell Dubilier) and mount it behind the firewall, not in the hot engine compartment. These things do fail and when they do, they generate smoke. But they are a whole lot less likely to fail if kept cool. The higher the voltage rating, the less likely to fail, so go with a 40 VDC rated cap.
Keep in mind that caps. are additive , so two 11K ufd tied together in parallel will give 22K ufd. The more the better. watch polarity! plus to plus to plus and minus to minus to minus, etc.

Wes
Kit #95, yet to install my filter cap.

KevinC · Posted: Tue Apr 29, 2008 12:37 am Post subject: capacitor

Wes,

I noticed the B&C Specialties site recommends a 47000 uF capacitor @ 16V for the Rotax (http://www.bandc.biz/cgi-bin/ez-catalog/cat_display.cgi?6X358218). What up?

k

scubarider2 · Joined: 28 Jan 2008 Posts: 224 Location: Cleveland, GA

I have all the standard gauges, Lowrance 2000c airmap, stobes, nav lights, icom radio and intercom. No hum, no problems at 50 hrs. Still need a capacitor? If so, what do you recommend?
Dennis

stede52 · Joined: 06 Sep 2007 Posts: 209 Location: Carnation, WA

According to the 912 wiring schematics and the HomeBuilt help DVD you will need 25v 22000 mf capacitor and you can get it at http://www.tedss.com/2020004642/, or just google it and you'll find a number of places that sell them.

Steve

Wes · Posted: Tue Apr 29, 2008 5:30 pm Post subject: More Caps

Kevin,
I guess I would feel uncomfortable with a Voltage rating of 16V. The voltage rating is the max operating voltage that the cap is designed to withstand on a continuous basis. These caps are probably conservatively rated, but I would feel much better with one that could tolerate a higher voltage (I think the next available is 25VDC). Recall that the AC 'ripple voltage is riding on top of the 14VDC and the peak of the AC voltage is added to the DC level, so potentially with a ripple amplitude of 4 VAC (peak), the cap could see 14 + 4 or 18 volts peak. I admit, I have never measured the ripple voltage coming off the Rotax systems (takes an oscilloscope and an unfiltered bus), but my guess is it could be at least 4 V peak. The failure mode for these aluminum electrolytics is usually a 'punch-thru' of the very thin chemically formed electrolyte. The 'punch-thru' is caused by voltage spikes and is a cumulative process that eventually results in catastrophic failure of the cap thru internally generated heat and cooking of the electrolyte chemicals (say SMOKE).
If anyone has a scope they could get near a "running" Rotax (mine will be someday), it would interesting to measure the ripple voltage coming off that rect/reg. under different load conditions. That is data that is not in the Rotax documentation.

Dennis, Interesting that you see no symptoms of the AC in the system. My guess is that it is certainly there, but your avionics are somehow immune. Count yourself lucky!

Wes
Kit #95 (soon to have a big ol' fat filter capacitor)

gme9261236 · Joined: 27 Jan 2007 Posts: 95 Location: Hampton, NH 7B3

I am having a hard time finding a cap that meets the exact specs you guys have mentioned. Would this do the trick?

It is 23,000 UF (not sure what that is in relation to MF) 35 Volts and is relatively small at 4" tall by 2" round with easy screw terminal connections. Cost of this unit is 15 bucks which is reasonable. If you experts think it would do the trick say the word and Ill get it ordered!

Wes · Posted: Thu May 01, 2008 7:46 pm Post subject: Cap

Hey, that cap looks like a 'winner'!
And $15 is a good price.
The industry is kinda lax about units nomenclature, but it should be "MicroFarad" abbreviated UFD (where the 'U' s supposed to be the micro symbol) or MFD.
Grab a couple of them at that price.

Wes

Wes · Posted: Fri May 02, 2008 4:51 pm Post subject: More on Capacitors

Been doing some more research on caps. and offer the following:
-aluminum electrolytic caps have a limited life span due to their construction.
- the life span is limited by the semi-liquid electrolyte that is used to chemically produce the insulating skin between the two plates.
- life is shortened by heat, and overvoltage.
- the Rotax recommended capacitance rating of 22, 000 mfd is a minimum value (if using a single cap) and anything up to a practical limit of 100,000 MFD can be used. (practical, based on cost and size)
- the voltage rating should easily exceed any peak bus voltage (~15 volts) plus the AC ripple (maybe up to 4 V).
I have kinda settled on a Mallory cap. rated at 24,000 MFD at 50VDC. I found them at Mouser (http://www.mouser.com/Search/ProductDetail.aspx?qs=OFbHmfmvrg1aPmyhY2NDLA%3d%3d) for $26 in lots of 1 ea.
There are plenty of caps around that will do the job, but I wanted to start with a known brand and a "fresh" cap., knowing that they do have a limited shelf life also.
Think of this cap as a 'miniature' battery that stores energy when the bus voltage goes up and gives it back when the bus volts drop. It obviously doesn't have the storage capacity that the big aircraft battery has, but it can absorb fast changing voltages (such as coming out of the engine alternator) and smooth out the DC present at the rect./regulator B+ pin.
OK, enough already.

Wes
Kit #95

b1x4nqb · Joined: 14 Aug 2006 Posts: 199 Location: Orwigsburg, PA

Wes,

Thanks for that research. I went ahead a couple of them.

Paul, PA