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Rated Capacitor Voltage

 
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rexjan(at)bigpond.com
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PostPosted: Thu Mar 01, 2007 6:49 pm    Post subject: Rated Capacitor Voltage Reply with quote

Last weekend when running my engine to check static RPM before first
flight my noise capacitor blew. The engine has run several hours with no
problems. I was told by one "airport bum" it was undersized (came from
Skystar- 25 V). He indicated I needed at least 50V.

David,
  you already have a lot of good advice on this but as a retired electronics tech I'll try to explain a bit more.
First years ago all electrolytic capacitors [ that's what you are really talking about ] were marked not only with their capacity in microfarads [ mfd ] but the voltage. This voltage rating was actually two. One was working voltage and the other peak. Now that 12 volts you talk about as yours planes voltage is just really a figure of speech because even a good battery is about 12.6 volts or slightly more for an Odyssey type [ Absorbed Glass Mat ]
When your battery is being charged by the alternator on the plane the voltage should be approx 13.8 volts however that 13. 8 is really an average. It's all happening many times a second but it's not a rock steady 13.8 volts that it might appear to be on your gauge on the panel if you have one. A graph of that voltage would show a lot of very fast or short duration spikes considerably above 13.8 volts. The 13.8 figure is an average. That's why electrolytic capacitors were marked working voltage [ 13.8 or more required ] and peak voltage. Generally twice 13.8 would be a good guide for peak requirements but it does depend on that waveform ie:- the size of those peaks. I would think one could not rule out seeing a 70 volt peak for a split second. Also we talk about 13.8 volts as controlled by our regulator but our club Jabiru for instance is often charging at 16 volts. Now this is not good for our battery if it happens all the time but we do need to allow for it when choosing our voltage rating for our capacitor plus we always go bit more for a safety margin like 20 volts rating. Electroytic capacitor manufacturers have stopped putting the two ratings on and my understanding is that the stated rating is the working voltage and if you want the peak you need to find information from the manufacturer or just guess as we all do from experience.
OK so why wouldn't a 20 volt capacitor suffice. Well it rightfully should except we do have those high peaks on our waveform out of our alternator and regulator. Most regulators don't act quick enough to cut these sharp peaks. Also the crude method of shorting being used in the regulators these days does nothing to most of these peaks. So let's say I'm about right when I talk about 70 volt peaks and I'm also right in saying the peak rating of a capacitor is at best double the stated [ working ] voltage. What voltage do we then need ? Well as I said a miniumum of 20 volts but due to peaks that might be 70 volts we need to cover 70 plus a good safety margin so we will opt for 100. Now if the peak rating of a capacitor is double the stated voltage that means a 50 volt [ 1/2 of 100 ] rated capacitor is what is required. So why does a 50 volt capacitor sometimes still blow. Well normally it shouldn't and I guess normally it doesn't but if we get an odd higher peak than we expect perhaps due to a loose battery terminal or a million other reasons then we stand a good chance of blowing the capacitor. I think you will find a lot of one Farad capacitors used as noise suppressor and filter and reservoir in car stereo systems are rated at 100 volts for this reason. I don't think they state it but the 100 volt figure is peak. I guess they just choose to quote the higher figure and not define what it is because the higher figure looks better. They do that with wattage output figures on stereo systems all the time.
Anyway if you want to do really well you could go for a one farad 100 volt capacitor. Be warned though they usually cost about $100 AU. On the other hand a 50 volt capacitor will probably be OK although you could lose it sometime and for that reason connecting in circiut with a fuse is a sensible precaution.
Someone also mentioned reverse voltage spikes. Well it's generally considered an electrolytic can handle 10% of it's rating in this regard but it's not easy to know what might be required so I'd say in your case ignore that. In any case if that was a consideration I'd go for a data sheet rather than guess this one.
Also as some one said "Are you sure you need the capacitor" I haven't got one but I do use a little circuit instead. A lot have no cicuit or capacitor and they are just fine.
Hope all this helps.
    Rex.
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Float Flyr



Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

PostPosted: Thu Mar 01, 2007 8:49 pm    Post subject: Rated Capacitor Voltage Reply with quote

Rex:

I liked your explanations. I've seen caps with several thousand uF but I've never seen a one farad cap. When I was in school they told me you couldn't move a one farad cap with a semi rig. While this was probably a gross exaggeration isn't a one F cap a monster!

What I was getting at was to be sure the capacitor was installed after the rectifier (new installation) and to be sure there wasn't something wrong with the rectifier bridge.

Generally, when a lytic pops I go looking for a reason.... sometimes I find one, sometimes I don't.

All the times I've looked at wave forms on a scope all the nasty voltage spikes all but disappeared when a battery was installed in the circuit.



Noel [quote]
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PostPosted: Fri Mar 02, 2007 7:06 pm    Post subject: Re: Rated Capacitor Voltage Reply with quote

1 Farad Capacitor with Digital Monitor 3.2 lbs

http://www.amazon.com/1-Farad-Capacitor-Digital-Monitor/dp/B0009DTJBO


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PostPosted: Fri Mar 02, 2007 8:02 pm    Post subject: Rated Capacitor Voltage Reply with quote

Wow... That would jump start your day!

Thanks

Noel

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