Capacitor

[Ockham4230]

This is not a very "DIY" question - sorry but I figured if anyone would know an answer people in this group would.

I have a general idea what a capacitor does, but I don't know how this plays a role in suppressing radio noise which can interrupt transitions. My problem is my ESC came with two .10uf (104) capacitor one of which is now broken and the wire broke too close to the base of the capacitor to re-solder. I have a spare 102 capacitor but I don't know the difference between the two so I don't know what the effect will be of using it as a replacement.

Should I use it? If I use it should I also replace the other capacitor on the other motor pole? (These capacitors go from each pole of the motor and ground on the side of the motor). It is a 400 speed brushed motor. It also has a ,47uf (473) capacitor going from pole to pole and a "Schottky Diode" between the poles.

If I shouldn't use the 102 where are places I can get a 104? My LHS is a 40 minute drive. I hate to burn $8 in gas to get a capacitor. Will Radio Shack likely have it? Other options?

What can I expect if I fly with only one of the two 104 capacitors? Right now it's raining so it won't matter, but I hate to miss a good day of flying at this time of year because you never know when the next possibility will be. Is it imperative that I wait until I get a replacement? This goes in a e-powered sailplane, so it gets some hight and distance from me, but I also don't run the motor constantly. Will this cut down significantly on my range or is it either going to interfere or not regardless of range?

I would appreciate any advice you can give.

Thank.

David

[vintage1]

Those caps are there to 'short circuit' RF noise. Without shorting the DC!!

102 is only 100pF - not a lot ofuse,.

Any scrap radio will have tons of 104 disc ceramics in it.

If you are in the UK I have a few hundred ;)

[fm_head]

The motor generates "radio noise" when it is running. This noise can affect your receiver in flight, so it is not recomended to fly your airplane without the capacitors soldered on to the motor. The 1nf caps you mention will not be enough so replace it with a 100nf ceramic disc type capacitor only. You can order some from digikey or as vintage 1 mentioned try desoldering one from any scrap electronic item.

[Ockham4230]

Thank you for the replies. This raises a further question.
Vintage1 said the 102 is only 100pf and Fm head mentions 1nf and 100nf. The way it was written in the ESC manual was what looked like a greek 'mu' then an f (word has the symbol -symbol character code 109 - but I can't paste the symbol here). so I used uf. What is this measurement. How do you know what a 102 is? Why did vintage1 us a 'p'?

Finally if I remove it from an old radio, do I need to worry about getting a bad one? Should I check it? I can use a multi-meter, but other than that I have virtually no electronic testing equipment.

Thanks. Sorry to be such a novice.

David

[fm_head]

Thank you for the replies. This raises a further question.
Vintage1 said the 102 is only 100pf and Fm head mentions 1nf and 100nf. The way it was written in the ESC manual was what looked like a greek 'mu' then an f (word has the symbol -symbol character code 109 - but I can't paste the symbol here). so I used uf. What is this measurement. How do you know what a 102 is? Why did vintage1 us a 'p'?

Finally if I remove it from an old radio, do I need to worry about getting a bad one? Should I check it? I can use a multi-meter, but other than that I have virtually no electronic testing equipment.

Thanks. Sorry to be such a novice.

David


104 = 100000pF = 100nF = 0.1uF (micro farads)
102 = 1000pf = 1nf = 0.001uF

So now it should be clear to you that the values that I and vintage1 mentioned are the same only the units are different. Yes you can check the capacitor using a meter. Your basic knowledge of electronics is not good so I recomend that you ask a technical person to help you out. Go to any person who repairs electronics he should be able to solve your problem.

[slipstick]

p=pico (10 to the power -12)
n= nano (10 to the power -9)
u = micro (10 to the power -6)

So 1,000pF = 1nF
and 1,000nF (or 1,000,000pF) = 1uF

Capacitors codes give the value in pF as a number followed by a number of zeros

So 104 = 10 and 4 zeros, 100,000pF, 100nF or 0.1uF
102 = 10 and 2 zeros which is actually 1000pF or 1nF. It is NOT 100pF.

No wonder poor David is confused if we can't get the numbers right :(.

Steve

[Ockham4230]

Thanks for the help. With the explanation of the values I now see why there is such a big difference between a 102 and a 104.

When testing the capacitor is it merely a matter of measuring resistance?

David

[phil_g]

102 is only 100pF - not a lot ofuse,.
Not so.
102 = 100pf = 1nf = 0.001uF
And wrong again!
The three-digit '102' notation matches the three colour band notation, a cap usually has one or the other.

Brown, black, red is the same as 102, ie it translates as one, nought, 2noughts, or 1000pf.

So, do you have any more 102's you could parallel up, or just the one? (mind you, you'd need 100 to make a 104!)

If not, can you scrape the paint or wax (could be either) off the end of the broken cap, clean it up, dab of flux & solder a new lead on?
Silver paint a new lead on then epoxy over once thats dried?
Speaking for myself, I'd sick the 102 on if thats the only option, 1nF is still quite an effective RF shunt. Make sure its the same type, some are quite inductive & are less suitable for suppression use. IMHO.
You wont measure any resistance in the cap - if you do, throw it away!
You can test caps by putting your analogue meter leads across it for a sec then reversing them, when a slight kick will be seen on the needle, probably not with a 1nF tho, too small. Some digital meters measure capacitance.
Cheers

Phil

[fm_head]

102 = 10 and 2 zeros which is actually 1000pF or 1nF. It is NOT 100pF.



Human beings do make mistakes!

[SNIPE_53]

Did you get a replacement Ockham? :)

[slipstick]

Human beings do make mistakes!
Me too, and I'm always pleased when someone corrects them for me.

Glad to help ;).

Steve

[phil_g]

Human beings do make mistakes!
Me too, often big style. Fm & vintage, sorry for jumping on a typo...

David, you may find when you strip an old radio or whatever that the caps are stripy, maybe having more than 3 bands of colour... just read the first three, starting from the 'top' ie furthest from the leads. Your 104 should be brown, black, yellow, then the remaining colour bands are voltage & tolerance, ignore them.
If you find anything looking like the pic, you're in. These are all the correct value, just different flavours:
http://www.mccrash-racing.co.uk/pics/cap104.jpg
Cheers
Phil

[vintage1]

Sorry - had a senior moment there ;)

10nF is more than enough for suppression: 100nF MAY be worse, especially mylar/wound types.

[Ockham4230]

Ok, I think I have got pretty clear on this. I haven't gotten a replacement yet, but will probably do so today. I have a couple of other projects running competition and some are job related so they tend to take priority. I will probably look first in an old radio I have, so thanks for the pictures. If I don't get a positive match I will probably try to replace the 104 from a retail source then failing that I will go with the 102. Vintage 1 thanks for the follow up comment. Regarding the idea that it may be worse, I am going on the assumption (and I realize it is a big jump) that since the 104 came with the ESC that it would be the preferred suppression. Unless Great Planes is just trying to dump some extra 104 capacitors I figure the put the 104's in for a reason. I also know from experience that using the 104 I can get significant range without interference. If I need to rethink this line of thought I will need further information on what factors I need to consider. Let me know what you have in mind.

David

[Ockham4230]

Does the use of a ESC increase the "noise" the motor produces? Does the type of motor (brushless/brushed) matter? Does the size matter?

Just curious.

David

[phil_g]

Yes, yes & yes - a brushed motor on its own will produce commutator noise as the coils are energised in turn by the brushes, in the form of a spikey irregular waveform superimposed onto the DC voltage. You would therefore expect that a bigger motor drawing more current would create more RFI noise, however if the motor is better built, ie cleaner brush contact, more accurately spun commutator, smaller commutator gaps, etc it may even produce less.
A brushless obviously doesnt create comm noise because it hasnt got one, however both types when connected to an ESC will produce some switching noise. Basically, to give a variation in speed, an ESC switches the coils in the motor either fully on or fully off in varying ratios.
Without going into too much detail, a coil is happy when its 'on' and its happy when its 'off' but it will resist changing state from one to the other. Every time the ESC switches a coil 'off' for example (and there could be up to 20,000 switches per second) the coil will try to maintain the current flow through the 'off' switch in the ESC, where, if not for the caps, it would momemtarily create quite high voltage spikes, again superimposed on the DC supply.

The purpose of your suppressor capacitors is to shunt these spikes to ground so that they cannot interfere with the ESC circuitry or with your reciever, both of which are powered from this supply. Mounting RFI caps on the motor hits the problem at source, but there will be RFI caps inside the ESC too.

A few liberties taken with the explanation there but you get the idea :-)

Cheers
Phil

[Ockham4230]

That was helpful. Thank you.

David