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Dont know nothin bout no caps
- Thread starter Splash1
- Start date
Well, 10,000pf equals 0.01uf, for starters. I don't know what type of noise you're working with, or what's causing it, but that might not be enough capacitance.
Assuming the problem is a motor with sparking brushes (typical), I wouldn't use a capacitor rated less than 50V. Such a spark can have some very high (and very short) voltage spikes.
Typically, the only type of capacitor that has polarity markings is an electrolytic - the large ones used as filters in power supplies. Ceramic disc capacitors are not polarity sensitive.
You can use electrolytic caps to help suppress ignition noise by putting it inline with your power feed to the radio. However, you need much bigger caps than the type you're suggesting. I would use at least twice the rated voltage of your device (bigger if you can, like Beetle said), and you're going to need something in the range of 30,000pf. That's just an off-the-top-of-my-head guess. The cap is going to be the size of a beer can.
The other thing you can do with a cap of smaller value is to combine it with an inductor like this:
http://www.worldwidedx.com/home-bre...-simple-alternator-whine-ignition-filter.html
The other thing you can do with a cap of smaller value is to combine it with an inductor like this:
http://www.worldwidedx.com/home-bre...-simple-alternator-whine-ignition-filter.html
Again, 30,000pf = 0.03uf. Don't confuse the prefixes! You won't find any 0.03uf electrolytics, but 30,000uf would be pretty easy to come by.
Also, the capacitor doesn't go "in line" (meaning "in series") with a DC line unless you want to block all the DC and just pass the AC component (being the noise). Call me picky, but I'd install such a capacitor ACROSS the line, rather than IN line.
Pico = 10^-12
Nano = 10^-9
Micro = 10^-6
Beetle, you're absolutely correct, obviously. Not pf...but uf, of course...just typing without thinking there. Good catch. Maybe "inline" isn't a good choice of words? The point is, you hook up the positive lead to the positive terminal, and the negative lead to the negative terminal.
Zactly - putting the capacitor ACROSS (sort of "in parallel" with) the power supply, ideally as close to the radio as possible.
My turn to be picky.
You want the caps as close to the noise source as possible in order to prevent radiation that can be picked up by the antenna. If the noise is present on just the power leads then placing the caps close to the radio will work. 
My turn to be picky.
Fully agree - I was assuming (I know) that it was conducted rather than radiated noise.
In all my 30+ years of operating mobile radio I have yet to be blessed with a case of conducted noise. ALL my noise issues have been of the radiated kind.