Radio Interference supression

Leonidas said:
About to replace the old FM car type radio with a more modern one. I always experienced some interference when the f.w. water pump was running. Any ideas as to how I can suppress this interference ?
Thanks.
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For an easy reading tech bit see https://www.pololu.com/docs/0J15/9
If thats too deep then add a few of: http://www.ebay.co.uk/itm/100nF-0-1uF-104-50v-Ceramic-Disc-Radial-Capacitors-UK-Free-Postage-/131439765769?var=&hash=item1e9a6ba909:m:m0ShFCD_LbvHONIfEZo0a3Q across the pump terminals. These capacitors are not polarised, so dont worry about which way round you connect them.
 
Interference from the pump will probably occur with the new radio as well. If at all possible use an entirely separate 12v battery to run the pump for test. Or try the radio on a separate supply. This will confirm interference is coming from the pump motor and is being transmitted via the 12v supply. Interference is a result of stop start current of the motor (very rapid as commutator switches many times per revolution. The current voltage changes can be smoothed out with a capacitor. A bit like a small capacity battery that takes the extra voltage when it rises and replaces voltage when it drops. Capacitor is usually about .1 microfarrad with a voltage rating well over 12v. A different type of capacitor is the electrolytic which must be connected with correct polarity and might be rated at 100microfarrad or so.
Any piece of wire exhibits some inductance. ie it looks like a coil. This inductance can inhibit the flow of very high frequency voltage/current pulses. So if you use an electrolytic capacitor it will exhibit some internal inductance and may fail to bypass the very high frequency changes that produce the interference. Fit a small ceramic capacitor perhaps .001microfarrad in parallel with the large capacitor to be sure of bypass.
We then use this inductance effect to inhibit the pulses from passing along the supply wires. This can be done by having all current pass through a coil or choke. Often with a ferrite (black ceramic like material) core. However a simple trick is to buy a ferrite core and wind some of the feed cable around or through it. You see this kind of thing in computer equipment cables. ie key board or mouse wire and these can be a source of ferrite core.
So the capacitor bypasses at the pump +ve to-ve and the inductor (choke) goes in the feed cable. Just how much effort you need to go to to suppress your interference only you can tell by experiment. In extreme you may have to make similar filters for the radio supply although usually they have these built in. good luck olewill
 
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