Capacitors to reduce electrical noise

28 Sep 2005 Capacitors to reduce electrical noise

Jim Skala wrote:
>> I will use a lot larger capacitance value, but a 50 volt ceramic type is
>> exactly what you want. At work, I have found that soldering a 10uF, 50V
>> multilayer ceramic 1210 SMT capacitor chip between the leads of a 0.1
>> 50V radial leaded capacitor works great for noise suppression for severe
>> noise sources. This part is extremely small and rugged, but packs a big
>> 10uF capacitance. You can’t buy a single part to compete with this for
>> size or performance.
>>
>>
>>
>> 10uF cap: Digikey part # PCC2308CT-ND
>> <http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=146597&Row=440234&Site=US>
>> The .1 uF radial leaded cap: BC1165CT-ND
>> <http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=146835&Row=264866&Site=US>
>>
>>

I just soldered a 10 micro farad capacitor that Jim Scala suggested between the leads of a .01 micro farad capacitor that my tech counselor suggested, and hooked that ‘assembly’ between the ground and high speed leads at the fan motor. Reinstalled the heater fan and found no more noise problems affecting the supercharger controller. It didn’t matter any longer whether the controller was grounded on the airframe in addition to the ground wire, and it didn’t matter whether I used the existing controller chip or the new one. I haven’t checked any other systems for noise yet, but I think this will do it.

>It seems to me that you guys are just adding two capacitors together to get
>10.1 (or 10.01) mfd. I think the general tolerance for the 10 mfd capacitor
>would overwhelm the additional .1 (.01?) mfd and make it unnecessary.
>I.e. just use the 10 mfd cap. You’re only talking 1% or .1% difference
>depending on whose values below are correct.
>OK, I’m ready to be slapped. 🙂

The reason that frequently capacitors are used in parallel this way is for
Impedance. No capacitor is perfect, in that it also has resistance and
impedance. These prevent the large capacitor from being able to suppress
high frequency spikes. In the mechanical world this is the same as why many
applications use multiple springs, because each spring has a natural
frequency at which it is pretty much useless as a spring.