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Capacitor and ignition noise..
- Thread starter Splash1
- Start date
B
BOOTY MONSTER
Guest
just for future reference........ what size cap do you use for a typical 30 watt cb radio . does the value change for high powered radios ? does it work for amps too ?
like i said , no noise problems here but who knows what the future holds . lol
like i said , no noise problems here but who knows what the future holds . lol
When you say "jump the leads", do you mean the DC power to the radio?
Depends if the noise is being picked up by the antenna, or coming via the DC wires to the radio.
If you disconnect the coax from the radio and you still get noise, then noise is coming thru the DC. Verify your wires are directly connect to car battery.
If noise vanishes with antenna gone, then your vehicle is radiating the noise.
Could be poor grounding of engine block. If its coil-over-plug, then could be missing/faulty bypass cap on the v+ rail. Or if its the older distributor type, the high-voltage cables could be old.
It also helps to have new sparkplugs to reduce voltage required to spark.
Depends if the noise is being picked up by the antenna, or coming via the DC wires to the radio.
If you disconnect the coax from the radio and you still get noise, then noise is coming thru the DC. Verify your wires are directly connect to car battery.
If noise vanishes with antenna gone, then your vehicle is radiating the noise.
Could be poor grounding of engine block. If its coil-over-plug, then could be missing/faulty bypass cap on the v+ rail. Or if its the older distributor type, the high-voltage cables could be old.
It also helps to have new sparkplugs to reduce voltage required to spark.
dudmuck..ya,its sparkplug noise mostly and it is still there with antenna reoved..wanted to know where to place the cap..by the back of the radio or by the battery..when i did the fuel pump it was the antenna picking up the noise now its the power leads..i take it its ok to ''jump the leads''by that i mean hook up one leg of the cap to the hot and the other leg to the neg.
Put the cap as close to the radio as is reasonably possible. Use a big electrolytic cap...at least twice the voltage you're using for safetly cushion (in case of spikes, etc.). Something like a 30v, 33000uF. Theyr'e going to be pretty large. Remember, the polarity matters with electrolytic caps!
Using both capacitors shouldn't hurt a thing, in fact, the more capacitance the better to some ridiculous point. Watching the voltage ratings and polarity are both good things to do. Polarity sort of explains it's self, backwards means a bang. Twice the normal voltage, or even three or four times the normal voltage won't hurt anything at all. Costs a little bit more, but that's just cheap insurance. Like 'Mole' said, as close to the radio as possible is nice. Got to be practical about that so that "as possible" has some leeway in it.
And just to make sure, this capacitive 'by-passing' thingy is only for DC, not AC. Different way of doing AC. Not that I didn't think you knew that, but just to make sure...
- 'Doc
And just to make sure, this capacitive 'by-passing' thingy is only for DC, not AC. Different way of doing AC. Not that I didn't think you knew that, but just to make sure...
- 'Doc
In this case, more capacitance is not what you need.
You need capacitors of the low-inductance type, like metal film.
You are trying to prevent an RF signal from coming down the DC line.
What can also help is routing the wires from the battery to radio as far from the engine as possible. If you run those dc lines nearby ignition wires, then your asking for it. Also some twisting the red and back together can reduce noise pickup.
You need capacitors of the low-inductance type, like metal film.
You are trying to prevent an RF signal from coming down the DC line.
What can also help is routing the wires from the battery to radio as far from the engine as possible. If you run those dc lines nearby ignition wires, then your asking for it. Also some twisting the red and back together can reduce noise pickup.
They should be entirely adequate. That's not a guaranty, by the way. They are larger than the typical by-pass capacitors used by lots of people, so if that's where you're getting noise, it should certainly reduce it.
All noise is a pulsing or alternating current type signal. Capacitors can 'pass' AC just like a short circuit depending on the amount of capacitance and the frequency of that AC, has to do with the charging/discharging rate of the capacitor, which depends on the amount of capacitance. If you 'match' that charge/discharge rate to the frequency of the AC, it 'sees' a short circuit. The 'catch' to that is that noise can have a huge range of frequencies, not just one. So, more capacitance is better, sort of. Put that capacitor across a DC supply line and all/most of the AC component goes straight to ground, never gets past the caps. That's an exaggeration to some degree, not all AC goes to ground, wrong frequency kind'a thingy. That's also why sometimes there are more than one by-pass capacitor size used. That's not a very 'scientific' description of what's actually happening, but it gets the point across well enough to give an idea of what's going on. Sort of.
Good luck.
- 'Doc
Think about it, it's the same kind of 'filtering' circuit used in power supplies.
All noise is a pulsing or alternating current type signal. Capacitors can 'pass' AC just like a short circuit depending on the amount of capacitance and the frequency of that AC, has to do with the charging/discharging rate of the capacitor, which depends on the amount of capacitance. If you 'match' that charge/discharge rate to the frequency of the AC, it 'sees' a short circuit. The 'catch' to that is that noise can have a huge range of frequencies, not just one. So, more capacitance is better, sort of. Put that capacitor across a DC supply line and all/most of the AC component goes straight to ground, never gets past the caps. That's an exaggeration to some degree, not all AC goes to ground, wrong frequency kind'a thingy. That's also why sometimes there are more than one by-pass capacitor size used. That's not a very 'scientific' description of what's actually happening, but it gets the point across well enough to give an idea of what's going on. Sort of.
Good luck.
- 'Doc
Think about it, it's the same kind of 'filtering' circuit used in power supplies.
Depends whether we're talking "bypassing" here, or "filtering". For most of the bypassing I've had to do, disc ceramic capacitors in the range of 0.01 to 0.10 microfarads (at least 50 WVDC) have done just fine. Attach them as close to the offending motor or other component as possible.
Now, if you're trying to filter the incoming DC power, you'd gain a lot more ground by using an inductive-based filter like this: New Page 1. This installs right at the radio, or as close as possible.
Lots of folks have built these in an hour or less and have been very happy with the results.
Now, if you're trying to filter the incoming DC power, you'd gain a lot more ground by using an inductive-based filter like this: New Page 1. This installs right at the radio, or as close as possible.
Lots of folks have built these in an hour or less and have been very happy with the results.
You can buy something like that pre-made from radio shack. I think the design that beetle posted probably has more wraps, though. I don't know...I never opened up the radio shack one. I like the idea of using that quick link, though. I think I have all that stuff laying around the house, so I'll have to make one for fun 
