electric fan causing cb feedback noise

[inacoma]

please help. I just installed an electric fan to supplement my condensor. Now when I turn the fan on (luckily I put a switch inline), my CB goes screaming.

i've tried many different grounds for the new fan and still no dice. the fan is wired directly into a relay that is powered directly from the batter post.

is there some sort of filter I need to get for the CB and/or fan?

thanks!

Kumar

 

[woody]

Radio Shack has an inline noise filter, as will most car stereo shops, perhaps even Walmart....

Also, try changing which power circuit you run the CB from, may make a difference.

 

[inacoma]

Radio Shack has an inline noise filter, as will most car stereo shops, perhaps even Walmart....

Also, try changing which power circuit you run the CB from, may make a difference.

cool, will try that. thanks! btw, my cb is also directly hooked into an 8 gauge wire that runs straight to the battery. Will probably get a noise filter for it too (as well as the fan), that should help things...hopefully.

Thanks!

Kumar

 

[MoJ]

My cheapy Uniden came with a built in filter. Works well.

 

[lurker]

please help. I just installed an electric fan to supplement my condensor. Now when I turn the fan on (luckily I put a switch inline), my CB goes screaming.

i've tried many different grounds for the new fan and still no dice. the fan is wired directly into a relay that is powered directly from the batter post.

is there some sort of filter I need to get for the CB and/or fan?

thanks!

Kumar

What you really need is a big honking 1 farad capacitor which should cost you about $50.

 

[inacoma]

What you really need is a big honking 1 farad capacitor which should cost you about $50.

I was googling last nite and actually read that a .01uF cap should do the trick. technically, i think I only need to filter out the CB frequency range (26.xxx to 27.xxx mhz). I would love to do overkill though we'll see what radioshack has for me.

thanks!

Kumar

 

[lurker]

I was googling last nite and actually read that a .01uF cap should do the trick. technically, i think I only need to filter out the CB frequency range (26.xxx to 27.xxx mhz). I would love to do overkill though we'll see what radioshack has for me.

thanks!

Kumar

I think you might want to isolate the fan frequency and it's harmonics from the CB and not the other way around?

 

[Bob_Garrett]

The Radio shack filter should help. If not try disconnecting the antenna, first at the radio and then at the antenna, and see if the noise goes away. Also, make sure that the power wire to the radio isn't running parallel to other power accessory leads.

Bob

 

[inacoma]

I think you might want to isolate the fan frequency and it's harmonics from the CB and not the other way around?

I went ahead and did the .01uF capacitor trick. I put it between the positive and negative of the fan wire, close to the fan motor. Worked like a charm! NOT a single bit of noise now.

Thanks! and I hope this helps others, it's not always a power problem. In my case it was fan interference coming through my antenna.

Kumar

 

[mabrodis]

You'd know why it worked if you READ MY E-MAIL!

I did the calcs and with a 0.01uF (micro-Ferad) cap, the effective reactants at 27Mhz is 0.56ohm, which is a pretty good short.

Forumla is: X = 1 / 2(pi)FC , where F is frequency in Hz and C is capitence in Ferads (0.01 uF = 0.00000001 F).

Also doing this you did correct by installing the cap directly across the positive and negative wires, not from the positive wire to chassis ground, that would create a huge current loop, which would actually radiate signal at that frequency, attenuated because of the low resistance of the ground wires, but there would still be a current loop.

 

[Beowulf]

> ... you this you did correct by installing the cap directly across the positive and negative wires <

Would this also work if you have run the pos wire to a motor alongside the CB coax? (Neg is frame grounded at motor)

-B-

 

[mabrodis]

If I get the picture correctly, then probably.

The thing with stuff like this is you want to create your short as close to the problem as you can. A cap is not a short with a DC signal, but as the frequency of a signal goes up it changes it's characteristics alot...so at 27 Mhz it looks like a short to that signal, but does not bother (much, everything bothers something some) the DC current powering the fan motor.

Are you talking about having a motor right beside the CB? Or having a CB coax antenna cable run right beside a positive wire driving a motor?

With Kumar's case, the wires (positive and negative) were acting as antennas for the pulses coming out of the motor. The actual noise was undoubtedly brush contact noise, on/off sharp transitions which create lots of harmonics, up even to the 27 Mhz range, which is MANY harmonics up there, but your CB has to 'hear' very weak signals, so it picks up all this stuff.

The ground loop in Kumar's case as it was just sitting there with no mods, was from the brushes through a positive wire (or negative, doesn't matter) to the battery, through the internal resistance of the battery, then back to the motor through the other wire. That loop was radiating the signal. By creating a short for that signal right at the motor, it's still going to radiate, but has a 1/2" antenna which gets nowhere, so you don't hear it.

If interference is a problem on a CB (other devices hearing interference might need different caps to kill those frequencies), then it's worth a try to play with a cap or two in different configuration, the difference will be immediate, so you can just hold it in your hand, touch the contacts and have the CB on and see if it helps or not.

Good Luck...
Mark Brodis

 

[Beowulf]

>> Or having a CB coax antenna cable run right beside a positive wire driving a motor? <<

This one. When the motor runs you can hear it through the CB speaker when the CB is on. The motor is 10' away. The +12v wire to the motor runs alongside the CB coax. The motor ground is 10' away from the CB head unit and never comes close to the CB antenna coax.

-B-

 

[ParadiseCruiser]

B -

Another - or perhaps an additional approach - is to shield the power leads to the radio.

These wires pick up an enormous amount of noise. This is good insurance against sources of noise that cannot be identified, or others that are difficult to get to (i.e., fuel pump), etc.

Yet another is to wrap audio cables (leads to the speakers) around small ferrite cores (available at Radio Shack, and any good radio shop), a neat little trick that helps absorb stray noise that would otherwise end up in the speakers (though admittedly, this would help more with stray RF voltages from the radio than with the type of interference we're discussing here).

Cheers, R -