Trolling Motor Interface on Sonar

[Bankc]

I recently installed a PWM controller for my trolling motor on my Kayak.  Like many, I started experiencing noise on my sonar when I ran my trolling motor off the same battery (a problem I didn't have before the PWM controller).  I run the TM wires on the starboard side and sonar wires on the port, so I knew it wasn't from the signal bleeding into the transducer's wires.  And the transducer and motor are about 8 feet apart so, I knew it wasn't EMI from the motor itself.

 

I hooked up a scope and saw a 15kHz signal being backfed into the battery from the PWM, into the Sonar's power supply.  To clear it up, I installed an RC LPF (resistor, capacitor, low pass filter) to filter out the high frequency noise.  The Sonar draws one amp at 12 volts, so it provides the resistor function of the filter by applying 12 Ohms to the circuit.  So on the Sonar's power cable, I soldered in a 100uf, 35v, low EMF audio grade electrolytic capacitor in parallel with the sonar and covered it in liquid electrical tape.  I calculated this would give me a cutoff of around 132 Hz and give me 35ish dB of noise reduction at the 15kHz point.  I tested it on my scope, and with the sonar, and sure enough, it worked!  The interference is gone!  And done with parts I already had!  A capacitor won't pass DC current, but will pass AC, so it shorts out the AC noise while blocking the 12v DC from shorting out.  

 

The capacitor is about $2 (plus shipping) and can of liquid electrical tape less than $10.  I used a bipolar capacitor, but that's not necessary.  A low ESR/ESL electrolytic cap is necessary though.  You might could get away with a 16 volt one, but it would probably be better to go up to 35 volts just to give yourself plenty of protection.  Also, I'm betting you could go as low as 10uF or a bit higher than 1000uF and still get ample noise rejection, though 100uF is a good size as it's not too large or expensive, yet gives you massive amounts of noise rejection.  It's a LPF, so as long as the filter frequency is well below the frequency of the PWM of the trolling motor, it should work.

 

So it's a simple and cheap solution to a common problem.  Just be sure to place the capacitor right before the sonar for maximum effect.  
 

It won't solve problems of EMI leakage into the sonar wires or transducer.  You'll need to ground the motor's housing for that.  And it won't solve interference from two sonars, as you'll need to get them on their own frequency for that.  But it should solve power supply noise created by infinitely variable trolling motors and other high frequency noise injecting circuits.

 

 

Oh. And if you don't use a bipolar electrolytic capacitor, then you'll need to make sure that you solder the negative lead on the capacitor to the negative wires of the sonar and the positive lead to the positive wires.  

[Way2slow]

Just a little terminology correction.  A capacitor and coil appear to pass AC voltage, but actually does not pass it.  The charging and discharging of the plates/windings on one side causes the opposite side to charge and discharge at the same frequency and close to the same potential, but the AC does not actually pass through them.  This happens to be one of the questions I used to ask my new technicians fresh out of tech school to see if they actually learned anything.  What components appear to pass AC voltage while blocking DC?

Actually a fluctuating DC will do the same thing as AC.  That's how an ignition coils on a 12VDC system works.

 

[Bankc]

16 hours ago, Way2slow said:

Just a little terminology correction.  A capacitor and coil appear to pass AC voltage, but actually does not pass it.  The charging and discharging of the plates/windings on one side causes the opposite side to charge and discharge at the same frequency and close to the same potential, but the AC does not actually pass through them.  This happens to be one of the questions I used to ask my new technicians fresh out of tech school to see if they actually learned anything.  What components appear to pass AC voltage while blocking DC?

Actually a fluctuating DC will do the same thing as AC.  That's how an ignition coils on a 12VDC system works.

 

You are correct.  There's actually a lot of mistakes that I thought about after posting.  Like it's not a true RC Low Pass Filter, because the load comes after the capacitor, and there's no resistor placed before it (other than the wire itself).  Therefore my math on the cutoff frequency is going to be off.  It should actually function as just a standard filter capacitor, like what you'll find on most power supplies following the rectifier.  At least that's what I'm thinking now.  I'm not an EE, but electronics is one of my hobbies.  

 

But in the end, all that really matters is that it works.  And I proved that both with a scope and with a real world test.  I was initially going to try an LC filter, but the inductor I ordered got lost in the mail.  So I opted for an RC filter since I had those parts on hand, but as I said, didn't really make a true RC filter.