Smoothing Cap for 12v Stereo

[DHutch]

On emilyanne we have a noramal 12v Car Stereo Headunit, the problem is, when ever the battery charger (24v sterling marine) is on, we get noise though to the stereo, even though the regulated 12v supply. Which is very noticable, especially when listening to the radio.

- This is a bit annoying as we often want the radio while we're working, and charging the batterys, so in though i might place a capaciter between the battery charger and the stereo.

 

- The question is, where can i get a suitable cap? i know the car audio people use large power caps, but this are almost certianly very over priced, and my well not be approprete anyway, seeing as there trying to smoothout power larger fluctations rather than electrical noise.

- Also would it make any diffrence which side of the 12v regulator i put the cap?

 

 

Just wondering,

 

Daniel

[Denis R]

Daniel, I wonder if your problem is RF from the charger, which is being picked up by your radio? Some designs of charger give off masses of RF (can't remember exactly what the system is, but it involves some form of triggered PWM to vary the charging input to the battery).

If this is the case it's often difficult to cure, short of shielding the charger itself. A capacitor probably wouldn't make a big difference if this is the case, but it would smooth any induced AC on the power feed to the radio.

[DHutch]

Daniel, I wonder if your problem is RF from the charger

- If this is the case it's often difficult to cure, short of shielding the charger itself.

- A capacitor probably wouldn't make a big difference if this is the case, but it would smooth any induced AC on the power feed to the radio.

Right, well that maybe the case.

- I would imagen it would be somthing with the PWMed output of the charger, but it didnt know how best to solve this. (hence though of using a cap)

- The charger is about 20ft away, just the otherside of a steel bulkhead and the cooker, if that makes any diffrence (you talked about shealging)

 

Thanks, i dont know much about this. Im more of and electrical person then electronics (fidley stuff)

 

 

Daniel

[Denis R]

But then again, if your charger is fairly recent and is CE marked, it shouldn't be giving off RF to any great extent anyway... And I would have thought that a steel bulkhead would have provided enough shielding for any 'low level' RF that might be acceptably emitted.

Hmmm..., sorry, can't think of anything meaningful to offer here.

However, a cap would be worth trying, to kill off any AC in the line which might be acting as an antenna.

Here's a thought - have the radio and charger got a common ground?

[DHutch]

Here's a thought - have the radio and charger got a common ground?

Yeah, they do i guess.

- I guess maybe i've got an earth loop then.

 

The -ve of the 24v and the earth of the 240v is commoned, and conected to the hull.

- And so the charger will be earthed to the 240v and the 24, which would make a loop.

 

 

Daniel

[NB Willawaw]

Daniel,

 

Its very easy to check. Pop in to Radioshack/Tandys and get a 50-200 uF electrolytic capacitor (50V working voltage). Fit this between the positive and negative supply to the radio (on the radio side of the 12V regulator and making sure you observe the polarity of the capacitor - the end marked + should connect to the positive supply). If you want to be doubly sure, get a small radio inductor from a car audio shop and put that in series at the same point. Between the choke and the capacitor, they should reduce or kill the interference.

 

Mark

[DHutch]

Its very easy to check, get a 50-200 uF electrolytic capacitor (50V)

- Fit this between the positive and negative supply to the radio (on the radio side of the 12V regulator).

 

If you want to be doubly sure, get a small radio inductor from a car audio shop and put that in series at the same point. Between the choke and the capacitor, they should reduce or kill the interference.

Ok thanks, Ive got a few caps i could try, some 100uF 25v ones, and some 1200uF 16v ones, if eather of thoughs would be ok?

-Also, the bit about the inductor, and putting it (in series)between the cap and the choke.

 

From my limited (just gained) knowlage, and inductor (often called a choke), is a large coil of copper wire, sometimes around an iron core, which is put in serise with any wire which may be carrying unwanted interference.

- which makes sence, but i still dont see what you saying.

 

[edit]

Is the last line of your post suppost to read "Between the 12v regulator and the capacitor, they should reduce or kill the interference" ?

[/edit]

 

Sorry to be dumb, ive tryed electrnics, but im not good at it (no patiance)

 

 

Daniel

Edited June 10, 2005 by dhutch

[NB Willawaw]

Try the 100uF one.

 

The inductor, sometimes called a "choke" is, as you say a coil of tightly wound, low resistance wire. When the DC passes through it, it offers little resistance and there is no significant voltage drop (at DC anyway).

When the noise, which is high frequency AC and superimposed on the DC tries to pass through, it creates a magnetic field which when the produced lines of flux pass over the turns of the coil, will induce an opposite current and oppose the noise signal. This is why its called a choke because it will choke the noise signal.

 

Most DC power supplies carry combinations of inductors (L) and capacitors ( C ) on their outputs to smooth and filter the output.

 

Mark

Edited June 10, 2005 by NB Willawaw

[DHutch]

Ok, so i'll put the (100uF) cap accross the 12v side of the regulator, and then would the inductor be between the cap and the regulator, in the +ve?

- does it matter much what type/size the inductor is at all?

 

 

Thanks, Daniel

Edited June 10, 2005 by dhutch

[pagan witch]

I agree with the capacitor and choke ideas but wonder if we are missing the obvious here.

 

Does this interference only happen when listening to the radio or does it also appear on the cd / cassette option ?

 

If it is only the radio then it may be worth checking out the aerial system. Unless you are using a seperate 'ribbon' type then the chances are that there is also an earth loop with the aerial.

 

When I was working as an aerial rigger I sometimes found interference caused by the ever growing birds nest of wires at the back of the average tv/vcr/hi-fi/dvd combinations. Sometimes simply shuffling the wires about helped or cured the problem, sometimes coiling up the wires (effectively making a choke) helped.

 

My C & G lecturer suggested making a quit tight coil in the aerial lead, stripping a small area of outer sheathing then connecting the two outers togther. This seemed to have the desired effect. Perhaps it was successful because it altered the phase between the centre core and outer ?

[NB Willawaw]

Capacitor from +ve to earth/-ve and inductor in series

in the +ve line. The inductor will have a value in H but I can't remember

what it is off the top of my head. Just ask for a car radio inductor/filter in the shop and they should know which one you need. It is usually a set value which is the most appropriate for this purpose. Mark

 

Ok, so i'll put the (100uF) cap accross the 12v side of the regulator, and then would the inductor be between the cap and the regulator, in the +ve?

- does it matter much what type/size the inductor is at all?

Thanks, Daniel

22699[/snapback]

[DHutch]

We do get the interference while using the CD, although it is much more with the radio.

- The aerial is a cheap one from a halfords (or simular) it just has a meter of coiled up co-ax, and then goes though the roof, just about the radio, and terminats in a car-type flexable aerial jobbie.

 

 

I havent actually been to the boat since i started this topic, but im spending all of july on there, so i'll take a few diffrent caps and pick up a choke from somwhere and have a go with them.

 

 

Daniel

[Hondaman]

As you get interference with CD it is most likely coming on the supply wires, but could be radiation of RF interference picked up by them.

Square(ish) wave inverters and switch mode chargers are great at producing harmonics to very high frequencies. These harmonics should be filtered in the unit but ----

 

The use of a fairly large capacitor is a good idea on the supply wires but often these high value electrolytics or even the rolled up mylar film type have a high impedance at RF so a small disc ceramic capacitor (47 to 100 pF) should be wired in parallel.

 

Pick up of induced/radiated interference is also possible on speaker leads which can cause feedback into the amplifier (particularly as leads on boats can be longer than in cars). Try to run the leads as twin wire (both wires will pick up the same interference and cancel out), away from any power carrying wires and close to the metal hull. It is also possible to add a small capacitor across the leads close to the radio or a small RF choke in series. Most decent modern car radios will have this sort of protection built in.

 

In car audio I have often found poor arial earthing to be a problem rusty steel where the base clamps underneath ( clean off and smear with grease), aerial plug outer to boat hull should show less than 1ohm on resistance meter.

 

Why are you using dropper from 24 volt? does your boat only have 24v system?

 

And finally go on please get one of those ridiculously overpriced 1Farad capacitors, it won't do any good but they have a pretty little led on top to justify the price and prove to the world that some people haven't the faintest idea what it is all about.

Edited June 23, 2005 by Hondaman

[DHutch]

Ok, thanks for all of the honda.

 

- I'll rumage out a small ceramic capacitor to go with the electrolitic

- And i'll have a look at the aerial earthing while im at it.

 

The leads for the speakers are quite short 2 & 5m (for left and right)

 

And yes, we run a 24v system, with the 24-12v regulator for the sterio (and also the phone charger)

- I think i'll give the "overpriced 1Farad capacitors with pretty little led on top" a miss!

 

 

Daniel