smoothing the output from a battery charger

[Alexma]

Hi everyone, I have a touring caravan with both 12v dc & ac mains. I have installed an LCD combo tv & the sound quality is very poor due to the speaker sizes.So I made up a 25w stereo amp from a module (*bay) this works fine except for a buzzing noise which I have discovered is coming from the onboard battery charger(?). If I turn on any 12v appliance it gets louder/worse as the charger tries to compensate for the loss/demand. Even an LED lamp causes this. So I guess a smoothing/ripple circuit is needed, but where? In the output from the charger, or the input to the amp? And if so how would I make this? I'm guessing some large capacity Electrolytic caps 50v? might be the answer. Also, the output from the TV is via the 'Headphone out' socket and is way way too loud, even with the amp gain set at it's lowest setting (there is no volume control on the amp, only gain) we have to use the TV with only 1 or 2 'bars' of volume which is still quite loud. There was a suggestion that a 50k dual pot might be used to aleviate the problem, on the line in.However I would not like to compromise the TV as it is still under warranty. I would be eternally grateful if some of you experts could advise me. This is my first visit to the forum so I hope I have not infringed any rules. Best wishes to all Alex

 

[CocaCola]

You might be best tackling the noise at the alternator, they have nice filtering caps and other designs just for this purpose... Google 'alternator noise filter'

As for the volume control, the easiest way to deal with it is to get an inline volume control, Radio Shack even carries one http://www.radioshack.com/product/index.jsp?productId=2102975

You can certainly approach these solutions with DIY versions but sometimes it's easier and quicker to pick up already done solutions...

 

[davenn]

Ummmm .... he said when the batter charger is in use, this would presumably be when at a caravan park and the charger is plugged into the site mains power outlet.

@ Alexma .... its pretty common that a lot of mains powered car battery chargers have a very raw output .... transformer and rectifier .... there isnt even usually a smoothing capacitor ...... you could try some serious sized smoothing caps across the output of the charger say 22,000uF to 33,000uF @ 25VDC rating
KNowing that you could be running lots of things off the system ... TV's sound gear, fridge etc there could be a respectable current drain
So I have err'ed on the high side for values .... you may get away with 10,000 to 22,000uF

cheers
Dave

 

[CocaCola]

Ummmm .... he said when the batter charger is in use, this would presumably be when at a caravan park and the charger is plugged into the site mains power outlet.

Well I read it as unsure of the source thus the "(?)" and didn't specify when running, generator or plugged in...

 

[CDRIVE]

I'm surprised that you describe a "buzz". I would expect it would be 100Hz hum and 120Hz hum on this side of the pond. Since a large wet cell will usually smooth out LF ripple as well as many capacitors I wonder if something else is going on here.

Is the charger circuit a dedicated line or wired like this?

 

[CDRIVE]

Or is it wired like this?

 

[Alexma]

Thank you all for your replies. I have deduced that this is a problem with the output from the tv related to RF interference, as there are no problems when an Mp3 player is connected. I feel the output from the headphone socket is too high thus requiring a low volume setting on the tv. I think an attenuator in the line in is needed. Can anyone suggest a suitable circuit please, or any other solution? Best regards Alex

 

[CocaCola]

I think an attenuator in the line in is needed. Can anyone suggest a suitable circuit please, or any other solution? Best regards Alex

Read my previous post, I linked you to a Radio Shack offering... You can of course do it yourself it you want with a dual pot or fixed resistors...

 

[BobK]

Most likely the noise is cause by having the TV volume turned so low. If you solve that problem, I think your noise will go away, at least some of it. Since there is a gain control on your amp, you could simply use a two resistor divider to reduce the TV output by, say a factor of 10. I.e. Put a 10K and and 1K in series across the headphone out, with the 1K to ground, and then take the output from the junction of the two resistors.

Bob