Ground loop problem?

[carbongixxer]

Hey Guys,
I think I'm having a ground loop problem with a video system in my car. Maybe you guys can help.

I have a 4 channel video system in my car that is wired the 12volt battery in the car. I have cameras connected to the 4 channel processor. The processor is then connected to a separate digital video recorder that runs on internal batteries. I just recently got a car charger that will connect to the digital video recorder to supply continuous power to it. When I plug it into the cigarette lighter and then into the video recorder I start to get hum bars moving across the screen of the recorder. If I disconnect the car charger they go away. Quick note the car charger converts 12volts down to 5volt at 2A.
Is this ground loop problem? Any help you could give me would be great.

Thanks.

 

[MrDEB]

sounds more like a power supply filtering problem maybe.
an automotive battery charger (low cost) have no exacting standards for how clean the output voltage is. you buy an expensive car charger and they put out cleaner voltage.
old addage, you get what you pay for.
a couple of capacitors might clear up the problem but ?? how good is the output voltage regulation?
what description of the charger, model, etc. any specs?

 

[smanches]

Do you get the same thing when the car isn't running? If it's nice and clear when the car is not running, it's a filtering issue.

If you still get the "hum", then it might be the charger itself.

 

[carbongixxer]

Hey guys,
Thanks for your fast response. The charger is made by the same company that makes my recorder. Here is 2 pics of the charger. One is the label on the back and the other is the internal circuit board.
Smanches: The bars are there when the car is off and on.
**broken link removed**
**broken link removed**

 

[smanches]

Since it happens with the car off, it's most likely the charger itself. The 470uF cap can't handle the current ripple needed by the device, most likely. Either that, or the chargers voltage ripple is really high.

Can you place another cap on the other end of the charger where it connects to the system?

 

[carbongixxer]

I'm sure I could solder one inline with the charger. What size cap should I use?

 

[Hayato]

I think 16V is too low. Use at least 1000uF x 25V caps.

Looks like the capacitor exploded or leaked. So remove the old ones first.

 

[carbongixxer]

You think I could just pull that 16v one and place a 1000uf 25v there?

 

[Hayato]

I see no problem doing that.

 

[carbongixxer]

Cool I will give it a try. Thanks for your help.

 

[smanches]

Well, the datasheet shows 1000uF for a 3A load. Estimating a 666uF for a 2A load would say add another 200uF or so. Going higher than that won't hurt anything at all, and might help in case the switcher is acting badly and causing more ripple than it should.

 

[carbongixxer]

how about the 35v 100uf capacitor next to it?

 

[Hayato]

This 35V is connected directly to the 12V car's output?

 

[carbongixxer]

Correct, when I volt meter it. The 35volt cap has 12volts at it and the 16v 470uf cap has 5volts at it.

 

[Hayato]

ok.
What is the regulator IC?

 

[carbongixxer]

on the regulator it reads:
JM76RP
LM2576T
-ADJ P+

 

[smanches]

It's an all contained buck regulator. Just requires an external inductor.

 

[carbongixxer]

It's an all contained buck regulator. Just requires an external inductor.

I'm new to this, so can you explain what you mean by a external inductor?

 

[Hayato]

smanches, the inductor is there.

 

[smanches]

With this chip the only external parts you need are the two filter caps and an inductor. It's about the most simple SMPS you can build. If you were to build it from discrete components, there would be much more to do.

Most likely, the chip is not behaving exactly as expected and has more voltage ripple than the video system can tolerate. Adding a newer larger cap, or an additional cap (on the output) could help a lot.

Hayato: I know it's there. I'm not sure what you were pointing out? I was talking generically about the chip itself, not the specific charger.