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Old 04-10-2014, 06:25 AM   #1
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AC Interference With DC Instrument

I installed a new Garnet See Level waste tank monitoring system in my old Trade Wind. It works normally when I'm on 12v DC power only.
But when I connect 115v shore power, the display goes wacky. I'm getting AC interference in my DC instrument.
My AC to DC converter is a 1999 vintage unit. I'm told I need to add a electronic device to my converter that will "clean up" the DC from the buzz of AC.
Does anyone know what this electronic device is and have had success with it after installation?

David
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Old 04-11-2014, 08:56 AM   #2
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It sounds like the converter might have some issues. The modern converters are switching power supplies that use very high frequencies to convert from AC to DC. I expect something like a cap has gone bad in the converter. Also check to see that your grounds are good. The switching power supplies are much lighter and more efficient but the can produce more noise than the old linear power supplies like the old Univolts (boat anchors).

Perry
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Old 04-11-2014, 10:37 AM   #3
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It isn't necessarily the converter's fault. But if it is, you can put a bypass capacitor, or a ferrite bead, or both, on the output.

For a bypass capacitor I would suggest a 35 volt, 4700 uF electrolytic. Be sure you get the polarity right. You can get them for a dollar or two from electronics supply houses. There are also fancy chrome plated ones from places that sell car stereos. Either way they would connect between the + and - output from the converter.

For a ferrite bead just use anything that will fit over the output wires, either the + or both together.

But these sorts of problems can also be caused by bad grounds. In some cases a ground connection can conduct DC but, due to corrosion, not AC. (I'm simplifying)

Start by being sure that all the ground connections are clean, tight, short, and suitably sized. It can be helpful to make ground connections with braid rather than wire, because braid conducts AC better. (again, I'm simplifying)
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Old 04-11-2014, 07:21 PM   #4
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Garnet Tech Support suspected a bad ground. I measured 2 ohms from the display connector to the ground spot I selected. Since I had cleaned the trailer frame (ugh!) with a die grinder and stiff wire brush, I paid special attention to the DC ground and the AC ground spots on the rear of the frame. Nice bright metal on both sides of the bolt. None the less, I ran a separate "test wire" from the instrument to the battery ground and get the same erratic display when on shore power.


The instrument works normally while "boondocking" on the battery power only. As soon as I plug in 30 amp shore power, it gets erratic.


Thanks for the specs on a capacitor. I think my 1999 converter is making RF interference in the DC wiring.


David
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Old 04-11-2014, 08:15 PM   #5
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Well it is 15 yrs old and stuff does not last that long now days. Try a different power supply/battery charger/converter. Hook a car type battery charger to it and see if the problem goes away. Let's say you put a filter in the line and the problem is covered up. Well maybe the next time it will start smoking instead of sending out noise.

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Old 04-12-2014, 06:09 AM   #6
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Thanks a bunch for your input Perry and Jammer. It's great you folks spend time helping out the great mass of the "unknowing" like me.
I don't know yet if new, modern converters have RF noise suppressors built in. I know they are much better at battery management. I have 5 or 6 DC circuits initiating at the converter. Maybe I need a capacitor or ferrite ring on each one? The "kids" at Radio Shack were clue-less to my needs, but they did check the internet for me to no avail. I also stopped in a local RV dealer who has a big parts and service business. They did not have a "noise filter" for converters and wished me good luck.
I'm not sure, but maybe the copper ground wire for the converter is connected to the 115v ground bolt to the frame. Is that a factor in RF noise generation?
I need to get my new See Level display to work on shore power. Do I need a noise suppressor on each of the 12v wires coming out of the converter? I have AC "ripple" in my DC circuits. The Airstreamers who are Ham operators may have fought and won this battle.
David
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Old 04-12-2014, 10:17 AM   #7
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You know, I'd try one of these big ol' capacitors that they use now to stabilize DC voltage in cars when they add a 2.2 giga-watt bass thumper system. Those would go between the converter + output and ground.

The next step would be a choke coil in series with the positive output of the converter. I don't have any decent specs on the choke coil, which would have to be a pretty sturdy/heavy/expensive gadget. Do me a personal favor, though, and don't use both the choke and the capacitor at the same time. It's conceivable that you could get into a harmonics situation and let some smoke out of something. (Unless you really know electronic circuit design or have a friend who does.)

The next step after that would be to get in touch with the guy at bestconverters.com and have him set you up with a new one that wouldn't do that.
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Old 04-12-2014, 05:21 PM   #8
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It is going to have to be one mother of a choke to handle 60 Amps or whatever the current capacity is.

Perry
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Old 04-13-2014, 05:33 AM   #9
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I researched new converters and see some make mention of "filters" that provide good operation of TVs and stereos and other electronic devices. I haven't found anyone who provides RF noise suppression devices that are retrofitted to existing converters. I suppose an oscilloscope would track the "ripple" in the DC voltage caused by the converter and show improvement with capacitors or ferrite rings.


I'll bet some Airstreamer has installed a Garnet See Level display unit and had this same problem, and then solved it. Steel shamelessly, it saves headaches!


David
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Old 04-13-2014, 09:51 AM   #10
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A scope will tell you what the frequency of the noise is and then you can select the proper size cap to put in there. Computer power supplies are of the switching type and they don't put out a ton of noise.

Perry
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Old 04-13-2014, 11:17 PM   #11
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My old Magnetek converter had about 7VAC ripple on the DC output. My new fridge with DC controls wouldn't work reliably until I directly connected it to the battery.

A new Powermax Boondocker fixed the problem permanently and upgraded the charger from a single stage to a 3 stage.
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Old 04-14-2014, 06:13 AM   #12
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Thanks for sharing your experiences Kyle401. I was looking at those converters on line. Interesting new fridge with fancy controls wouldn't function normally without going directly to the battery posts. My Garnet 709 See Level display jumps between 7 volts and 17 volts when the AC power is on. I assume this is just garbage, but I wonder if my voltage ripple is that great?


Maybe I need a new, modern converter with noise suppression. More money spent! But it may be the best option as opposed to hanging capacitors and ferrite rings on the DC wiring. Besides, my old converter over charges my battery at 13.8 volts continuously.


David
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Old 04-14-2014, 11:38 PM   #13
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Your converter appears to be a 3230. I had a 3240 (have it in the garage still actually). If yours is similar there is a trim pot on the battery charger section that you can adjust to lower the fixed charging voltage. Lowering the voltage can prevent overcharging, but it will also reduce charging speed.

Parallax has several pretty good resources for these considering they are an obsolete model. I have attached a couple of troubleshooting guides. The charging troubleshooting guide that details the presence of AC ripple as "good" is what finally convinced me to upgrade. Testing the AC ripple is pretty straight forward and is described in the trouble shooting guide.

You could move circuits that are sensitive to AC ripple to the charger section outputs. I was going have to spend 1/2 the cost of the new converter to rewire a couple of circuits since my filtered output didn't have any unused circuits. I bought the PM4-45.
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Old 04-15-2014, 06:23 AM   #14
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Kyle401, thanks so much for your research and response. I'm leaning to a new Power Max Boondocker in the 45 amp range as they advertise good ripple suppression. I hope the install is straight forward. I believe all my DC circuits radiate from the converter. I read the Boondocker is easy to install as all you do is connect the large battery wires and that's it. Did I miss understand something? I assume there is some type of fuse panel, or connection posts inside the Boondocker to connect my 7 DC circuits, one of which charges the battery.


Was your Boondocker install pretty easy and did it involve all of your DC circuits?


David
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