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Old 10-20-2007, 10:20 PM
robbie53 robbie53 is offline
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Default battery charger hum

Hello everyone! I'm new to this so please bear with me. I have a battery charger connected in my RV to maintain the batteries. Problem is: If I try to use the 12v radio, I get a hum if the charger is on. Sounds like 60 cycle, but I'm not sure. There is a .22v AC component with the charger on the low (3A) setting, ~.5v AC at the 10 amp setting. Is there a way to filter this, or should I just buy one of the new "smart chargers"? I've looked at the Rectifier/Filter circuit in the Volume VI experiments. Will this work? Any help would be appreciated!
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Old 10-20-2007, 11:05 PM
GS3 GS3 is offline
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I would definitely try filtering the power supply as a first measure. A good sized choke and a couple capacitors should be a good starting point.

In the end though it could be that some or most of the hum is coming in the antenna and that would require other measure. To test this you can power the radio momentarily from a separate battery which is not being charged and see if the noise disappears.
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Old 10-20-2007, 11:55 PM
wireaddict wireaddict is offline
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Besides an L-C AC ripple filter, make sure you have a good battery or batteries & the connections are clean & tight; the battery provides some filtering itself. One way to check the battery operation & connections is to unplug the charger with some lights on; if they immediately dim then the battery isn't doing its job, either because of poor connection[s] or it's nearly dead & won't charge.
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Old 10-21-2007, 07:53 AM
GS3 GS3 is offline
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Quote:
Originally Posted by wireaddict View Post
Besides an L-C AC ripple filter, make sure you have a good battery or batteries & the connections are clean & tight; the battery provides some filtering itself. One way to check the battery operation & connections is to unplug the charger with some lights on; if they immediately dim then the battery isn't doing its job, either because of poor connection[s] or it's nearly dead & won't charge.
I think you are missing the point here which is that the battery is not being charged by DC but by a pulsating rectified AC and this causes the ripple as the battery goes from charging to discharging at twice the mains frequency.
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Old 10-21-2007, 04:53 PM
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thingmaker3 thingmaker3 is offline
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Try a filter on the radio itself. Filtering 100% of the current used by the RV is not needed (and not easy). Only the current drawn by the radio need be filtered.

Get a pair of electrolytic capacitors and a small "step-down" transformer from a surplus electronics dealer. Cover the transformer secondary leads with wire-nuts or heat-shrink, so that they will never make electrical connection with anything. Hook each transformer primary lead to the "+" terminal of each of the capacitors. Hook the "-" leads of the capacitors to each other, and to the "-" of your 12V power cord. Hook "+" of power cord coming in to one of the "+" terminals of the capacitor. Hook "+" of power cord out to radio to the other capacitor's "+" terminal.

This should greatly reduce the hum.

To learn how this circuit works, run a Google search on the exact phrase: "pi filter."
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Old 10-21-2007, 08:17 PM
robbie53 robbie53 is offline
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Quote:
Originally Posted by GS3 View Post
I would definitely try filtering the power supply as a first measure. A good sized choke and a couple capacitors should be a good starting point.

In the end though it could be that some or most of the hum is coming in the antenna and that would require other measure. To test this you can power the radio momentarily from a separate battery which is not being charged and see if the noise disappears.
I checked out the antenna - same problem. Thanks for the info!
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Old 10-21-2007, 08:20 PM
robbie53 robbie53 is offline
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Quote:
Originally Posted by thingmaker3 View Post
Try a filter on the radio itself. Filtering 100% of the current used by the RV is not needed (and not easy). Only the current drawn by the radio need be filtered.

Get a pair of electrolytic capacitors and a small "step-down" transformer from a surplus electronics dealer. Cover the transformer secondary leads with wire-nuts or heat-shrink, so that they will never make electrical connection with anything. Hook each transformer primary lead to the "+" terminal of each of the capacitors. Hook the "-" leads of the capacitors to each other, and to the "-" of your 12V power cord. Hook "+" of power cord coming in to one of the "+" terminals of the capacitor. Hook "+" of power cord out to radio to the other capacitor's "+" terminal.

This should greatly reduce the hum.

To learn how this circuit works, run a Google search on the exact phrase: "pi filter."
Makes perfect sense to me! A friend just told me that Radio Shack sells a "Noise Filter" which is exactly what you described. Supposedly available in a 15 - 20 amp version. That should take care of my radio and amplifiers. Thanks for your help!
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