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Ground loops and breadboards

post #1 of 9
12/7/09 at 6:20pm
Thread Starter 
So...I decided to test a project amp of mine on a breadboard.
I'm using a toroid in a center tapped configuration with the center tap connected to safety ground (earth).

Well, this worked fine with my ipod, but as soon as I connected it to something into the mains I got terrible hum and noise.

First I thought the source was just that noisy, but after trying with a usb dac and getting even worse results, I knew something was not right.
Tried leaving safety ground unconnected and all the hum and noise was gone...just silence. So, I figure I have a ground loop.

Well, my question is this: is this usual with breadboard connections, and is it expected to go away with proper casing, isolation, grounding, pcb w/ gnd plane, etc?

If it does not, I suppose I'd have to put a ground loop breaker in-between.
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post #2 of 9
12/7/09 at 6:37pm
Quote:
I'm using a toroid in a center tapped configuration with the center tap connected to safety ground (earth).
Why?
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post #3 of 9
12/7/09 at 6:39pm
Thread Starter 
Center tap connects to circuit ground, so circuit ground to safety ground...no?
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post #4 of 9
12/7/09 at 7:03pm
Quote:
Originally Posted by wap32 View Post
Center tap connects to circuit ground, so circuit ground to safety ground...no?
Safety ground to metal chassis.

Circuit ground can go to chassis via an RC network. 51 ohms in series with 0.01uF.

If you're breadboarding, don't bother with the safety ground.

se
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post #5 of 9
12/7/09 at 7:13pm
Thread Starter 
Yes, that would be with a ground loop breaker.
But couldn't any potential failure blow up the resistor before the breaker kicks in, leaving potential ac voltages exposed?

I've seen something like this, which seems safer.

Still, I've built amps with circuit ground to safety ground without any grounding problems whatsoever, hum or otherwise.
My question was if I should expect the issue to go away once I have the circuit off the breadboard and into a chassis and pcb.
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post #6 of 9
12/7/09 at 7:44pm
Quote:
Originally Posted by wap32 View Post
Yes, that would be with a ground loop breaker.
But couldn't any potential failure blow up the resistor before the breaker kicks in, leaving potential ac voltages exposed?
The purpose of the safety ground (which is to be connected to the chassis at the nearest point possible to the point of entry), is to provide a return path for fault currents in the event that the AC hot comes into contact with the chassis which can expose the user to potentially lethal voltages.

Of course to be an effective shield, the chassis needs to connect to circuit ground. But since the shielding is primarily against RF, an RC network can be used to provide a low impedance connection to the chassis at RF frequencies, but a very high impedance at AC line frequencies.

And because it has a very high impedance at AC line frequencies, it's not going to blow up the resistor.

Now, if the chassis meets Class II (double insulated) specs, then you can do away with the safety ground altogether. And indeed, most mass market consumer audio gear uses just a two prong plug, which is why those who use mass market consumer audio gear rarely have ground loop issues.

Quote:
Still, I've built amps with circuit ground to safety ground without any grounding problems whatsoever, hum or otherwise.
It's often a crapshoot given that it depends on a number of factors such as how the other equipment is designed, where you have them plugged in, etc.

Quote:
My question was if I should expect the issue to go away once I have the circuit off the breadboard and into a chassis and pcb.
If you're getting it now, you'll get it when it's in the chassis as long as you have the same direct connection to circuit ground.

se
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post #7 of 9
12/7/09 at 7:59pm
Thread Starter 
Quote:
Originally Posted by Koyaan I. Sqatsi View Post
[...]
And because it has a very high impedance at AC line frequencies, it's not going to blow up the resistor.
But then what would happen if say, an ac line got shorted to circuit ground?
The filter would either not conduct (high impedance to AC freqs), leaving me with ac exposed, or conduct and maybe blow up from the high currents, leaving me with ac exposed. Am I seeing this right?

Quote:
Originally Posted by Koyaan I. Sqatsi View Post
[...]
If you're getting it now, you'll get it when it's in the chassis as long as you have the same direct connection to circuit ground.
But aren't ground loops also dependant on board layout, case grounding, etc, etc?

Still, I see I'll probably have to have a ground loop breaker of some sort, any comments on the circuit I linked earlier?
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post #8 of 9
12/7/09 at 8:25pm
Quote:
Originally Posted by wap32 View Post
But then what would happen if say, an ac line got shorted to circuit ground?
The filter would either not conduct (high impedance to AC freqs), leaving me with ac exposed, or conduct and maybe blow up from the high currents, leaving me with ac exposed. Am I seeing this right?
Nothing's perfectly "safe."

Even if you implemented Rod's scheme with the 35 amp bridge rectifier, unless all your circuit ground wires and circuit board traces were sufficient to handle the fault current, they would just blow like fuses and leave you in the same position.

The simplest way is to just implement the AC side of the power supply in such a way that hot could never come into contact with circuit ground.

Quote:
But aren't ground loops also dependant on board layout, case grounding, etc, etc?
They can be, yes. But in those instances, you'd have noise even without anything else being connected to the unit. The problem you're dealing with is interchassis currents brought about by way of safety grounds.

Quote:
Still, I see I'll probably have to have a ground loop breaker of some sort, any comments on the circuit I linked earlier?
It'll work, but with the caveat I mentioned above.

se
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post #9 of 9
12/7/09 at 8:45pm
Thread Starter 
Quote:
Originally Posted by Koyaan I. Sqatsi View Post
Nothing's perfectly "safe."

Even if you implemented Rod's scheme with the 35 amp bridge rectifier, unless all your circuit ground wires and circuit board traces were sufficient to handle the fault current, they would just blow like fuses and leave you in the same position.
Yes, that much is true, any connection on the way of the fault would have to handle such currents and not blow up.

Quote:
Originally Posted by Koyaan I. Sqatsi View Post
The simplest way is to just implement the AC side of the power supply in such a way that hot could never come into contact with circuit ground.
I wouldn't think of planning otherwise
It was more of a hypothetical question.

Quote:
Originally Posted by Koyaan I. Sqatsi View Post
They can be, yes. But in those instances, you'd have noise even without anything else being connected to the unit. The problem you're dealing with is interchassis currents brought about by way of safety grounds.
That hadn't occurred to me, but it makes sense; if the problem was intrinsic to the circuit, it would have showed up on it's own, not just when connected to other grounded devices.

I might still try using a closer outlet (it's plugged into a different extension cord, separated by a few meters of cable) to try to reduce the ground path between devices to a minimum, but I'm guessing I'll need a ground loop breaker anyway.

Thanks a lot for the help!
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