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Old 14th July 2012, 01:59 AM   #1
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Default Earth Lift circuits and electrical safety

So you build a nice low distortion oscillator, or other piece of equipment, and you come up against the issue of earth loops between unbalanced mains powered circuits. The classic solution is the Earth Lift switch. The chassis of the oscillator (say) remains earthed (in case of transformer or power switch breakdown to frame or a wire coming adrift), but the transformer secondary circuitry itself floats, picking up its earth from whatever it's attached to. Even so, there are risks. Supposing the transformer breaks down to the secondary?

Back in my day [cue creaking door sounds], we used to put a pair of biggish power diodes, in reverse parallel, across the earth lift switch, on the premise that if the transformer arced over, the diodes would last long enough to take out the fuse. I don't remember anyone ever testing the premise!

That, in theory, limits any deviation from ground to +/- 0.65V. We also put a 100nF disc ceramic across the diodes, so that the floating circuitry was increasingly "earthed" at higher frequencies, but not at mains frequency.

What is current thinking about all of this? Are there better ways, or is the old way good enough?

Terry
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Old 14th July 2012, 02:38 AM   #2
jcx is offline jcx  United States
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Default Class II

double/reinforced power tansformer isolation - pass 4 kV hipot, meet spacing, redundant fixing of hot side components and you don't need no stinking safety gnd

still have pri-sec leakage C - split bobbin better than layer wound/toroid


or there's always batteries
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Old 14th July 2012, 04:19 AM   #3
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Quote:
Originally Posted by jcx View Post
double/reinforced power tansformer isolation - pass 4 kV hipot, meet spacing, redundant fixing of hot side components and you don't need no stinking safety gnd
The kind of information the suppliers provide doesn't translate easily into English. EG: for a typical split-bobbin transformer:

"Insulation: Class E (120C)
AS/NZS 61558.2.6 approval: Q042028
AS3108 approval: CS/813/Q"

Quote:
still have pri-sec leakage C - split bobbin better than layer wound/toroid
Yes, whatever happened to the good old electrostatic shield? I'd feel pretty secure on the other side of an earthed shield. And cleaner.

Quote:
or there's always batteries
That's getting a bit inconvenient, but not out of the question. I suppose another thought is two transformers in series, and earth the secondary of the first one. "Double insulation" gone mad!

Terry
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Old 15th July 2012, 12:00 AM   #4
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For low powered units, just use a commercial wall-wart transformer or DC power supply. It meets the safety regulations.
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Old 15th July 2012, 12:20 AM   #5
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Yeah, good point. A good way to make some use of the pesky things left over from USB hubs, modems, etc! And keeping the mains right out of the box can't hurt.

I have tended to avoid these because they are so often left on 24/7, consuming energy wastefully. But in this case, I intend to have a "master on/off" switch for my test gear, so it would get fully powered down when the test gear is not in use.

Combine that with a standby switch on the unit, and one of those pretty blue LEDs. I don't know why, but they seem to glare at me in a very accusatory way.

Terry
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