Metacelsus
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The weirdest thing happened today with my oscilloscope . . .
I was working on a boost converter designed to convert from 12V DC (lead-acid batteries) to 330V DC (for capacitor charging). I was testing out a
different inductor, and was about to measure the input voltage waveform with my oscilloscope. When I did so, the GFI tripped on the outlet that the
oscilloscope (and only the oscilloscope) was plugged into. However, the oscilloscope and the circuit showed no signs of damage.
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Burner
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Doesn't the GFI trip when current on the "hot" fails to match the current on the "neutral" within a couple of milliamps (http://en.wikipedia.org/wiki/Ground_Fault_Interruptor)?
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Metacelsus
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Yes, that is true, and it is why I am mystified. It's not like an overcurrent due to some malfunction within the oscilloscope.
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Burner
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Is it an older scope? I have a 1960's Tek and I am sure that it is starting to show its age. It would not surprise me that my unit might not pass
the "GFCI test". 
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Metacelsus
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Yes, it is quite old (it uses vacuum tubes ). However, this was the first (and
so far only) time that it caused the GFCI to go off.
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Burner
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I am guessing that you may have a minor, intermittent leak to ground internal to the chassis of your scope. That would trigger the GFCI. I work in
the basement and have only GFCI near the sink so I have not run across a similar issue.
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macckone
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High frequency spike in the circuit under test can cause GFCIs
to trip. The solution is to put a RFI filter on the power cord.
Surge suppressor bus strips usually have these built-in.
If that doesn't fix the problem you may want to have the
scope checked (or check it yourself).
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Metacelsus
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That must have been the problem, then. It's not surprising, given the nature of the circuit being tested.
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Varmint
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I wouldn't expect any problems with the scope. As has been established, current flowing in the ground connection needs to match that flowing in the
neutral.
So, how can you have current flowing through ground? Well, if the circuit under test is "grounded" at any point, then that point must be your only
coonection to your ground clip from the scope. People fall into this trap a lot when using variacs for a "soft start", not realizing the variac is an
autotransformer, and thus provides no isolation like you would expect from a genuine transformer. In essence, "ground" can be quite "hot" in these
situations.
Another possibility is based on a properly isolated circuit radiating enough energy (remember, "RF" starts in the mid KHz for all pratical purposes),
and enough radiation from any other point in the circuit or attached load might induce enough current in the ground leg to look like a ground fault.
If providing a ground path for radiated energy is the fault mode (example 2 above) I woulld expect you could connect ground from a completely
different power cord (no O-scope) to the same point the scope's ground was connected, and have it cause the same GFI fault condition.
The exact opposite condition could be in play too, where the scope input is attached but the ground lead is not, or is experienceing a poor
connection. In this case, "RF" from the circuit can pass though the input, and through capacitive coupling also induce current in the ground lead.
In this case though, the input would need to be of high enough voltage and frequency so the coupling is efficient enough to generate the level of
current required to cause the trip.
Either way, until further troubleshooting takes place, I wouldn't be placing the blame on the scope just yet.
FWIW, I have a 2W 5MHz battery powered oscillator I designed as a kid that will trip GFI if ground from the GFI outlet is made to contact the BNC
center (output). The case and leads from the battery are fully isolated from ground, but form enough of an antenna that pretty good coupling is
achieved and the GFI fires immediately.
DAS
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Metacelsus
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At no point (except the oscilloscope lead) was the circuit grounded. The circuit operates at 30 kHz. Tonight, I'll try to replicate the fault without
the scope.
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aga
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It is quite normal to find a second hand 'scope to have the earth lead removed.
The reason (especially for TV power supplies) is that the 'earth' of the circuit you're trying to fix is not at 'earth' potential, and can be 100v or
more above it.
Suddenly earthing said circuit by attaching the probe clip will make something pop, so the 'scope earth gets cut to prevent that.
Unsafe for sure, but is common, and works just so long as you never touch the scope body and a proper earth at the same time.
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Metacelsus
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probably not the scope after all . . .
I just replicated the ground fault this afternoon without the scope; it seems as if the frequency that the circuit uses is enough generate significant
interference. The total power used by the circuit is 200 W, but little of that should be radiated.
By the way, the reason I'm messing around with the circuit is that it's only giving me 190 volts, not the expected 330.
[Edited on 19-5-2014 by Cheddite Cheese]
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Varmint
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Yep, I was pretty certain of it.
Now, take your scope, don't attach anything, and just get NEAR the areas you expect should have high frequency, with just the probe tip.
Get enough signal for deflection, sync on it, and measure the frequency. I suspect your inductor is far too low a value and is reaching saturation
before the circuit thinks it should be done dumping current into it.
You should be able to get a pretty good signal just using the radiated signal and figure it out from there.
DAS
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The Volatile Chemist
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Mind if we see the schematic? I'd be interested!
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Metacelsus
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The schematic isn't actually that interesting; it's just a basic boost converter driven by a 555 timer.
Timer component values (subject to change):
R1 = 36 kOhm
R2 = 1 kOhm
C = 1.2 nF
The MOSFET that the 555 drives is an IRFP460.
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The Volatile Chemist
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I'm not familiar with boost converters, I've never had a formal education in electronics... What are they/look like?
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Varmint
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Volatile:
You can gain a rhudimentary understanding of converter topologies by doing a wiki seach for boost converter, buck converter, and flyback converter.
These are the primary switching power supply designs currently in use, each offering their own advantages. Using readily available parts and
single-chip converters, switching power supply design is within everyone's reach these days.
No longer do you need massive iron cores and heavy copper windings to achieve a given output, a handful of parts and some quick study can get you the
supply you need in short order.
In the old days of strictly linear power supplies, you might have an enormous transformer, one or several very large capacitors, and a rectifier of
some sort to get an unregulated ouput, to which you would add a string of pass transistors, a voltage reference, and a driver circuit to maintain the
voltage at a regulated level. The more voltage/current out, the bigger everything had to be since it was all running off the line voltage at 50 or
60Hz. A 1000W supply might have weighed 50lbs or more, and been well over a cubic foot.
Now that same 300/500/1000W power supply could weight 3 or 4 lbs, and be no larger than a box of crackers. All of this brought to you by the
efficiency of higher operating frequency and switching elements (FETs, MOSFETs, etc..) whos performance approaches perfection with almost zero "on"
resistance and almost infinite "off" resistance, coupled with very low switching time between off and on, and just as important, on and off!
DAS
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The Volatile Chemist
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Quote: Originally posted by Varmint  | Volatile:
You can gain a rhudimentary understanding of converter topologies by doing a wiki seach for boost converter, buck converter, and flyback converter.
These are the primary switching power supply designs currently in use, each offering their own advantages. Using readily available parts and
single-chip converters, switching power supply design is within everyone's reach these days.
No longer do you need massive iron cores and heavy copper windings to achieve a given output, a handful of parts and some quick study can get you the
supply you need in short order.
In the old days of strictly linear power supplies, you might have an enormous transformer, one or several very large capacitors, and a rectifier of
some sort to get an unregulated ouput, to which you would add a string of pass transistors, a voltage reference, and a driver circuit to maintain the
voltage at a regulated level. The more voltage/current out, the bigger everything had to be since it was all running off the line voltage at 50 or
60Hz. A 1000W supply might have weighed 50lbs or more, and been well over a cubic foot.
Now that same 300/500/1000W power supply could weight 3 or 4 lbs, and be no larger than a box of crackers. All of this brought to you by the
efficiency of higher operating frequency and switching elements (FETs, MOSFETs, etc..) whos performance approaches perfection with almost zero "on"
resistance and almost infinite "off" resistance, coupled with very low switching time between off and on, and just as important, on and off!
DAS
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Thanks, I'll do so. I might order some Lead Acid Batteries (From sciplus.com), so this'll be mighty convenient information.
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Metacelsus
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OK, I just got a satisfactory measurement of the input waveform. The frequency is 34.28 kHz and the duty cycle is 94.2%. Based on the duty cycle, I
should be getting 207 V from 12 V input. 190 V (the measured output) is not far off, so I think I should increase the duty cycle. It should be 96.4%
if I want 330 V.
How far can I meaningfully increase it? For example, will 97.5% really give twice the voltage as 95% (as predicted)?
I might look into making this a flyback converter instead of a boost converter (replacing the inductor with a transformer) if I'm hitting a duty cycle
wall.
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aga
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@Volatile
Buck and Boost converters basically take the DC voltage coming in, and use that to store Energy in an inductor.
Depending on whether you want to Buck (step-down) or Boost (step up) the voltage, you either put the inductor in series or in parallel with the
incoming voltage source.
Then you switch it off/on really fast (with a mostfet usually) to generate a waveform that you then rectify and end up with what you want.
Schematically it's simpler to understand :
http://ww1.microchip.com/downloads/en/AppNotes/01015a.pdf
Skip the chip specs and scroll down to "Theory of Operation"
that describes a Buck converter.
@Cheddite
If it's Voltage you're after, decrease the inductor value and increase the frequency
AT 90 odd % duty you'll fry either the coil or the mosfet.
[Edited on 21-5-2014 by aga]
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Metacelsus
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Finally, the boost converter is working as planned. I ended up increasing the frequency, reducing the duty cycle, and center tapping the inductor (in
effect using it as an autotransformer) to maintain the same voltage. I managed to also get my efficiency up to 89.2%.
By the way, shorting a 330V 1900 uF capacitor is quite the experience!
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Varmint
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Off topic, but your shorted capacitor "sparked" an old longing of mine, inductive coin shrinking.
This guys equipment is truly top notch, but so are the results.
Your voltage and capacitor aren't suitable for this sort of thing, but I thought you'd find it interesting just the same.
http://www.capturedlightning.com/frames/shrinkergallery.html
DAS
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