This lowpass-filters high frequency noise out of the power supply current, so I can easily see the average current change vs time.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Image successfully displayed! Well done, John! :-D
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The images that people have whined about not seeing, weren't images.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I have not seen the previous posts of this topic. But, the image looks like it was taken from LTSpice.
So, the resistor is 1 milliohm and the capacitor is 1 millifarad?
Right. This is feeding a switcher working at 4 MHz, and the ripple current is huge. This lets me see the averaged current vs time, without all the hash.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
John Larkin wrote on 1/17/2018 7:55 PM:
Why does the actual power supply current need to be filtered rather than treating the voltage on the sense resistor as a signal with a simple filter with much smaller components (in fact, I don't think the inductor is needed). Just how high is the ripple current? Are you worried that the voltage on the sense resistor will muck with the switcher?
-- Rick C Viewed the eclipse at Wintercrest Farms, on the centerline of totality since 1998
The sense resistor isn't totally frequency-independent, there's skin effect. An inductor in a low impedance path IS a small component, as power elements go.
I guess it was more the 1000 uF cap that seems like it should go. But since no one is trying to measure higher frequencies, quite the opposite, it makes no difference to the measurement circuit. Are you suggesting the skin effect will perturb the operation of the switcher? I mean, you will get skin effect in the conductors as well.
I was looking at using copper tape for a loop antenna where the high currents make the losses in copper very significant. It was *very* hard to find much info on the impact of the skin effect in rectangular conductors.
-- Rick C Viewed the eclipse at Wintercrest Farms, on the centerline of totality since 1998
Yes, this doesn't kook like a typical optimized John Larkin solution. The cap is low-voltage, nice, but in a quick look, Digi-Key's largest MLCC was 470uF, a 10x shortfall for 5m.
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Thanks,
- Win
I don't understand what he's trying to do, either. A 5mF capacitor isn't going to look much like a capacitor at 4MHz. From the datasheet, even your 470uF MLCC capacitor looks like it has a SRF of about 200kHz.
LT Spice has better, and cheaper, caps than Digikey.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
It's better with one more cap, to make it 3rd order.
This was just fiddled, but I guess there is a class of LC current filters, rather than voltage filters. Maybe all I need is a single-end-terminated LC lowpass, right out of the Williams book. Gotta try that.
I guess it depends on how you look at it.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Snark!
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Thanks,
- Win
Sorry, I assumed it was obvious, with these values, that this circuit would never be physically realized. It has no output.
This lets me see my pulse outputs and my supply current, all with proper scaling on the graph.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
The trivial is the source of endless argument here. Next they'll spend hours arguing that a relay does not have infinite gain.
NT
That would be a *very* good capacitor, with an ESL of just 1.3nH! In fact it's better than I'm willing to believe.
It doesn't matter however. Even if the capacitor is inductive above
200kHz, the filter still attenuates a lot. It's just that the attenuation no longer increases with frequency.Jeroen Belleman
It's measuring the current in a Spice simulation, guys.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Check the datasheet. ;-)
Point taken.
Right! I saw 470uF but missed the MLCC.
OK, now I believe.
Jeroen Belleman
Yikes, $4.55 in onesies and $1.85 in reels!
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net https://hobbs-eo.com
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