Home brew 120Vac LISN - Page 1
Forum section: electronics / testgear

author: Jester
author_rank: Frequent Contributor
timestamp: August 28, 2016, 03:28:27 pm
content: I'm considering making one of these:Are there any obvious recommended improvements that I should consider?Perhaps some protection on the output, or an integral attenuator?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2016, 07:27:39 pm
content: Beware that the 1uF caps draw quite a lot of ground current, so you won't be able to use this on a GFCI/RCD circuit.The power line itself is probably pretty noisy, so you should add extra filtering. That's what the two common mode toroids are doing on mine,The black wire and stacks of ferrite cores are the 50uH chokes, which saturate beyond 20A peak or something like that. Peak has to be at least 1.4 times the RMS current rating, otherwise your frequency response will vary with load current, which is weird. Air-cored inductors would be even better, but they'd be huge. I don't have that much wire, but I do happen to have a pile of loose ferrite cores. YMMV.The FCC Part 15 network is very similar to the CISPR spec shown, except it uses 5uF + 5 ohm bypass caps. Which seems reasonable, as far as keeping impedances nominal. Rather than trying to bypass the hell out of it, which doesn't work out very well in practice (a very low impedance reflects rather than absorbs power), the ESR allows some damping. The practical effect is how the specified impedance and attenuation curve drops off in the <300kHz range (you'd have to read the standard in detail, to see how tightly the curve is specified), and how the impedance interacts with the proceeding filter (if used).For safety and for reducing ground current, you should probably use it with an isolation transformer. In a testing situation, this is a good idea anyway.You can also add MOVs. Make sure they're rated for the voltage you'll use; I had initially installed 130V MOVs on mine, then forgot that fact when I went to do a 240V test...The H-N MOV got rather crispy... I quickly discovered the problem, and removed it for that test!For further protection, a clamp/limiter on the RF terminal might be a good idea, but be careful that it doesn't interfere with normal operation -- i.e. a really noisy device might produce a sizable fraction of a watt, which while being rather generously over the allowed limit... would still be important to measure linearly. More importantly, you want negligible IMD at more nominal signal levels, which means the clamp needs to act with as high and sharp of a threshold as possible.You might also want the limiter optional, or perhaps as an in-line attachment, for when using the LISN in reverse, for susceptibility testing.A highpass or bandpass filter might be cool, too. I put a 3rd order highpass filter on mine, so the low frequencies are very well attenuated. That keeps 60Hz and harmonics away from the spec. You want the cutoff somewhere below 150kHz, not right at it, so it doesn't disturb the specified attenuation curve.If you add a lowpass section as well, mind that it should be constant resistance towards the LISN. This can be designed from tables:http://jeroen.web.cern.ch/jeroen/reports/crfilter.pdfYou could set the cutoff for 30MHz, but it's likely the LISN is good for higher frequencies than that, especially if built well (I don't claim mine as a pristine example of wiring quality...). You might want it 100MHz or more, especially if, again, you'll be using it for susceptibility -- those tests often run up to 80MHz, conducted.Tim

author: joeqsmith
author_rank: Super Contributor
timestamp: August 28, 2016, 08:00:02 pm
content: I have a small circuit breaker in mine. Also two connector taps and then just terminate the one not in use.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2016, 10:21:29 pm
content: Quote from: joeqsmith on August 28, 2016, 08:00:02 pm^^ That's what I did.Tim