Self-powered Pulse Sharpener - Page 1
Forum section: electronics / projects

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2021, 04:39:39 am
content: Someone may find this useful --V1 is a signal generator, e.g. 0/5V (measured open circuit), 50 ohms. It may have a somewhat sluggish edge rate, up to 200ns say.Now say you wanted to drive a pulse transformer, very sharply (LO1 as primary). You'd have to do a whole song and dance to speed it up -- external power, get out a breadboard or something? Nah, do it like this instead. This is a schmitt trigger circuit, biased normally-on, and triggered by falling dV/dt (C1, C3). Using a differential pair, biased with resistor dividers, it presents an approximately resistive load to the signal generator -- around 50 ohms, making it a crude termination resistance as well. As such, VIN is 2.5V while on. Also as such, it's biased to around 50mA when on, which is about all those poor transistors can manage (hFE is quite low at this current), and about what goes into LO1 -- if you need less current, increase resistors proportionally. If you need more, you'll probably have to settle for slower operation, or more expensive transistors (2N3866 is about all that comes to mind?).In my case, LO1 is 12 turns of #37 AWG on a typical (3mm long) mix 43 ferrite bead. Secondary, 4 turns, driving a B-E junction of an avalanche transistor.The KTC1027 happens to avalanche around 300-350V, and is easily synchronized by driving it in this way.The rest of the circuit (to the right) is biasing, to get the 300V to the transistor (via 10k current limiting resistor), and couple the pulse out (via a pulse forming network at the header, and some resistors). The two wires are avalanche supply, and output bias (it's capacitor coupled so I can freely set the baseline voltage).I didn't save a scope shot of the schmitt trigger output, but suffice it to say it's reasonably fast, firing the avalanche transistor within say 10ns of the input falling below 90%. (The R+C across the primary is determined empirically; leakage inductance isn't great in this transformer, even as small as it is. Probably, fewer turns would be acceptable as well.)Tim

author: doktor pyta
author_rank: Frequent Contributor
timestamp: February 20, 2021, 10:20:13 am
content: Interesting.Thanks for sharing!

author: mawyatt
author_rank: Super Contributor
timestamp: February 20, 2021, 03:28:16 pm
content: Agree, interesting circuit! Curious as to the use?If you don't have higher current NPNs you could parallel the MMBTH10 Collectors & Bases with another MMBTH10 diff pair and separate emitter resistor like R8. So just two MMBTH10s and another R8.Best,

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 20, 2021, 03:55:09 pm
content: Not a fan of NLTL?https://www.anritsu.com/en-us/test-measurement/technologies/shockline-nltl

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2021, 10:14:54 pm
content: Quote from: Alex Eisenhut on February 20, 2021, 03:55:09 pmI don't have a monolithic GaAs schottky fabrication line; I would if I could.Not that picosecond methods are needed for mere nanoseconds. But more to the point, silicon diodes aren't available in a whole range of values, nor is the C(V) in general known for assorted type 2 capacitors. I've done a few NLTL experiments before and never got an attractive waveform, or had any insights as to what would clean it up. Example,https://www.seventransistorlabs.com/Images/Magnetic_Compressor_Sch.pnghttps://www.seventransistorlabs.com/Images/Magnetic_Compressor_SlowPulse.pngA simulation because I don't have ferrites in such flexible values, and screw trying to adjust it by hand.Have also tried a chain of ferrite beads (including tens, hundreds of pieces), which can sharpen the wave to a point, but does no multiplication. Ideally I'd have an IGBT (modeled here as a relatively slow step source), driving a mystery network which both sharpens and heightens the pulse -- making not just a sharp wave, but the electrical equivalent of a great wave crashing ashore.Quote from: mawyatt on February 20, 2021, 03:28:16 pmThe schematic, is what it is; the avalanche pulser is for fast pulse, and pulsed power, experiments.Tim