Joerg wrote in news: snipped-for-privacy@mid.individual.net:
I'm all for pushing the envelope, there's no hide at stake here. :) It sure looks like it will work. It's 'competing' with systems that can do
100 or 200 KHz at best generally, and none of these devices are needed to go further. They work alongside laser systems incapable of more that 10 K, often as not.
The main point is to see what's possible, and to establish the safe operating area so when I really want to use this thing I can have a very good idea how to make it right, at more reasonable figures. And I'll know it will draw a very clean line, which is what the people most likely to use it would want it to do.
But I still want a good laser diode electrical model.. :)
Phil Hobbs wrote in news:zamdnaOrPvgQguzXnZ2dnUVZ snipped-for-privacy@supernews.com:
I've not used them often enough. Always liked the idea though, and I agree that predictable base current and voltage helps a lot. But unless I'm missing something, they make great switched but lousy proportional controllers, no?
Do you have some examples and pics? That would be interesting, I have use the 1N4007 as a RF PIN diode but never as SRD.
One of my next projects needs the opposite, briefly and automatically disconnecting an RF amp from a several kV pulse path so it doesn't go up in smoke. The kind of stuff where people often say "you can't do that". I love it when a project like this comes along.
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You can only try that on a breadboard, not SPICE. And then you find out that it can do 250kHz but after switching to a different manufacturer's LM317 ... *POOF*
I'd rather prefer a nice bottle of Chateau d'Yquem :-)
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Phil Hobbs wrote in news:AY-dnczxb411gOzXnZ2dnUVZ snipped-for-privacy@supernews.com:
Once I'd never have considered it because it has other troubles to overcome too, I'd used it solely as a fixed constant current drive in every laser I'd built in the previous 7 years..
One problem is that the series resistor must be on the high side of the load, most designs using sense resistors put it on the low side. Another is that
1.5V is dropped across that resistor, other designs use far less, so are more efficient. And another, as you say, is that to damp the feedback for an ideal response to changes in a supply or load, they fixed conditions that may or may not be troublesome to us tinkerers who do weird things with it.
The large voltage drop on the sense resistor is actually a boon, it's far easier to dupe the LM317 accurately when you have that 1.25V to use. I have modified the idea to drop 1V across the voltage control network leaving just
0.25V on the resistor, saving one watt per amp, pretty cool if you're doing this with an LM338. >:) It sucks for fast mod though, stability and solidity is dire, but it's great for a dimmable low-volt light though, a string of LED's and that trick, and the efficiency approaches that of a decent switched power converter.
The high-side sensing seems at first awkward, but it's not that bad, just feed it to the normal input of a differential amp and do the modulation on the inverting input. A second stage is needed to set input protections and an agreeable polarity for response to signals but it's still just one 8 pin chip.
That internal componentry for response to line.load changes was the thing I thought would make the concept useless, but that's why reaching 500 KHz with respectable square waves is so nice a surprise.
Viva the LM317. If I can have 100 of those to do entertaining stuff with it beats having to agonise over Mouser's lists and paying big tax on many small boxes. I really aim to do things with a minimal range of parts types, it's a kind of thing I have.. :)
Google "Grehkov diode". He's a Russian guy who discovered the DSRD and impact avalanche effects in cheap power diodes.
Also look for papers and patents by Thomas E. McEwan.
We did one water-cooled DSRD pulser that makes -2KV pulses, about 2 ns wide, at 500 KHz. We forward-bias a secret diode at +48 volts for about 80 ns, to let the current build up to 50 amps or so, then reverse-bias it from a 400 volt supply and wait for it to snap.
Here's the pulser head...
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with the serious parts bolted to a gold-plated copper block. A water-flow cold plate gets bolted to the bottom.
I can show you innards privately. The HV, high power PIN diode turned out to be unusual.
Joerg wrote in news: snipped-for-privacy@mid.individual.net:
Not the diode that bothers me. The laser's fast ability is what makes the idea easier. The question is whether the complexity of other parts of the system is worth it, and whether in practise a fast graphic scanner shows a dashed line. That sets limits on ranges for carrier speed. No need to go faster than is required to avoid that.
Joerg wrote in news: snipped-for-privacy@mid.individual.net:
Sure, but again, that's part of the appeal of making widgets that all use a single device or small number of parts. Means such adjustments don't have to be made that often. And again, this is as much as exercise in establishing safe operating areas, as much as anything else...
Joerg wrote in news: snipped-for-privacy@mid.individual.net:
Or any number of one-offs. That's actually the idea. Where labour is the biggest obstacle (lack of automation, and a desire to do as much of it here as possible), it pays to make a few widgets for a small demand, as one-offs, and test them individually too. And an LM317 is very convenient, a lot of function hiding behind three pins, and enough heft to do serious work of its own. It's a very attractive device and saves a LOT of board layout work.
Careful what you wish for! >:) I have a hair trigger as this thread shows..
I don't really see a connection there. Certainly there's plenty to be learned from applying ICs such as the LM317 in unorthodox ways, but there's also plenty to be learned from studying the "right" way to solve a specific problem. Indeed, often the "new" right way comes about from someone experimenting with the old ways and having a flash of insight.
I'm all for people experimenting and figuring out interesting new applications for circuits originally designed for different applications, but I'm also all for examining the works of the likes of Phil and Joerg and Jim since they have a demonstrated track record of building rock-solid designs.
In a hobbyist environment, one should certainly try out whatever wild ideas pop into their heads. In a work environment, while engineerins shouldn't generally be discouraged from trying out wild ideas :-), they need to be prioritized based on the perceived risk, time/resources needed to experiment, other project priorites, etc. I've seen firsthand someone's clever idea costing lots of money because the idea turned out to have unforeseen limitations -- any time you're proposing something wildly different from the traditional approaches, be sure to budget for plenty of testing time. On the other hand, you have John's company, where many of his products are completely unique based on his ability (and his employees') to successfully incorporate a few wild new ideas into viable products.
Yes. We did wind up with a reel of FR804's that snap nicely, maybe decent PIN diode material, but we went to a bigger part in the end, higher voltage and more power dissipation.
It was fun, but we only sold a few.
You should probably use real RF power PIN diodes, like Skyworks/Microsemi/Aeroflex. They have doping profiles that give absurdly long recovery times, so it takes very little DC to control a lot of RF. I used exotica because I needed a huge amount of power dissipation capability when we were running at 500 KHz.
"Joel Koltner" wrote in news:3Wlcm.493461$ snipped-for-privacy@en-nntp-02.dc.easynews.com:
That's exactly how I improve code when I write it, I look at what aorks best and always revisit to see if there;s a further improvement because there often is. But there is also context. In several posts here I've accounted for the context of my decision to try this LM317 wheeze. There's no reason not to try it, and a few good ones to try it.
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