apzx
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You forgot a rectifier in your schematic.apzx said:..... Essentially, what I'm thinking here is to use an appropriately rated bridge rectifier and followed by the filter capacitors.....
I have never heard of using that, and I like discrete transistor tricks. I found the Wong references but that's about it. It seems too process dependent to rely upon for production unless a manufacturer sells selected parts for that attribute.apzx said:The transistor is used in a reverse avalanche condition. Kerry D. Wong did a short little article on it. The oscillation time is set by the the RC time constant set by (in my simple schematic) R1 & R2 and C1. This makes a sawtooth-ish oscillator. I've seen this particular configuration be called a "Negistor" as well, at least it seems to pull more results on the Googles.
And yeah not every single NPN transistor will actually work, but in my quick test board I made I just grabbed a random 2N3904 I had lying around, 10k pot, 1k resistor, 10uF elctro, and a white led. It worked without a hitch, but I did cut the transistor's base lead off (apparently that can help in making the transistor work in this configuration more readily). I had to supply it with about a minimum of ~+12.5VDC for it to start oscillating. Any lower and the circuit would not begin and as I increased the supply voltage it started going faster and lowering the voltage made it a bit slower even to the point where it would not oscillate at all. I believe the main reason for the current being so high is because with say a supply voltage of about 12V and a 1k resistor gives ~10mA or so flowing through the led. Then adding some support circuitry to the mix and the current goes up a bit.
apzx said:Without a doubt the parts would have to be selected because it relies on the B-E breakdown which is probably not very controlled in the manufacture of the actual devices. Though enough transistors seem to work just fine in the actual circuit. In trying to understand some more about how these things actually work (just for curiosity sake more than anything) I found a paper from Raymond Arthur Kjar at Iowa State University titled Avalanche mode operation of transistors from 1964. In my searches the most common thing I've found about this particular configuration is in making a circuit with extremely fast fall time and making something with an extremely fast rise time is also possible. It is an interesting circuit if all sorts of flawed.
The Norton amp is pretty much the back half of an op amp with the input long tail pair bypassed. They do somethings well mostly due to the shorter path (with less delay) but fell out of favor when real op amps are so cheap and widely available.apzx said:Indeed, anything I've read on power amp design forward avalanche is taken into consideration or should be at least.
Now, before I saw this I was not aware of a BJT being able to be used in this condition. Frankly, just looking at the schematic and I I would just think that the capacitor would charge up and cause the led to illuminate briefly and that is about it. But having built it and trying to get a better understanding of things it seems to behave more like a relaxation oscillator.
Circuits like this remind me of the sheer ingenuity of some folks. A couple more that I've scratching my brain with are VCFs in particular the Steiner Parker filter wherein he used 8 diodes and their dynamic resistance with respect to their forward voltage. The other is the ARP 4072 filter and its uses a LM3900 (I believe in another thread you mentioned something about Norton Opamps) plus a bunch of differential pairs to operate. Still trying to wrap my head around how that one works.
digikey shows many thru holes are obsolete, but has some SMD parts.abbey road d enfer said:Are UJT's (unijunction transistors) still in production?
Really? It's ages I haven't seen any in actual use...JohnRoberts said:digikey shows many thru holes are obsolete, but has some SMD parts.
JR
abbey road d enfer said:Really? It's ages I haven't seen any in actual use...
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