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Just one active component in this LED flasher circuit - a thyristor or silicon controlled rectifier (SCR). With a couple of resistors and an electrolytic capacitor, the LED can be made to flash at a few Hertz.
https://en.wikipedia.org/wiki/Thyristor
https://en.wikipedia.org/wiki/Silicon_controlled_rectifier
https://www.schematicsforfree.com/archive/file/Oscillators%20and%20Generators/Misc/SCR%20Relaxation%20Oscillator.pdf
Just one active component in this LED flasher circuit - a thyristor or silicon controlled rectifier (SCR). With a couple of resistors and an electrolytic capacitor, the LED can be made to flash at a few Hertz.
https://en.wikipedia.org/wiki/Thyristor
https://en.wikipedia.org/wiki/Silicon_controlled_rectifier
https://www.schematicsforfree.com/archive/file/Oscillators%20and%20Generators/Misc/SCR%20Relaxation%20Oscillator.pdf
I managed to find an SCR in a dollar store nightlight controlled by an LDR. It was actually one of the first bits of electronics I ever played with. It's a fairly low power device for an SCR. It only came in a to93 package.
Eleven minutes of Julian not understanding a damn thing about thyristors – or voltage drops across resistors.
It's not bloody latching if you can turn it off by lowering the damn gate, Julian! Plus, seems to me that the reason it oscillates is because the cathode voltage is rising above zero during the conduction phase.
Relaxation oscillator, or a low-tech blink sketch.
Do you know if anyone sells those spring connectors in bulk?
It would be fun to make an Arduino uno with components in a kit like that.
Very interesting! I encountered this by accident, when designing a "5 part laser laser alarm" (scr, ldr, buzzer, r) for students. Some circuits would not stay on. It never appeared to me that I could use this "bug" as a LED flasher 🙂
Or as my mom refers to them the "thigh master rectal fires"
Thyristors are still used in high power industrial situations. For example in the HVDC link between the two islands of NZ, they are arranged as rectifiers. But unlike normal diodes they can also be reconfigured to operate as as an inverter, depending on the direction of the power flow required. I believe that you were able to turn off your SCR because you were reverse biasing the gate against the cathode, due to the presence of the 1K resistor. That's something that never seems to be mentioned anywhere when these devices are described, and can create issues in designs when you don't allow for it. Hope you had a great Winter Solstice..! 🙂
Flashing LEDs will never catch on. 😉
+Julian Ilett
I believe that a thyristor can be viewed as two SCRs coupled in antiparallel, i.e. it will allow current to pass in both directions, depending on the state of the gate. It is, of course, implemented on a single die, and not as two separate components. I vaguely remember seeing an illustration where it had an NPNP structure – or perhaps it was PNPN; I can't remember since it was way back in the late 70s or the 80s I saw this image.
Anyway, Merry Christmas and a Happy New Year to you and yours, my Good Sir, and greetings from Denmark.
A smart design.
Merry Christmas and a Happy New Year .
Reason the SCR turns off in the later part is that you are drawing current out of the gate, and at the low anode current the current drawn is enough to remove the base bias for the lower transistor in the equivalent circuit, thus turning it off and the top one as well. For higher current most SCR devices have too high a gate resistance and thus will not turn off this way, though Sony did make the Gate Controlled Switch, which was a SCR optimised for being able to be turned off during conduction by a negative voltage current pulse applied to the gate.
Drawback was that they typically ran at around 5A, and you had to pull this current out of the gate to turn it off, thus your gate drive was roughly 2V at 3A to turn it on fast, and then a -10V pulse at around 5A to turn it off. In the most common application of these, television line output power device and television vertical driver circuits, the current pulse was relatively easy to get using a simple transformer coupling to the device gate, and they did serve the purpose, in a time when designing a high power, high voltage, fast turn on and off and rugged power transistor was both a brand new science, and difficult. Later devices were much more rugged and even Sony stopped using these, and there were many modifications to Sony Trinitron sets ( the biggest used of GCS devices) to replace the now no longer made GCS devices, involving a lot of modification to the set, a new larger heatsink for the replacement part, some resistors, diodes and capacitors and a BU508D line output transistor ( and later improved ones as well, more process enhancements made better devices) to replace it. Turns out that GCS devices are prone to fail with time, due to the gate itself breaking down and the device going short circuit. however, Sony did put in plenty of fuses in the sets, so they would just stop working when one failed, and not have a massive trail of fried silicon and wound parts.
ah I miss the days when we would name things with the suffix "-tron"
i'm curious why a thyrister would be used over a MOSFET at high voltage? Is it just a ratings issue?
Also. Merry Christmas! 🙂
Funny you draw European capacitors yet American resistors
Happy birthday and merry new Easter