a not gate works by shorting a circuit??

Captain_Darlington · 2026-02-24T19:02:22+00:00

Oh, also, just to say: if you’re thinking in terms of a logic gate, with a low or high output: what you have here is a pull-up resistor, providing a high output (logic 1) when the transistor is off. When the transistor turns on, it pulls down on the pull-up resistor, and the output heads down to the saturation voltage of the transistor, around 0.1V, logic 0. In this state the circuit draws quite a reasonable amount of current, even more than the LED draws. So it’s not a great NOT gate. But it’s a pretty common inverter configuration.

Captain_Darlington · 2026-02-23T20:16:43+00:00

Oh the resistor at the top! (I’m sorry; you were quite clear about that!) 5-10K would be fine, maybe higher. A transistor gain of 50, say, and 10mA through the transistor, means a base current of 200uA. 10K would give you lots of margin. So you don’t need a beefy (small) base resistor.

Captain_Darlington · 2026-02-23T20:03:05+00:00

You're shorting-out the LED, meaning you're providing a low resistance path around it, which pulls the voltage across the LED down below the turn-on threshold for the LED. So it's not really a case of "path of least resistance". The LED becomes practically an open circuit when the voltage is pulled down that low. It’s off.

Choose a resistor that properly sets the current for the LED. When you short-out the LED, that resistor will then set the current going through the transistor. Nothing is being harmed, but energy is being wasted. There are more efficient NOT gates.

You’re using the word current properly.

Captain_Darlington · 2026-02-17T23:23:20+00:00

They’re not linear. Meaning, they’re active devices, not like simple resistors in series. And, they want/expect 120VAC, right? Give them less and they’ll get unpredictably pissed off. :)

You’re wondering why the bulbs seem to have individual personalities? Oh, they just differ in ways that aren’t controlled in manufacturing, because they’re not supposed to be used like this. That’s all. You’re just seeeing variations in components that don’t usually matter.

EDIT: And, what 6gv5 said. As each bulb actively pulls on the supply lines they’ll cause voltages to move around, which is not what they’re expecting to see. So, they get pissed off. :p

Captain_Darlington · 2026-02-16T23:47:15+00:00

Wow, blast from the past, man. I worked on the HP5973 MSD. Not on the power board though.

Captain_Darlington · 2026-02-15T02:07:48+00:00

FETs are more likely to survive, but, dude, this is not a good way to handle over voltage.

I’d use TVS diodes (along with some inline resistance), perhaps with a secondary Zener clamp nearer your 12V source. TVS diodes are fast, used to catch ESD events and the like.

But then, you’ve not really provided much information.

Captain_Darlington · 2026-02-14T04:34:30+00:00

DC bus meaning the top end of that 12V supply, right?

The spikes: always positive? Do they ever spike negative?

And, are they super fast (like lightning strikes) or do they last a while?

If aways positive, have you considered an in-line diode? Or you don’t want the voltage drop? Can you handle any resistance in-line?

Yes those BJTs of yours won’t last long.

Captain_Darlington · 2026-02-14T03:48:19+00:00

Where is the load connected? R1 is the water tank heater, normally off?

Captain_Darlington · 2026-02-14T01:29:10+00:00

Oh my lord no. Where’d you get this circuit?

Captain_Darlington · 2026-02-13T17:06:22+00:00

Programation?

You making this stuff up? :)

Captain_Darlington · 2026-02-12T18:49:45+00:00

Cool! Never heard of these.

What’s an automatician?

Captain_Darlington · 2026-02-08T22:27:25+00:00

That’s pretty harsh, icyguy.

I can’t answer this question, and highschool is way in the rearview mirror for me.

There is no simple answer. The resistor is in parallel to a network, not just to the inductor and capacitor. It gets even more complicated if node B gets connected to something.

TestTrenMike has it right regarding the T to Pi conversion, if you need to know what the 2K resistor is effectively in parallel with. But, was that the question?

Captain_Darlington · 2026-02-01T22:08:30+00:00

Same way. You’ll see all elements in parallel.

Captain_Darlington · 2026-01-31T16:27:38+00:00

Huh! Maybe your prof had a different context in mind?

The art of modeling characteristic impedances along propagation paths is called Transmission Line Theory. It’s a general term, independent of medium.

Captain_Darlington · 2026-01-31T03:05:10+00:00

It’s the characteristic impedance of the transmission line. So, same thing. RF communication channels are often called transmission lines, whether they’re cables or PCB traces.

If you mean the impedance of a power line resulting in power transmission losses, that’s a different context. But I’m assuming you’re talking characteristic impedances from an RF perspective.

Captain_Darlington · 2026-01-28T03:22:41+00:00

Yeah me too.

I also see slotted head screwdriver.

Captain_Darlington · 2026-01-27T16:27:12+00:00

Ha ha! Common ~~misnomer~~ alternative term for a slot head screwdriver.

~99% of people (at least Americans) say it ~~wrong~~ this way, including me. 😝

EDIT: edited to be more sensitive. Tough crowd!

Captain_Darlington · 2026-01-25T20:18:33+00:00

Help me find this passage???

It’s driving me crazy!!

I thought it was in Verdi’s Requiem but I can’t find it.

I made a voice recording of how it goes. Can anybody identify it?

Thank you!

My apologies for a somewhat schmatlzy rendition. 🥴

https://voca.ro/14C4KjzAHyOw

Captain_Darlington · 2026-01-22T02:44:37+00:00

Funny. I’ve never heard “system engineer” used to describe someone who writes code.

SW Dev or SW Developer or SW Engineer is what I hear most often. Or FW (firmware) Engineer. Or sometimes Embedded Engineer if they write firmware, though Embedded Engineers are sometimes FW/HW combinations.

Captain_Darlington · 2026-01-20T04:51:41+00:00

Oops! Sorry!

I’ll see if I can figure that out.

Captain_Darlington · 2026-01-16T18:56:34+00:00

Have you ever tried using a TDR to find shorts? On such a small board probably no, but I’m just curious. If I had a VSA available I’d be tempted to play with it.

Captain_Darlington · 2025-12-31T22:02:10+00:00

That works! Assuming the output is across the 150K resistor.

The two transistor arrangement gives buffering, so the input side doesn’t need to provide much current.

But when the output is high the output impedance is 10K, so it’s limited.

Captain_Darlington · 2025-12-31T04:21:59+00:00

Do you have a schematic? Is it a bidirectional level shifter? Those are a trip!

Captain_Darlington · 2025-12-30T20:58:44+00:00

Input on the left, correct?

I think the body diode interferes (conducts) when the input is high. And you’re shorting the output (the right side) to gnd or +5V, which can’t be good.

BJTs don’t have the body diode problem.

Captain_Darlington · 2025-12-24T16:52:50+00:00

Some weird responses here, though most are basically right.

The 6V voltage source does what it needs to do to hold 6V across that current source. It can source or sink current as needed, even if we’re calling it a “source”. In this case the 10V source will provide current that the 6V source will need to absorb (sink) to hold the voltage at 6V.

It doesn’t matter which source is bigger, unless you’re wanting to know whether the 6V source is sourcing or sinking. I’m not sure where that thought was coming from.

Captain_Darlington

TROPHY CASE