Electrical Engineering

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11:47 AM

Hello folks!

Just to confirm my findings, I would like to ask you a quick question:

I noticed that diode datasheets show max forward voltage drop for a given (test) current. After a few simulations, I concluded that when you have bigger currents through, the maximum forward voltage drop in the datasheet is exceeded.

So if I want to work with the V_forward in the datasheets, I should operate my diode under a stated current value, correct?

JRE

@DanielTork If you want Vf to be at (or below) the rated Vf then you need to stay at (or below) the given test current.

Daniel Tork

@JRE Fair enough, makes sense, thanks

Otherwise, how's your day ?

JRE

12:16 PM

A good diode datasheet will have a Vf/If curve. Like this:

Other than that, I'm stretching my lunch break instead of returning to my current battle with an Infragistics Ultragrid.

5 hours later…

W5VO

4:49 PM

@DanielTork You'll also find that any forward voltage they give will be "typical", and will depend on manufacturing, temperature, power dissipation (temperature again)...

Daniel Tork

@W5VO sounds like temperature is an annoying parameter

Yeah, that's what influences the thermal voltage

W5VO

Simplistic diode models are for making calculations easy. You can get fairly decent ballpark value with saying "above Vf, it's a voltage source, below Vf, it's an open circuit", depending on the application. Improving that to have a series resistor gets a lot more cases

I can't think of any diode models I've seen in simulators where they do anything other than the diode equation, though I have seen some omit the series resistor.

Daniel Tork

I get what you're saying, simulators don't take temperature into account

W5VO

sometimes they do, but you need to make sure it's set up for it

and that means the model has a method for incorporating self-heating and you have an accurate power dissipation implemented

Daniel Tork

So does LTspice have that option?

W5VO

5:01 PM

Q: LTSpice: Can I vary schematic temperature over time?

I'm trying to simulate a temperature sensing schematic with a latching function (meaning that the schematic should go to overtemperature mode at let's say 75C and go back to normal at 50). I can do a temperature sweep function with the command .temp 25 50 75. I see that the schematic responding ...

looks like not really

but I'm not certain

Daniel Tork

It's a bit complicated

JRE

5:19 PM

@DanielTork As I read that question (and answer) you can run a simulation at a particular temperature, and some models will respond to it

LTSpice can't change temperature in one run, though.

So, adequate if you want to compare diode voltage drops in the sahara with diode voltage drops in antarctica - but not capable of reacting to changes in temperature while a simulation running.

The models have to make use of the temperature, though, and not all do.

Daniel Tork

@JRE I guess it depends on the kind of application you want to simulate

JRE

Yup. Simulating how your circuit reacts to changing temperatures is out, but simulating operation at various fixed temperatures is possible (if your models are complete.)

Daniel Tork

5:45 PM

I needed to replace some fuse and took the opportunity to search the web to see if there are fake cartridge fuses out there...and there is actually one like that out there

Do you deal with this/hear about this a lot?

2 hours later…

jippie

7:22 PM

so ... did I miss anything good since the last time I was here?

Sunnyskyguy EE75

An electronics engineer typically graduates with his mind filled up with formal analysis methods, theorems, and derivations. He is well-skilled in solving simplified,
sanitized analysis exercises that have unique answers: one answer is correct, all others wrong. The system is this way for good reasons, namely that most such exercises are graded by teaching assistants who have neither the time nor the experience to evaluate an answer that doesn't match the one provided by the instructor @ C. Middlebrook MIT Sometimes I wonder if this also applies to the critics who downgrade my answers

W5VO

7:45 PM

@jippie Hey man, good to see you again!

jippie

@W5VO have to figure out all the workings on the site again :-)

laptop2d

@SunnyskyguyEE75 From what I've seen you have some good answers, I think you could write more towards your audience and put yourself in their shoes, some of them don't need a shotgun loaded with equations.

Sunnyskyguy EE75

I guess I see the audience as more than just the 1 person asking the question to give a broader scope , but I see your point. Students don't want that

yet need to learn it to understand fully

W5VO

@SunnyskyguyEE75 You sound like a professor I had that insisted that I needed to know differential equations to be able to do anything with a transistor.

Sunnyskyguy EE75

hobbiest technicians dont need to know it, but engineers do. This is why one's profile ought to mandatory

W5VO

7:57 PM

and it really just depends on where they are in their education

Sunnyskyguy EE75

nah but even the professor doesn't know that Rs =k Pd a handy relation between diode and BJT bulk series resitance

exactly which is why I think their education level ought to be mandatory in the Profile to ask a proper question here. I know PhD's who can't perform an optimal realistic design because they don't know how to write specs ( acceptance critieria for must have vs nice to have)

I think C Middlebrook had the best concepts for teaching, but that isn't being practised here except for Verbal Kint and jonk . They are excellent . I try to be practical and show theory to back it up. but I guess I need to try harder

W5VO

I will agree that jonk gives excellent answers

Sunnyskyguy EE75

they both follow fundamentals in theory expressed well

W5VO

His writing is also very good, which helps make his answers more understandable

Sunnyskyguy EE75

agreed

I'm so disappointed in myself. I just proved to my self a Butterworth Filter is underdamped in higher (>2) order fi.lters

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