Electrical Engineering

1:31 AM

I don't want to make an entire question about this... can somebody help me figure out where this guy is getting his current calculations (1.5A) from? electronics.stackexchange.com/questions/261629/…

The Photon

@Helpful He's smoking something. I calculate 20 mA.

Or 140 mA if you use OP's numbers of 3 V operation with 21 ohms coil resistance.

Meaning the answer that was commented on wasn't using appropriate resistor values for use with the relay described in the question.

But still I don't see 1.4 A happening anywhere.

Helpful

Photon, thank you very much. Could it be some combination of the 3.3V source and the voltage divider?

Roronoa Zoro

Good people of EE. I have a question about the initial output value for a 'Gated D Latch' flip-flop.

Shalvenay

@RoronoaZoro ask away

Roronoa Zoro

1:49 AM

We have four NAND gates, without providing any input (in D or E), each of the NAND will have two 0s as their inputs, which means that each NAND will have output of 1. However, this means that the two NANDS that are connected to each other (crossing) will both have output 1, which then will change to 0 at the same time (since both of their inputs is 1 now) and this process will repeat. The two crossing cables will start 0 then 1 then 0 and repeat... Is that correct!

The Photon

@RoronoaZoro "this means that the two NANDS that are connected to each other (crossing) will both have output 1"...no not really.

A NAND with one input as 1 might have a 1 or a 0 at its output.

What will happen is during power up one or the other will power up slightly faster and you'll end up with either Q=0, Qbar=1, or Q=1 Qbar=0.

Which one you get depends on slight errors in geometry of the transistors in the two nand gates, random noise, etc. You can't predict it unless you deliberately design asymmetry between the two gates (at the transistor level).

Roronoa Zoro

2:05 AM

@ThePhoton I see. Thank you so much for the answer. So is this what really happens when we power up the circuit? Or do we usually provide 1 (from E) and another value from D, so that we know that Q will match D?

The Photon

@RoronoaZoro Depends on the technology. I don't think the simple gate you show is actually used very much

More likely you design the flip-flops that matter with a reset function, and make sure to have a reset cycle before the whole system needs to start its normal operation.

Roronoa Zoro

@ThePhoton I see. Thank you again. So this 'Gated D Latch' flip-flop is not how a bit is implemented in main memory after all? I remember reading that it is, but I'm starting to assume now (based on what you said about this gate not being used much) is that a one bit is much much complicated than that!

The Photon

@RoronoaZoro In main memory you don't necessarily need to know what the state will be on power up. So long as your state machine comes through and initializes it (or the parts of it that matter) to some appropriate value later.

Assuming you mean main memory connected to a microprocessor of some kind.

Roronoa Zoro

@ThePhoton Yes that's what I meant; and you're absolutely correct about not needing to know the state at power up.

@ThePhoton I have a second somehow different question about the speed that a gate (such as the NAND) processes the two inputs as opposed to the speed that the electrons move from one output (of one gate) to one input (of another gate). Is it fair to assume that the gate will take much longer to process the inputs (to a proper output) when compared with the speed that the electrons move in a wire, say from the output of the top left gate to one of the inputs of the top right gate?

The Photon

5:35 AM

@RoronoaZoro 1, electrons actually move really slowly in the wire, maybe about as fast as you can walk. 2. But the signal in the wire moves much faster, similar to how a wave on the ocean moves faster than the water molecules that are moving up and down to form the wave.

@RoronoaZoro 3. Whether you even need to worry about propagation delays in the traces depends mainly on the rise and fall times of your signals. 4. The propagation delay of a discrete gate should be specified in the datasheet. It will depend somewhat on how much capacitance is loading the output.

Asmyldof

12:40 PM

Entertainment!

?

Shalvenay

3:35 PM

where would one go looking for an inductor that is in the 36uH range at 20A?

Gregory Kornblum

mouser.co.il/Passive-Components/Inductors-Chokes-Coils/…

One hell of inductor

Guys, i have a question. Who worked with LeCroy oscilloscopes? How are they compared to Tektronics or Agilent? I am thinking of buying Wavesurfer 454, is it a good one?

JRE

@Shalvenay Mouser has 33µH and 39µH, but no 36µH listed.

Shalvenay

@JRE -- yeah, I looked -- and most of the "33uH" inductors they have there wouldn't be 33uH at operating current

Gregory Kornblum

Why do you need such accuracy?

So anyone, about LeCroy?

Shalvenay

3:54 PM

@GregoryKornblum 20A buck converter :P being off by a factor of oh 1.5 on the inductor value doesn't sound like it'd bode well in such an application, no?

Gregory Kornblum

Unless 20A is significantly different than 2A, it should make no difference.

Shalvenay

@GregoryKornblum hrm. perhaps.

Gregory Kornblum

What is the switching frequency and output capacitance?

Shalvenay

100kHz, 1000uF

Mathav Raj

hi all

i am building a 20 Mhz RF receiver out of the NASA radio JOVE instruction manual and they have recommended the use of LM386 as an audio amplifier. But I want to know if there are any good alternatives for that IC?

JRE

4:05 PM

@MathavRaj Hi.

That part is pretty uncritical. Just use the LM386. Don't complicate your task. Getting the RF parts right will be difficult enough. The LM386 is about as simple an amplifer as you will find. There aren't any "drop in" replacements that I know of.

2

The 386 doesn't seem to be part of the circuitry given for the Jove 20MHz receiver (I've got the PDF open.) That uses the LM387. If you are talking about an external audio amplifier, then pretty much any external amplifier should be good. A pair of PC speaker with built in amplifier should do well.

The copy of the manual I found doesn't mention the LM386 at all.

Mathav Raj

sorry its lm387

JRE

OK. That gets more interesting. The LM387 is obsolete. You may be able to find them, maybe not.

Gregory Kornblum

@Shalvenay i think you can take 33uF or 39uF and see no difference.

Mathav Raj

actually the shop keeper gave me a NE5532x and when i compared its datasheet with that of LM387 I found that the former has a unity gain bandwidth of 15 MHz while the latter has 10 Mhz.. does that matter?

JRE

Are you doing the layout yourself? The 5532 has different pin connections than the LM387.

Mathav Raj

4:23 PM

yes i have to now i guess

JRE

They aren't doing anything outrageous with the LM387. If it were me, I'd be inclined to try it.

But, this is actually a fairly decent question that you could ask on the regular EE site. "Is the 5532 a good subsitute for the LM387 and are there any traps to watch out for?"

Mathav Raj

oh actually i put it under amateur radio section

JRE

Maybe have it migrated over here? The question itself is a better fit for EE than amateur radio. When you post it, include a link to the manual for the Jove receiver, and post the section of the circuit that uses the LM387.

Mathav Raj

sure :)

Shalvenay

@GregoryKornblum makes sense.

Gregory Kornblum

4:36 PM

By the way, tolerance of those components alone is probably 10% or higher.

Gregory Kornblum

5:37 PM

So any thoughts about lecroy?

Nick Alexeev

6:36 PM

@abdullahkahraman That's not a bug, that's a ~~feature~~ lizard.

Helpful

6:50 PM

Shalvenay, I found some close on digikey:
http://www.digikey.com/product-search/en/inductors-coils-chokes/fixed-inductors/196627?k=&pkeyword=&pv19=2596&FV=fff40003%2Cfff80013%2C1080017%2C1080018%2C108001d%2C1080020%2C1080021%2C1080075%2C1080076%2C1080088%2C108008b%2C1080090%2C108009c%2C108009e%2C10800a4%2C10800a5%2C10800a7%2C10800ab%2C10800ad%2C10800b5%2C10800b6%2C10800bd%2C10800d3%2C10800d6%2C10800d8%2C10800d9%2C10800e5%2C10800e9%2C10800ea%2C10800ee%2C1080130%2C1080132%2C108017c%2C1080185%2C108019a%2C10801b8%2C10802cb%2C1080555%2C1080556%2C1080558%2C108055d%2C1080570%2C108058c%2C108068d%2C…

Shalvenay

@Helpful discontinued

Helpful

Of course, they're discontinued. Which isn't helpful. But there might be others in the range, if you're willing to go to 33uH - tinyurl.com/hmunjrh

Shalvenay

@Helpful yeah, but my understanding is you are not supposed to run SMPS inductors saturated

Asmyldof

7:58 PM

@Shalvenay Why so low frequency?

Asmyldof

8:09 PM

And also, if your converter needs to output 20A, your inductor needs to saturate quite a bit higher

Shalvenay

@Asmyldof I suppose I could go higher -- most of my concern is dissipation in the switching controller IC due to the high input voltages involved

Asmyldof

8:25 PM

@Shalvenay I'd be more concerned with saturation and run-away than having to spend $0.20 more on the MOSFET

Shalvenay

@Asmyldof well, even with a low Qg part, at say 1MHz, it'd still be awful high dissipation

Asmyldof

@Shalvenay There's a whole decade between 0.1 and 1.0

Just for the reference, I see no appropriate inductor in any of the links supplied above

Shalvenay

I haven't seen one either :P

Asmyldof

To give you an idea of how great an idea is bucking 20A through a 36uH inductance

Shalvenay

8:54 PM

@Asmyldof -- is it better to have an inductor that's a bit larger or a bit smaller than specified if you can't get the specified value in a buck converter?

Asmyldof

9:21 PM

@Shalvenay Depends on how the converter works

Lower inductance means it needs to respond quicker

Shalvenay

9:42 PM

it's a constant-current/constant-voltage PWM controller using current-mode control

Asmyldof

10:48 PM

@Shalvenay Fixed Off Time -> Doesn't matter much. Fixed Frequency -> Depends on PWM ranges. Fixed On Time-> Probably smaller, but depends on whether that risks over-current

Shalvenay

11:34 PM

@Asmyldof ah, fixed frequency -- so it depends on the duty cycles?

Transcript for

Electrical Engineering