EEVblog #1285 - How to do Design By Inspection

What is "Design by Inspection"
Dave answered what he thought was an obvious forum question, but to beginners it's not so obvious. Rather than build something up and test, let's do design by inspection.

#Design #Tutorial

Support the EEVblog through Patreon!

Buy anything through that link and Dave gets a commission at no cost to you.

Donate With Bitcoin & Other Crypto Currencies!

[FruitMuffn]

Love it Dave -- as a electronics hobbyist I don't have any of the formal engineering education. Always nice to learn something new :)

[Seiskid]

Good little explanation. Taking something simple and making a nice tutorial out of it. Think newbies will really enjoy this.

[peterpade]

As an old mechanical engineer. I would call this "design by calculation". Thanks for your help in applying engineering principles within electronics.

[shakaibsafvi]

Someone educate the fire alarm design companies not to use the zener design. Also educate the gov not to approve the high power consumption design.

[spacenomad]

Based on your previous videos, I'd call this design-by-inspection method "doing your homework"

[Moonbase]

Nicely pointed out. You have to know where to look, and a little experience helps a lot. Not even the need for firing up any old SPICE like Micro-Cap here. Talking about Micro-Cap, I wonder if you’d like to make a video one day showing us in which situations it makes sense to actually use a beautiful monster like that (since it’s now free).

[Dave]

This video is a classic example of one reason we all love watching your videos. For those of us that are new, or, like me, returning to electronics after decades away, we get to be reminded that even long-time designers make mistakes, and we get to see how those mistakes happen. Always an excellent learning experience.
It is quite unbelievable (or easily believable) that a company would get SO cheap with their design that they would use the old zener rectifier instead of a proper solution like a high voltage linear regulator that can be quickly found and easily implemented. Zener's still have their place in designs, but NOT (typically) in rectification of the input voltage today. Thousands of available linear and switching regulators make them obsolete for that purpose, and I'm sure the designer of that smoke detector knew that.
It is unfortunate that for many companies the money matters far more than quality.

[jbrown]

Thank you, Dave! It's so so so valuable to understand the thought process.

[slowpokejpg]

Phew! Finally my ultra-low Iq comes into good use!

[threeMetreJim]

I guess that linear regulator is going to have a fair amount of dissipation if you try to draw 10mA from it (Vin-Vout * Iout). It's probably meant for very low power loads like a remote sensor of some kind, that sits most of it's time idle and only draws a current of a few mA in short bursts (for a short range radio transmission, remote smoke sensor to central alarm?), so the average dissipation is very low (smoke alarm sensing fine, but not when the buzzer sounds). Note 7 seems to confirm that.

[p_mouse]

This is also called doing a thought experiment (term is often used in physics). In fact, most of the time 80% to even 95% of your whole design can basically just be done on paper.

[AgentElectronics]

Isn't this the kind of pre-design sanity checking one would often do anyway, to make sure a circuit concept makes basic sense?

[pbuongiovanni]

Thank you very much Dave!!! I follow you from Italy and your videos are very interesting for me.

[_ATHONOR]

As a 'young whippersnapper' I really appreciate videos like this even if I already knew what you were on about!


Side note: Young Engineers like me really prefer YouTube to books so keep making content like this when you can please Dave!

[AndyFletcherX]

If you use a series input capacitor then the effective input voltage will drop when the sounder starts to draw current. This will ensure that the linear regulator doesn't start to cook :) Thanks for the video.

[jkobain]

Sorry, I'm a silly newbie to your channel, I got no engineering education, nor knowledge.
But I like how you explain things. Thank you for the efforts!

[crafoo]

"... by inspection", "... by analysis", and "... by test" is standard terminology in my previous engineering field. They are used in contractual, technical specifications for product. I think the term "design by inspection" is quite common and used throughout engineering, not just electrical engineering. They have very precise definitions so that the scope of the work to be done is clear to all parties.

[bostedtap]

Very useful, I was looking at ground current, and trying to understand the difference. Never heard of the video title for a design protocol?,
Have a great Saturday, and thanks for sharing.

[thatengineeringchannel]

I've always called this "common sense". But I'll probably start using the "design by inspection" terminology.

[nrdesign]

Makes me wonder how they can sustain a suitable creepage distance with a SOT23-part at those voltages. Thanks for the heads-up tough, that there are indeed high voltage regulators like these!