Author Topic: Things derped engineers say.  (Read 35558 times)

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Offline T4P

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Re: Things derped engineers say.
« Reply #50 on: May 03, 2013, 03:31:59 pm »
I did one year of graduate course. They basically start again at zero ( ohms law) because a lot of people come in from other directions like latin-greek etc... Having had already 6 years electronics i was bored out of my skull . So i filled my time doing the seniors graduation projects, and annoying the non-teccies.

We just had seen the thermal calculation. Rthjc rthca et al... Dodo number one had a power supply with a 7805 and heatsink. It kept going in thermal protection . So he came to me for advice. Always up for a bit of fun i explained that increasing the surface of the heatsink would decrease the thermal resesitance rthca. . Case being the heatsink, a being ambient. So far so good. He nodded agreeingly..

But didnt have a larger heatsink. So i pulled in some other mathematics. Look at the thickness of the aluminum. If we were to drill a hole in there of the same diameter as the thickness. Lets say it is 1mm thick. The inner wall surface of that hole is thickness x diameter x pi . We only lost a bit of material in the hole ( on purpose i did not calculate what we lost , or it would have punctured the theory.. )

So dodo went off drilling lots of holes in his heatsink trying to get it to work. He spent an entire lab session (4 hours) on that.
Dodo indeed
 

Offline c4757p

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Re: Things derped engineers say.
« Reply #51 on: May 03, 2013, 03:45:30 pm »
Those are speed holes. They make it lose heat faster.
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Offline peterthenovice

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Re: Things derped engineers say.
« Reply #52 on: May 03, 2013, 08:55:20 pm »
 

                dodo: Is the heat sink big enough
  the other guy: yes! but add a car water cooling system for good measure
                dodo:all i have is an industrial cooling system
  the other guy: that"ll do

          :-DD
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Offline staxquad

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Re: Things derped engineers say.
« Reply #53 on: May 03, 2013, 09:51:11 pm »
Quote
"It's not the voltage that kills you, it's the current"

Yeah, one of my friends has used that statement many times in conjunction with

Quote
100 Amps?!?! [lead acid car battery] that'll blow your arm off!!!

I always thought it was the resistance (or lack thereof)  that kills you.
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Offline lemmegraphdat

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Re: Things derped engineers say.
« Reply #54 on: May 04, 2013, 12:28:58 am »
Quote
"It's not the voltage that kills you, it's the current"

Yeah, one of my friends has used that statement many times in conjunction with

Quote
100 Amps?!?! [lead acid car battery] that'll blow your arm off!!!

I always thought it was the resistance (or lack thereof)  that kills you.
Squirrels and birds must have a high resistance or the induction would surely kill them.
Start right now.
 

Online Dave

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Re: Things derped engineers say.
« Reply #55 on: May 04, 2013, 12:42:36 am »
I think he's saying that's what you need to do to get through school, not that that's all you need to do in school to learn properly. Think - your teacher would have been perfectly happy to pass you if you all remained complete ignoramuses with regard to the availability of TL072, right?
What I understood is that you are shown how to solve something and you just need to be able to repeat everything to pass.
So far, in my first year at the university, I've had an entirely different experience. We have been taught how to think like engineers and in order to pass the exam, you have to thoroughly understand the subject. On top of that, we are constantly encouraged to tackle more difficult tasks, which require a lot of thinking and are nothing like the tasks we encounter at lectures.
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Offline c4757p

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Re: Things derped engineers say.
« Reply #56 on: May 04, 2013, 12:45:03 am »
I must go to a shitty school.

Oh wait, I already knew that.
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Offline TimNJ

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Re: Things derped engineers say.
« Reply #57 on: May 04, 2013, 02:45:08 pm »
Speaking of chocolate percentages.

You should try 99% chocolate sometime, It's crazy strong.

Almost impossible to chew without being overcome with bitterness. You have to let it dissolve on your tongue.

http://www.lindt.com/swf/eng/products/excellence/excellence-99/



I like dark chocolate. Milk chocolate is a little too sweet for me sometimes. 70% is good for dark chocolate new comers. I eat up to 90% now. Tried the 99% percent. Not the biggest fan.

But to everyone else, dark chocolate in moderation is actually very healthy for you. My mother is a dietitian and she recommends everyone eat an ounce or two every day. Not that her word is law, but there is infact good scientific research to back that.
« Last Edit: May 04, 2013, 03:32:04 pm by TimNJ »
 

Offline SeanB

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Re: Things derped engineers say.
« Reply #58 on: May 04, 2013, 03:25:22 pm »
Havent had the 99%, but have loved the 85%. Used to buy the Cadbury's Cocoa powder in bulk 25kg bags, the lovely stuff that was so nice just in hot milk so it could dissolve.
 

Offline peterthenovice

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Re: Things derped engineers say.
« Reply #59 on: May 05, 2013, 03:10:17 am »
Havent had the 99%, but have loved the 85%. Used to buy the Cadbury's Cocoa powder in bulk 25kg bags, the lovely stuff that was so nice just in hot milk so it could dissolve.
sounds superbly brilliant
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Offline free_electron

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Re: Things derped engineers say.
« Reply #60 on: May 05, 2013, 03:42:03 am »
[chocolate] PFFF lindt ,cadbury and ghirardelli.. bunch of wannabe's

ANY serious scholcolate connoseur knows the real chococlate is Belgian !

Go take a look at chocolate from Callebaut. It comes with a datasheet including melting/solidifying/crystallisation points, processing temperatures for specific applications and viscosity curve. depending on the application they have different curves / mixtures. now THAT's serious chocolate. try finding one other manufacturer that specifies that.
they do have a little drawback though... it's only sold in 'slabs' of 5 kilo (11 pounds ) or 10 Kg bags..

http://www.callebaut.com/usen/products/chocolate/finest-belgian-dark-chocolate

Almost any chocolatier ( they guys making the fancy filled chocolates) in the world uses that as base ingredient.
Chocolate is defined by LAW in belgium ! can't throw just willy nilly in there. if you start mixing animal fats or other vegetable fats it must be labeled 'fantasy chocolate' or 'product containing chocolate' . the label 'Pure chocolate' is defined by law.

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Offline cwalex

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Re: Things derped engineers say.
« Reply #61 on: May 05, 2013, 03:51:36 am »
Go take a look at chocolate from Callebaut. It comes with a datasheet...

Chocolate that comes with a datasheet. Awesome  :-+
 

Offline ivan747

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Re: Things derped engineers say.
« Reply #62 on: May 05, 2013, 03:28:10 pm »
Aaaaand the thread has been derailed...
 

Offline Joules

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Re: Things derped engineers say.
« Reply #63 on: May 05, 2013, 08:09:15 pm »
Too many "Do not feed the engineers" signs leads to the inevitable discussion of food and or reproduction...  :palm:
 

Offline Jimbz

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Re: Things derped engineers say.
« Reply #64 on: May 05, 2013, 10:58:45 pm »
In this post, I want to hear all the stupid things either you, or a friend has said about electronics. Bonus points if you can confuse the higher engineers!

I'll start.
"it's a current limiting power supply, the circuit can't blow up!"

Hello, sorry to derail this thread a bit more, but what exactly is wrong with this sentence ? I am a seminoob when it comes to electronics, created an account just for this conondrum. I was always under the impression that if you limit your maximum current output via the current limiting or overcurrent protection on your PSU you can potential keep your components safe incase there happens to be a short ? If a circuit normally draws 75mA and you limit your current to 100mA would you not keep your circuit safe if there is a short ? if it exceeds the 75mA then you know for sure you have a short in your hands ? Can someone clarify this ?
 

Offline c4757p

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Re: Things derped engineers say.
« Reply #65 on: May 05, 2013, 11:02:16 pm »
Simple - if your circuit contains 10 transistors each taking 100 mA, the current limiter isn't going to prevent this from becoming one transistor taking 1A. Shorts aren't the only things that damage circuits.

Obviously the numbers aren't incredibly realistic, but the point stands.
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Offline hammil

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Re: Things derped engineers say.
« Reply #66 on: May 05, 2013, 11:22:17 pm »
Another interpretation could be if a constant current supply was being used... The current could be set to a low value, but that doesn't stop the voltage from rocketing up due to a high-impedance load. Semiconductors can easily be blown up with even relatively low voltages if applied in the wrong way.
 

Offline Jimbz

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Re: Things derped engineers say.
« Reply #67 on: May 06, 2013, 12:10:04 am »
Simple - if your circuit contains 10 transistors each taking 100 mA, the current limiter isn't going to prevent this from becoming one transistor taking 1A. Shorts aren't the only things that damage circuits.

Obviously the numbers aren't incredibly realistic, but the point stands.
I think I am missing something critical here... Why would it not limit the current ? You said that the transistors will draw 1A, fine and dandy. But why can you not limit the current to not take more than 100mA for example ? What is the point of a current limiter or overcurrent protector if it does not do that ? Isnt an OCP sort of like a reusable fuse ?

Another interpretation could be if a constant current supply was being used... The current could be set to a low value, but that doesn't stop the voltage from rocketing up due to a high-impedance load. Semiconductors can easily be blown up with even relatively low voltages if applied in the wrong way.

I have no arguments here but one, arent "current limiting" and "constant current" 2 different things ? Where on one you limit the current regardless of your voltage output and the other you set the current to a value and the voltage will be drawn automatically according to your load ?
 

Offline c4757p

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Re: Things derped engineers say.
« Reply #68 on: May 06, 2013, 12:12:53 am »
Simple - if your circuit contains 10 transistors each taking 100 mA, the current limiter isn't going to prevent this from becoming one transistor taking 1A. Shorts aren't the only things that damage circuits.

Obviously the numbers aren't incredibly realistic, but the point stands.
I think I am missing something critical here... Why would it not limit the current ? You said that the transistors will draw 1A, fine and dandy. But why can you not limit the current to not take more than 100mA for example ? What is the point of a current limiter or overcurrent protector if it does not do that ? Isnt an OCP sort of like a reusable fuse ?

Look at the numbers again. If you have ten transistors taking 100mA (the correct behavior) you have 1A. If you have one transistor taking 1A you have 1A. No change in total current.
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Offline tom66

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Re: Things derped engineers say.
« Reply #69 on: May 06, 2013, 12:13:54 am »
In addition current limiting takes some time to take effect, as the large capacitors on the power supply output may discharge into your low impedance load, damaging your semiconductor.

One example: I was playing around with a 7805 and got the + and - the wrong way around. Did it once, power supply limited at 1 amp, the regulator survived. Second time, it failed short circuit between input and ground.
 

Offline Jimbz

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Re: Things derped engineers say.
« Reply #70 on: May 06, 2013, 12:26:47 am »
Simple - if your circuit contains 10 transistors each taking 100 mA, the current limiter isn't going to prevent this from becoming one transistor taking 1A. Shorts aren't the only things that damage circuits.

Obviously the numbers aren't incredibly realistic, but the point stands.
I think I am missing something critical here... Why would it not limit the current ? You said that the transistors will draw 1A, fine and dandy. But why can you not limit the current to not take more than 100mA for example ? What is the point of a current limiter or overcurrent protector if it does not do that ? Isnt an OCP sort of like a reusable fuse ?

Look at the numbers again. If you have ten transistors taking 100mA (the correct behavior) you have 1A. If you have one transistor taking 1A you have 1A. No change in total current.


My concern is not about how much they consume the current, it is more about not being able to limit the current from the power supply. If you take on of those classic 12V adapters with 500mA rated current, will it not draw a maximum of 500mA in a circuit that consumes 1A regulary, meaning your circuit likely wont work ? Is it not the same for power supply "current limit" option ?
 

Offline IanB

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Re: Things derped engineers say.
« Reply #71 on: May 06, 2013, 12:32:42 am »
I have no arguments here but one, arent "current limiting" and "constant current" 2 different things ?

Not really. Both are in fact exactly the same thing.

If we look at a regulated power supply, "constant voltage" and "voltage limit" are equivalent; also "constant current" and "current limit".

Consider:

You set your power supply to 5 V and switch it on. The power supply increases the output current until the voltage reaches a limit of 5 V and then stops. You have hit the maximum voltage threshold (possibly with no current at all if no load is connected). Whatever you do with the load the voltage will never go above 5 V (but it might go lower, for example if you short the output).

Next, you set your power supply to 1 A and switch it on. Now it will increase the voltage until the current reaches 1 A and then stop. You have hit the maximum current threshold (possibly with no voltage at all if the load has zero resistance). Whatever you do with the load the current will never go above 1 A (but it might go lower, for example if you make the load a high resistance).

The power supply has two limit settings: maximum voltage and maximum current. Neither one of them will exceed the limit, but at all times one or the other of them is limiting.

So don't think of a "constant current supply", think of a "current limited supply". Don't think of a "constant voltage supply", think of a "voltage limited supply".

If you can find a "constant voltage supply" that maintains the set voltage at its terminals when you have applied a short circuit, let me know. I'd love that supply.
« Last Edit: May 06, 2013, 04:15:11 am by IanB »
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Offline IanB

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Re: Things derped engineers say.
« Reply #72 on: May 06, 2013, 12:35:50 am »
In addition current limiting takes some time to take effect, as the large capacitors on the power supply output may discharge into your low impedance load, damaging your semiconductor.

One example: I was playing around with a 7805 and got the + and - the wrong way around. Did it once, power supply limited at 1 amp, the regulator survived. Second time, it failed short circuit between input and ground.

And that depends partly on the quality of the power supply. See this thread, for instance:

https://www.eevblog.com/forum/testgear/comparing-two-power-supplies/
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Online Dave

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Re: Things derped engineers say.
« Reply #73 on: May 06, 2013, 01:04:00 am »
If you can find a "constant voltage supply" that maintains the set voltage at its terminals when you have applied a short circuit, let me know. I'd love that supply.
I have met one of those handsome devils that could do just that. It was 30V, 2.5kA. Yes, kiloamperes. 75kW. Kilowatts. It could maintain a constant voltage when faced with a short my sublimating the wire. >:D
This beast was in a lab I worked in last summer. It was used for testing high power DC electric motors.
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Offline Jimbz

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Re: Things derped engineers say.
« Reply #74 on: May 06, 2013, 01:08:27 am »
I have no arguments here but one, arent "current limiting" and "constant current" 2 different things ?

Not really. Both are in fact exactly the same thing.

If we look at a regulated power supply, "constant voltage" and "maximum voltage" are equivalent; also "constant current" and "maximum current".

Consider:

You set your power supply to 5 V and switch it on. The power supply increases the output current until the voltage reaches a maximum of 5 V and then stops. You have hit the maximum voltage threshold (possibly with no current at all if no load is connected). Whatever you do with the load the voltage will never go above 5 V (but it might go lower, for example if you short the output).

Next, you set your power supply to 1 A and switch it on. Now it will increase the voltage until the current reaches 1 A and then stop. You have hit the maximum current threshold (possibly with no voltage at all if the load has zero resistance). Whatever you do with the load the current will never go above 1 A (but it might go lower, for example if you make the load a high resistance).

The power supply has two limits: maximum voltage and maximum current. Neither one of them will exceed the limit, but at all times one or the other of them is limiting.

So don't think of a "constant current supply", think of a "maximum current supply". Don't think of a "constant voltage supply", think of a "maximum voltage supply".

If you can find a "constant voltage supply" that maintains the set voltage at its terminals when you have applied a short circuit, let me know. I'd love that supply.

Now if you help me clarify one thing I may have learned something today.

Let us say we want to have "constant current" of 100mA on a random load, and it will set its voltage to 25V, the voltage will not drop or go above it, all it wants is to maintain its 100mA. And now let us set a "current limit" to 100mA, am i to understand correctly, that it will not go above 25V on the same load ? Regardless what it says on the display screen ? Also am I correct to understand that you can fiddle with the voltage as much as you want, you can always go below in "current limit" but never above with voltage with the same load ? If you go below 25V you drop your 100mA to a lesser value, does this not differentiate "constant current" and "current limit" ? In where one you have set voltage and set current on a load, but in the other you can play around with the voltage as much as you want, but the voltage will never go above the current limit of 25V ?

Also could you please clarify for me why would the voltage increase in a short circuit ? I was always under the impression the only the current will increase ?

Sorry if I may seem like a dunce, but I was always under the impression that the voltage and the resistance determine the current, but from what I understand from your text its the current that determines the voltage.
 


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