Electro for newbie

Post by **KanedaFr** » Tue Feb 10, 2009 12:28 pm

Ok...I know I will see a lot of smileys in your reply but I need to ask this :

how in the world do you know which cap, resistor, diode you should use (and where) when you design your electronic projects ?!

I mean, how do you know you need to put a 10Kohms in pin 32 of this IC ? and a 0.1µF cap in pin 12 ?!

I first thought

ok, read the datasheet of the IC. If you see it need 200mA, select your resistor!

but if I follow this, I sould add resistor on every pin ?

then, reading a datasheet, I see the part 'Exemple of design' with a beautiful picture of the IC with 1 resistor and 2 caps...
ok...but why ??? If the IC MUST have this 3 components, why aren't they included IN the IC ? so it should mean it is, like the title said, an example.
But what components must I choose for MY design ?

Sorry for this post, you should now see me like the BIG newbie I am but, if I can design a project using this IC with this one and another one, I'm UNABLE to add the (passive?) components between them
Why it's not as simple as an optional voltage regulator (to avoid fire!), ICs and wires!?

Jorge Nuno · Post by **Jorge Nuno** » Tue Feb 10, 2009 1:42 pm

Well that 0.1µF seems like a Vcc filter cap: It's just that the wires have some internal resistance and inductance, when a chip toggles something, it has to charge/discharge the internal parasitic capacitance of the mosfets' gates, that produces a spike of current, but because the Vcc wire is a tiny coil the Vcc voltage will rise or drop (U = -di/dt), that's why there is a ceramic/poliester cap right by the chip, to sustain the voltage while di/dt != 0.
You just can't put a 0.1µ inside a chip, it's too big. Also smaller capacitors respond better to faster spikes, but provide smaller currents.

The 10kOhm resistor is a very typical value for pull-ups or downs. BTW the megadrive has 2.2k ones. Lower resistor values increase static current on the chips' IOs when they are outputting the complementary level of the pull (beware above 5mA currents), but the signal are toggled faster (the gate capacitor is charged/discharged faster).
Usually used in tri-state or unused input pins or open-drain/open-collector or NMOS (crap) logic.

Series resistors (those that connect to one input and one output) are usually to translate logic values (TTL<->LVTTL). Those are usually in the range of the cents of Ohms. Example: a megadrive flash cart with 3.3Volt flashes. If you supply 3.3V to the flash's Vcc but the signals are 0~5V you will kill the flash IO pin, unless it's "5V tolerant" or you add the series resistor so that the excess voltage is dropped across it. Bigger values worsen the rise/fall times.

There you go, I hope this explains.

HardWareMan · Post by **HardWareMan** » Tue Feb 10, 2009 1:58 pm

There are certain rules for the use of digital circuits. And they are different for logic TTL, Schottky and CMOS. There is relevant literature.

Post by **KanedaFr** » Wed Feb 11, 2009 10:49 am

...so it's not for me

I need to go back to school...I had electronic course 15years ago but totally forget them (what a shame! If I knew I'll need it for genny, I wouldn't forget it!)

HardWareMan · Post by **HardWareMan** » Wed Feb 11, 2009 11:07 am

KanedaFr wrote:...so it's not for me

Don't be so pessimistic.

TmEE co.(TM) · Post by **TmEE co.(TM)** » Wed Feb 11, 2009 11:16 am

just experiment... build stuff you see on schematics, do trial and error... I have fried a lot of stuff, and that way I know what you should not do

Post by **KanedaFr** » Wed Feb 11, 2009 10:02 pm

ah ah, I would like to avoid to fry my genny or my pc

Shiru · Post by **Shiru** » Wed Feb 11, 2009 10:15 pm

You can (re)start with electronics in retro fashion, directly from practice, with buiding blinking LED device (7400, resistor and capacitor), then learning some theory (why and how it works), then slightly more complex device (maybe dual-tone sound generator), more theory, etc. This way you'll not fry anything expensive.

Jorge Nuno · Post by **Jorge Nuno** » Thu Feb 12, 2009 12:35 am

Get some assortments of 74HC chips, a breadboard, some rigid core wires, a few 7805s and some LEDs + 200Ohm resistors, some 0.01µ caps and some 10k resistors, and there you have a lot of design choices!

HardWareMan · Post by **HardWareMan** » Thu Feb 12, 2009 3:27 am

Jorge Nuno wrote:Get some assortments of 74HC chips, a breadboard, some rigid core wires, a few 7805s and some LEDs + 200Ohm resistors, some 0.01µ caps and some 10k resistors, and there you have a lot of design choices!

Agree. And it's cheap.

Chilly Willy · Post by **Chilly Willy** » Thu Feb 12, 2009 3:50 am

A bit more fun is to make yourself (or buy if you aren't that good yet) a PAL or PEEL programmer, then use PALs/PEELs to make simple circuits. A PAL/PEEL is a small programmable gate array in a small DIP package. They were BIG 20 years ago. PALs can be programmed just once - PEELs are electrically erasable to be used over and over. A step up from that would be a full FPGA like from Xilinx or Altera. I recommend PEELs for small stuff. They're cheap and reusable and come in sizes comparable to most 74xx chips while being more useful.

HardWareMan · Post by **HardWareMan** » Thu Feb 12, 2009 6:43 am

Or just use an simulator and don't worry about frying some chips.

Proteus are very powerful simulator for this. For theory study it will be enought.

Post by **KanedaFr** » Thu Feb 12, 2009 12:11 pm

WOW!! Proteus is good !!!
I was looking for something like that and only find LiveWire

ok, now I'll have to make a 64pin PCB edge connector and bigger EEPROM

You can (re)start with electronics in retro fashion, directly from practice

yes, I plan to do that...but with the help of an Arduino board, it's more fun !

8bitwizard · Post by **8bitwizard** » Sat Feb 14, 2009 1:01 am

PALs are long gone, because the three main types of GALs can do anything most PALs can, and they use flash to store the matrix. PEELs are similar to GALs, but much less common in scrap equipment. GALs/PEELs are also the most complex PLDs which have freely available open documentation.