Voltage drop over capactors

Hi guys, I have a problem where my 5v over a 0.1uf cap drops to about 1.8v, i'm not sure if I have a bad batch of caps or I just don't understand some fundamentals about electronics yet.

I was advised to add these .1uf caps for the 328P vcc and avcc to reduce any noise, here's my schematic if that helps at all.

Thanks!!

C1 is wrong. One lead needs to be connected to Ground (it doesn't matter which one).

Oh I see where I messed up... How about C2 and C3 ?

C1, C2 is wrong, C3 is o.k.
Q1 wiring ?

driz:
Oh I see where I messed up... How about C2 and C3 ?

Yeah, you need 5V going directly to the Vcc and AVcc pins, and the caps from
there to gnd, not in series.

The capacitors "stiffen up" the supply voltage near to the chip - prevents/reduces high frequency current changes from making
the supply voltage noisy.

What the hell is Q1 doing there? Not only is it connected the wrong way around, but also: it will short circuit your power supply when activated...

yeah,i think so, it will short circuit your power supply when activated.

muddy:
What the hell is Q1 doing there? Not only is it connected the wrong way around, but also: it will short circuit your power supply when activated...

I think he meant to use it to provide power to the loads switched by the ULN2003,
but he wired the drain and source wrong. Should also be a p-channel, not
n-channel. I don't remember if the ULN2003 can take 5V inputs.

Thanks guys, Q1 is a L7805CV to reduce the voltage to 5V for the arduino and the ULN2003 is to switch the 12V from a 3S lipo to power 4 LED light strips

How about something more like this?

What happened to the decoupling capacitors - they are still needed, between Vcc and ground, next to each chip.
They reduce power supply noise in order to guarantee correct chip function, not because its a nice idea!

You shouldn't have removed C1 and C2, just connect them from the VCC pins to GND. This is "decoupling".


Rob

Oh okay I was thinking that I could just use the one 0.1uf decoupling cap straight off the regulator 5v. Can I still power bot the AVCC and VCC using just one .1uf cap like this ?

A schematic only tells half the story, the physical location of the cap is important. Decoupling caps should be as close to the chip's power pins as in possible, 2" away at the regulator is no good.

If the reg needs one as well (read the data sheet, some do some don't) then add another close to it.


Rob

Aref needs a cap from that input to ground.
Yes you can run one cap for the other but it is better with two.
However why use a symbol for a FET in place of a voltage regulator. That needs a cap on both input and output to ground. The wiring is still wrong on that.

With my UNO board I increase the ADC performances by soldering two extra capacitors (100nF SMD 0805):

The most important : dirrectly on the ATmega socket.

The second, more difficult to sold, can be an option : middle of the Aref track.

On Arduino board Aref track is filtered only at the input board but Atmel say it must be filtered as near as possible from micro (application note AVR042).

Forum12.png

Forum13.png

Datasheets... I should have thought to look at those. I'll add 2 more caps for the reg and 2 close to the 328. Thank you so much to everyone in this thread for your valuable input.

Still missing the cap on Aref.