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Topic: Voltage drop over capactors (Read 1 time) previous topic - next topic

driz

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!!


codlink

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

driz

#2
Jun 16, 2013, 08:17 am Last Edit: Jun 16, 2013, 08:19 am by driz Reason: 1
Oh I see where I messed up... How about C2 and C3 ?

pito

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

oric_dan


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.

MarkT

The capacitors "stiffen up" the supply voltage near to the chip - prevents/reduces high frequency current changes from making
the supply voltage noisy.
[ I won't respond to messages, use the forum please ]

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...

colormba

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

oric_dan


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.

driz

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

driz

#10
Jun 16, 2013, 08:36 pm Last Edit: Jun 16, 2013, 08:59 pm by driz Reason: 1
How about something more like this?


MarkT

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!
[ I won't respond to messages, use the forum please ]

Graynomad

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

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

driz

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 ?


Graynomad

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
Rob Gray aka the GRAYnomad www.robgray.com

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