Pages: [1] 2   Go Down
Author Topic: Voltage drop over capactors  (Read 1364 times)
0 Members and 1 Guest are viewing this topic.
Offline Offline
Newbie
*
Karma: 0
Posts: 22
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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!!

Logged

USA, FL
Offline Offline
God Member
*****
Karma: 14
Posts: 689
A life? Where can I download one of those?
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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

//LiNK

Offline Offline
Newbie
*
Karma: 0
Posts: 22
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Oh I see where I messed up... How about C2 and C3 ?
« Last Edit: June 16, 2013, 01:19:15 am by driz » Logged

Rapa Nui
Offline Offline
Edison Member
*
Karma: 60
Posts: 2071
Pukao hats cleaning services
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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

the land of sun+snow
Offline Offline
Faraday Member
**
Karma: 159
Posts: 2916
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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.
Logged

0
Offline Offline
Shannon Member
****
Karma: 206
Posts: 12062
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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

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

Eindhoven Netherlands
Offline Offline
Jr. Member
**
Karma: 2
Posts: 57
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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...
Logged

Offline Offline
Newbie
*
Karma: 0
Posts: 2
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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

the land of sun+snow
Offline Offline
Faraday Member
**
Karma: 159
Posts: 2916
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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.
Logged

Offline Offline
Newbie
*
Karma: 0
Posts: 22
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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
Logged

Offline Offline
Newbie
*
Karma: 0
Posts: 22
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

How about something more like this?

« Last Edit: June 16, 2013, 01:59:55 pm by driz » Logged

0
Offline Offline
Shannon Member
****
Karma: 206
Posts: 12062
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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!
Logged

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

nr Bundaberg, Australia
Offline Offline
Tesla Member
***
Karma: 126
Posts: 8497
Scattered showers my arse -- Noah, 2348BC.
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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

______
Rob
Logged

Rob Gray aka the GRAYnomad www.robgray.com

Offline Offline
Newbie
*
Karma: 0
Posts: 22
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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 ?

Logged

nr Bundaberg, Australia
Offline Offline
Tesla Member
***
Karma: 126
Posts: 8497
Scattered showers my arse -- Noah, 2348BC.
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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
Logged

Rob Gray aka the GRAYnomad www.robgray.com

Pages: [1] 2   Go Up
Jump to: