Decoupling Arduino’s power supply

arduinodecouplingdecoupling-capacitor

I am building a replacement for dishwasher's controller that burned, using Arduino Leonardo. Arduino is powered through standard 9V AC/DC converter.

Dishwasher uses quite large 220VAC electric pump and a few electromagnetic valves that I control using relay board. Because I was stupid I've bought 12V relay board, not 5V – so I had to power relay board from Vin pin – 9V is enough to do the job.

The layout is pretty tight – I use sealed lunchbox to store the components (wiring is removed at this photo, it looks as a ratnest when fully assembled).

The problem is that serial connection I use to debug my dishwasher code is quite unstable. Sometimes it hangs; if I use TTL wires hooked to pins 1&2 instead of USB I sometimes notice odd characters. Seems that either relays are causing short voltage drops sometimes or the noise from pump and valves are influencing the circuit.

My idea was to add few electrolytic and ceramic capacitors right on the 9V AC/DC's wire. But what worries me is that I've heard multiple times that decoupling capacitors should be installed as close as possible to the circuit. In my case the length of the wire between caps and Arduino's DC jack will be about 5cm. Is it gonna work or this setup will be useless?

Best Answer

In my case the length of the wire between caps and Arduino's DC jack will be about 5cm. Is it gonna work or this setup will be useless?

It is not feasible to know, without simply trying it. Yes, it's best to put the capacitors as close as possible. If 5cm is the closest you can get, than that's "as close as possible". Certainly adding the capacitors won't make anything worse.

We especially can't know if this will fix your problem because we don't know what your problem is. See Why does connecting a high-current device to my digital circuit cause weird behavior?

Related Solutions

Communication between 2 arduino crash when external power supply is plugged

Note: this answer is based on the comments above.

Possibly the cause of the problem is: * Long I2C cable length, combined with * Different power supplies causing current to flow in the ground wire of the cable, causing noise on the I2C lines.

The solution that worked was to reduce the I2C bus speed. It is usually 100kHz but one can reduce it to 10kHz by making the following code changes (taken from http://arduino.cc/forum/index.php/topic,12518.0.html)

Modify hardware/libraries/Wire/utility/twi.h:

Old:

#ifndef TWI_FREQ
#define TWI_FREQ 100000L
#endif

New:

#ifndef TWI_FREQ
#define TWI_FREQ 10000L  /* 10kHz */
#define PRESCALER_10KHZ 4
#endif

Modify hardware/libraries/Wire/utility/twi.c

Old:

cbi(TWSR, TWPS0);
cbi(TWSR, TWPS1);

TWBR = ((CPU_FREQ / TWI_FREQ) - 16) / 2;

New:

/* Select 4 as the prescaler value - see page 239 of the data sheet */
cbi(TWSR, TWPS1);
sbi(TWSR, TWPS0);

TWBR = ((CPU_FREQ / TWI_FREQ) - 16) / 2 / PRESCALER_10KHZ;
/*
  The documentation in twi.c is assuming a prescaler value of 1.  
  The actual equation (data sheet page 218) is 
  SCL Frequency = CPU Clock Frequency / (16 + (2 * TWBR * prescaler))
  For a rate of 10kHz with a clock speed of 16MHz, TWBR is 198.
*/

This post (http://arduino.cc/forum/index.php/topic,16793.0.html) also advises that ".. you also have to the delete the files:

hardware/libraries/Wire/Wire.o
hardware/libraries/Wire/utility/twi.o"

to make sure the changes are recompiled.

Electronic – arduino – Relay takes time to close

Assuming your supply isn't collapsing somehow, to me it's most likely that the CTR of the opto isn't sufficient to saturate Q1 and meet the pull-in voltage requirement of the relay:

The typical forward drop of an optoisolator photodiode is on the order of 1.5V
There's another LED in series with the path, dropping at least another 0.7V

The photodiode current is going to be on the order of:

\$I_D = \dfrac{5V - 1.5V - 0.7V}{1k\Omega} = 2.8mA\$

The vast majority of optoisolators that I know of only guarantee their CTR at 5mA current or higher. Even a high gain opto (100-300% CTR) will underperform at this current level. It's quite possible that many optos in this application will tend towards higher CTR and work without circuit modification. Also, some Q1s may have much higher \$H_{FE}\$ and handle the weak drive.

I would consider soldering another 1k in parallel with the existing R1 and see if the relay performance improves. Most optoisolators can handle up to 50mA photodiode current; that being said, the diode current should be set to the lowest possible current that allows your application to operate robustly, since optoisolators do age (the CTR degrades over time: faster as the photodiode current increases).

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Related Solutions

Communication between 2 arduino crash when external power supply is plugged

Electronic – arduino – Relay takes time to close

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