Capacitor on ethernet shield


I just finished a project (ArduXWeatherStation) of a weather station that sends data on the portal Xively remotely and locally on a second Arduino Mega 2560 with TFT touch screen 3.2 ".
The project runs smoothly, I had still the problem of the ethernet shield that inserting the plug of the power supply 9V 1A would not start, I just had the first time, press the reset button.
Now I solved the problem by inserting a capacitor on the RESET pin of the ethernet shield and GND.
On a post in this forum is listed capacitor to use and that is a 47nF, trying this ceramic capacitor I saw that the project starts regularly but led the shield flash intermittently for at least 5 to 6 seconds.
Trying instead an electrolytic capacitor 100uF or 330uF times were almost instantaneous and with that from 470uF in practice the project starts immediately.
Some of you know explain why? The electrolytic capacitor on the shield to act as a bridge between RESET and GND is more appropriate than ceramic or not?

I would be grateful if someone help me understand.

On youtube:


Your schematic shows you have a capacitor hooked from power to ground, not reset and ground...

If you meant power and ground then it makes more sense because I'd the Ethernet shield draws a lot of current before the capacitors on the mega fill up then it very well can cause a brown out and the mega ends up restarting when there is power again

The capacitor that works is four magnitudes bigger than the one that did work. It is nothing to do with the capacitor type and everything to do with the value.

Hi Ps991 What you say is a capacitor to the line of DHT22, the diagram posted on the forum is all complete project, but I have not designed the electrolytic capacitor for the ethernet shield, which is actually located on the PIN RESET its anode and PIN on the GND its cathode. I have not entered because I'm not sure that all the ethernet shield have the same defect. I have an ethernet shield compatible.

Hi Grumpy_Mike,

how to calculate the correct value for the capacitor? someone has understood the problem of the ethernet shield?

how to calculate the correct value for the capacitor?

It is complex and you need to know things about the circuit that you probably do not know. As you have not posted a schematic it is hard to say. What you have posted is a physical layout diagram which is useless if you want to understand a circuit.

Basically you need to know the required length of the reset pulse. You get time by multiplying resistance with capacitance. So in order to find the capacitance for a specific time pulse you need to know the resistance that is charging the capacitor and that which is discharging it.

However from the stupidly sized image you posted ( have you actually clicked on it? ) I can't make head nor tail of any reset issue. The only capacitor I can see is a rather ineffectual decoupling capacitor.

The ceramic capacitor you are saying and the sensor DHT22, actually it is only necessary if the sensor is disposed along a cable of more than one meter. unnecessary for long cables 20 cm, see the datasheet for the DHT22. I repeat, the electrolytic capacitor is not in the design of the project because I do not know if all types of shield have the same problem.

So where have you put this capacitor? You say:-

but I have not designed the electrolytic capacitor for the ethernet shield, which is actually located on the PIN RESET its anode and PIN on the GND its cathode.

But that is odd because a capacitor does not have an anode and cathode so I don't know what you mean.

Can you post a schematic of where you put this capacitor please.

is also clear from the description, I can not do better than that

Check out the links in my signature, the first row.

Grumpy_Mike Grumpy_Mike see also this that maybe you know …

thanks polymorph. But I not have the circuit diagram of my ethernet shield. I can only tell you that it is compatible R3.

I mean draw a schematic of your circuit, not a pictorial.