Although my project drives very low voltage on the far side of the relay/coil, and never have more than one relay tripped at time, only 5v load per channel no more. So I doubt I will have any EMF feedback issue, but I want to establish solid good habits that are best practice oriented, so if I ever do have a project that would drive higher voltage, or holding relays open for more than 1/8 of second, or multiple relays open the same time, etc., I am affirming good design.
Since you are providing power from the RPi to the LED side of the isolators on the board, each of which uses 20mA when on it would be better not to et 5V from the RPi GPIO connector. Sure it is good for this project where you only use one at a time, but many months/years down the line you might forget that restriction. Thats just "best practice" though.
I think though that you have not understood the EMF issue and the need
for a protection diode. The load on the contact side of the relay is not significant here, it is the fact that you have a relay that is controlled by on/off (logic) electronics. The coil in the relay is, well, a coil, an inductance. Inductances will resist any change
in the current flowing through them, when they are turned on they behave like a high resistance, when they are turned off they generate a voltage to oppose the current dropping, the faster you change the current, the higher the voltage. A logic signal changes very fast so high voltages are generated, just in the coil itself.
In simple terms:
A resistor opposes current flowing through it, generating a voltage to oppose that flow. The higher the resistance/ current, the higher the voltage
A capacitor opposes a change in voltage across it, providing a current. The higher the capacitance/rate of change of voltage, the higher the current.
An inductance opposes a change in current through it, generating a voltage. And, you guessed it, the higher the inductance/ rate of change in current through it, the higher the voltage.
This header is not
for power supply.
Its a jumper to set JD-VCC as VCC.
External power must be connect between GND (on input header) and JD-VCC. Or direct soldered to appropriate points.
Some similar module have a 3 pin header to select/input relays power source.
See this recent discussion about same relay module.
http://www.raspberrypi.org/phpBB3/viewt ... 63&t=35155
This is completely wrong, the pins on your header are connected to VCC and JD-VCC so they are definitely suitable. In fact there isnt another connector for JD-VCC so either you use a jumper here or you use it as a power supply connector.