I have one of these in my basement controlling a 4 pack sainsmart relay and it works fine on USB but when I try the power only jack with 9 volts it goes into endless reset. Anybody know what is happening here?
Can you please provide a schematic / wiring diagram; a reasonably sized photo (around 300 kB or so) of a hand drawn one is fine. Note that it should also show every power connection.
An Arduino is not a power supply 
You're probably overloading the on-board voltage regulator; at the moment that the power drops, the Arduino resets.
What is supplying the 9V?
the coils use power from a 5v pin as I recall. There is a jumper (red wire in the middle) connecting that to the drivers of the coils. I've done this in lots of cases and never had an issue.
Hi,
Do you have a DMM?
Measure the voltage between Vin and gnd.
This will measure the 9V when you plug it into the jack?
Also measure the 5V to gnd.
If you are powering the relays of the UNO 5V pin, then you may be overloading the onboard 5V regulator.
Thanks.. Tom... 
I do have a dmm. assuming black to ground and red to Vin or do you mean the 5v pin? I am using the 5v pin. Supposedly you can take something like 200ma from that and these coils supposedly do not need that much. I'll go down and try this.
Why not just power everything from a 5V wallwart? Then you are not stressing the 5V regulator, but are bypassing it instead.
Here's a good one
https://www.mpja.com/5-Volt-DC-Plug-Power-Supply-4A-Regulated/productinfo/18520+PS/
That's what I was using and it kept resetting. I think it was 2A. I'm going to measure voltages once I rig a bread board to split the ground and vcc.
Wait, no I was using a 9v version. I don't think you can plug a 5v into the barrel connector, at least not according to the documentation.
You can use cellphone charger and connect to USB; depends on your needs if that is an option. It however looks like you're already doing that so what is the problem with it?
I'm not sure if the 5V pin on the Uno WiFi rev2 can be used as a 5V input (like with normal Uno).
Oh good, you're here, actually, the wifi stopped working last night on USB so I don't know what is going on. So I guess it will start ok using USB but drift into la la land later. On barrel adapter it does permanent reset till unplugged. This thing is a charm! But getting back to the voltages, I should measure from between 5v and Vcc on the relay board, is that right? I will do with both USB and the other.
And the wifi can't complain about signal, I moved, at great hassle, the router to within 4 feet of the rev2.
Having looked at the schematic, I don't see a reason why it shouldn't work.
The Vdd and +5V nets are effectively connected through a 0R resistor and the 3.3V logic voltage is derived from it.
I did see that But I did not see the full power supply schematic (till now) 
But that now brings the question why a 9V power supply on the barrel will not work. I don't know enough about step-down converters though but the MPM3610 is good for 1.2A and temperature vs load graph does not seem to be too worrying (20 degrees C rise, max junction temperature 150 degrees C).
Yeah, you're right, that onboard regulator should easily be capable of handling those relays given Vin=9V and Vout=5V. I went through the datasheet and see no other very objectionable issues.
The only thing I can imagine is that somehow the 9V supply itself doesn't keep up with transients. The 47uF input capacitor on the Uno/R2 may not be sufficient to catch that. But it's a bit of a guess.
connecting a usb wall wart to the 5v would probably work.
This particular issue has evolved over the past year that box has been in use. In the beginning powered by thru the USB connector, it would work find for a while then lose connectivity to the internet. So I moved the router to within a few feet of it. Same result. Last month I tried using the barrel at 9v and that seemed to work well, ran about a week before needing a restart. Maybe it's because when it starts I turn all four switches on to verify they work. I'll take that part out because what it does now with non usb power is just continue to restart.
Hi,
Have you got a DMM to monitor the 9V supply when the problems arrive?
Thanks... Tom.
I have one, but how should it be wired? 5v to Vcc of the relay? Do we want current or voltage? I just realized I'm not sure what I'm doing here.
A DMM may be helpful (measure voltage across 5V that supplies the microprocessor), but a scope is probably more useful. A DMM won't pick up momentary sags.
That's what I was wondering, how much can a dmm show other than an instance. but I will hook it up and see what happens when I plug it in.
If you would have posted a schematic, we could have shown you were to measure 
But the interesting part would be the behaviour of the 5V against GND.
so from 5v to ground at startup on usb I get 5.1v dropping to 4.6 that's with the relays connected as usual stabilizing at 4.9, with 9v power into the barrel 2.6 and flickering. I wonder why that is. something is wrong here.