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FEZ Panda with FEZ Connect and 6 single relays



I’m building an home automation system to control my projector screens. I’m driving 3x 240v AC motors through 6 GHI relays connected with the 3pin e-block connectors on my FEZ Connect/Panda combo.

To control the relays I use the tcp webserver sample. Sometimes(but not always) when i switch off an relay it sort of resets my networking on the FEZ Connect. I can’t ping the device anymore; and i get an thread stopped message but no errors. When i pause the debugger i can see the networking thread is still running except the “server.LocalEndPoint” is bound to, which was before the thread gave a stopped message.

The weird thing is that this problem only occurs when I apply 240v to the relays and using them with a motor connected.

Is it possible that the current draw of the Fez Connect/Panda and switching relays is much for an USB port? Maybe it drops or something, just enough to reinitialize the network controller? Or are there any other obvious explainations for this? I’m suspecting some kind of backbleed of the relay?

Any help is very welcome :slight_smile:


Have you tried using power adapter. I think 9v will do the trick


Thanks Gus,

I haven’t tried yet. But the FEZ Connect Manual states that you can only use a 6,5 v or a 7v power supply. Is it safe to connect 9v?


Oh yes, FEZ Connect is power hungry and may cause regulator heat at 9V. It will not damage the board immediately for sure. Try 7V instead but you can try 9V for a minute and see how hot the regulator will get.



I bought an external power supply 1A. I’ve tried 6v, 7.5v and 9v but the network interface still crashes.

I really don’t know what to do now…


Strange! You can try adding a large capacitor on 5V and on 3.3V pins. You can also power the relays from separate power source (require some hacking to the wires but not the board)


Are there any tutorials for this? I really don’t have a clue :wink:

I’m a pretty good programmer, but my electronics skills needs some brushing up.


If things work fine when just switching relays when no AC is applied and if the problem only occurs when switching a load (i.e. when AC is applied) then it would point to an electrical noise problem. Switching an inductive load causes a LOT of electrical noise. There is arcing across the relay contacts when they open and close. This arcing is like a small arc gap transmitter. Look for devices called ‘quench arc’. They are special RC snubbers that go across the contacts of a relay that suppress the arcing/noise.



I guess i’m also able to supress the problem by using optocoupler and a seperate power circuit for the relays. That would prevent the noise problem too right? Is there any way to measure the noise?

Anyway i’ll try to get my hands on some of those “Quench arc” baby’s. Thanks for the suggestion!

Would it be possible to hack in a Quench Arc into the 3 pin cable? Or do i need to modify the circuit?


The quench arc goes across the relay contacts NOT the coil.


Just a thought, what about a ‘zero volt detect’ to switch the relay(s) off ?


Mechanical relays are way to slow for that, but SSRs have zero crossing detect built in…


btw, ssr is a solid state relay… :slight_smile:

the quencher that you must add will go into the screw terminals along with the motor wires.

the problem with the noise being generated is that it is radio noise. it can be picked up, and give problems, in any long wires, like network cables.

you have a few options, use a shielded network cable, all the way to the hub, or build the relays into a metal box that you connect to earth, or add a snubber to the skrew terminals along with the motor wires…



Good reads. I was at my parents house last week and my dad had similar insights, he told me to pack the panda in plastic and then aluminum foil and increase the distance between the controller and the relays.

He had some experience from those things, working in radio communications.

So i’ve ordered my quench arc’s and next weekend i’ll see how far i will get. Thanks for all the reply’s. I’ll try to update next week.