Pic reseting when switching mains current with relay

[lsteele]

Hi,

I'm trying to switch a mains current (240V AC) with a relay connecting via a transistor to my PIC (a 16f876a). The PIC is powered from a battery supply - 4 rechargeable AA NIMH cells providing 5V. That's powering the PIC directly - there's no regulator (I presumed there's no need for one).

At the moment the mains supply is hooked up to a light. Everything usually works fine when switching the relay ON, but when the relay switches OFF the PIC resets almost every time.

If I disconnect the mains supply the PIC can turn the relay on and off without any resets.

The schematic is attached. The relays sit on a separate PCB immediately adjacent to the board with the PIC, and the two are connected with 2 inch wires. The wires with the mains current travel close by the PIC board - this isn't really avoidable. The relays are switching the Live wire of the mains supply.

In addition to what you can see on the schematic I've added a 0.1uF cap between MCLR and ground. (Something I saw suggested somewhere.)

I've read a lot about decoupling capacitors, zero crossing, ground planes, and so on! But I don't know where to start, short of randomly trying everything. It would be great to be able to systematically diagnose what's creating the problem, but I don't know how to go about doing this.

Any suggestions are gratefully received.

Thanks,

Luke

[mat janssen]

Put a snubber network over the relaiscontacts.
(A snubber network is a resistor of 47 ohms in series with a 0,1 uF capacitor)
Dont forget to take a capacitor what is suitable for mains voltages (240V~)
And take a resistor of at least 1/2 watts.

[sayzer]

Luke,

What are the fuse settings? BOD ? WDT?
The code?


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[lsteele]

mat: If I try what you suggest won't the resistor allow a current to flow all the time? Or have I misunderstood how it should be connected?

sayzer: The code is definitely working ok - with no load across the relay it will switch on and off without any problems every time. It's only when there's a load across the relay that the pic will reset when the relay switches.

The fuses are everything off except PWRT (or 0x3F31). (Incidentally I'm using a bootloader, so this is the fuse configuration it defaulted to...)

[Luciano]

Won't the resistor allow a current to flow all the time? Or have I misunderstood how it should be connected?

Hi,


(Click to enlarge the picture)

See also this PDF:
http://www.redlion.net/Products/Grou...Docs/12027.pdf


Best regards,

Luciano

[mister_e]

Yes the snubber will help!

also

• Make sure you have connected both PIC VSS pins.
• Make sure you have disable the LVP mode
• Set all unused i/o to input and connect ALL of them to GND
• i would add a 47-220 uF on the PIC VDD line as well (close to the PIC)
• Place your PIC away of your relay to see what's happen.

How much current do you drive on the AC side?

[mister_e]

If you didn't change the default config fuse while loading the Bootloader firmware the first time, you can't modify them later in your code.

Default for 16F876_04.HEX are HS OSC,Watchdog off, Power up ON, Brown out ON, LVP OFF.

Maybe you could also try to reload the bootloader firmware but set the BrownOut to OFF... but, i think there's much more to check before...

[lsteele]

Thanks

>> If you didn't change the default config fuse while loading the Bootloader firmware the first time, you can't modify them later in your code.

I know! :-) But I'm quite happy with that fuse configuration right now... unless it's a factor in my problem.

>> Yes the snubber will help!
>>
>>also

>> * Make sure you have connected both PIC VSS pins.
>> * Make sure you have disable the LVP mode
>> * Set all unused i/o to input and connect ALL of them to GND
>> * i would add a 47-220 uF on the PIC VDD line as well (close to the PIC)
>> * Place your PIC away of your relay to see what's happen.

Ok, I'll try all of the above. But I'd like to understand why connecting a 47 Ohm resistor across the relay output (as part of the snubber circuit mat suggests) is a good thing -- surely it'll allow a current to flow all the time. Or have I misunderstood something?

Thanks again...

[mat janssen]

here is a drawing of a snubbernetwork .

[J_norrie]

Hi, There if ever I have to switch high current devices I always use an isolated supply on the pic side and a opto coupled darlington transistor to drive the power relay. This I think would solve your resetting problem

hope this is of some help.
kind regards john