PIC resets when powering fluorescent lights

Obviously, the large transient voltages from the relay power side are getting back into the PIC. A good starting point would be a complete schematic of the whole system, so that we can discuss coupling mechanisms and fixes. Can you draw something up , get it converted to GIF format and post to alt.binaries.schematics.electronic? Or email it to me and I will post it for you.

The networked connections can complicate things, since they are all antennas that can feed spikes to the PIC. I big no-no is having any connections to the PIC go outside the supply rails, even briefly. The specs for the PIC are damage specs, not malfunction specs. You might try adding an additional 1 Mohm resistor between the 10 Mohm resistor and PIC, with a 4.7 volt zener at the junction of the resistors ot the

What sort of circuit do you have connected to the reset pin of the PIC? Are any unused pins programmed to be inputs?

What sort of relay driver circuit connects the PIC to the lamp relays?

Good idea John - only my 'schematics' are more layout diagrams in my own 'style'! Moreover the controller device does a lot more as a slave device for a multi-node networked home automation system, I wouldn't want to complicate the question. I do believe, perhaps naively, that a simplified device would have the same problem. I could knock up something that represents the area of the circuit that deals with this, but in an attempt to expedite an answer, I'll try to describe the rest of the relevant circuit. The PIC, 2803 and relays are powered by a 7805 with recommended caps on both sides (1uf & .33uf) itself powered by a 1.2A 12v regulated supply. Another possibly relevant point is that the PIC determines zero-crossing using a dirty-but-working 10MOhm direct live feed to RB0. I'll check removing this doesn't fix it.

Does that help?

Cheers,

Richard.

Hi, several things spring to mind. 1. decoupling capacitors. 2 RFI filter on the ac supply to the psu. 3. DC filter to the pic vcc pin, I use 100r +01uf cap (low pass) twice, that seems to kill everything. 4. Are you using the RB0 input in interrupt mode? don't. 5. write your software so that it recovers from crashes, ie use the brown out interrupt.

John,

The network connections are via a MAX485 which itself has a common ground with the other nodes. I could try eliminating all the comms connections as well, I guess. I have eliminated the live connection via the 10 Mohm with the same results (grounding the input). All other inputs are in use - in fact every pin is in use.

I like the idea of the 1 Mohm + zener circuit you suggest - I'll add that at some point! I've used zeners for DC transient elimination on other inductive loads that I drive with the 12v line.

I'm glad you brought up the reset pin. I should have mentioned that. MCLR has a weak pull up (10k) to 5v. I know internally it is filtered for short low pulses, but perhaps there is a weakness here? Perhaps a zener here to prevent inverted transients? Maybe it would still be brought low and cause a reset anyway?

The relay driver circuit is very simple. Just direct PIC->2803 connections with direct delay driving from the 2803. Perhaps some filtering/protection here?

You'll gather I have limited knowledge of filtering and protection, so your advice is gratefully received.

Regards,

Richard.

1. Can you elaborate?
2. I could battery power the device first to determine if this would help I guess
3. Can you explain exactly how you do this - a snubber across the vcc and gnd? What do you mean by 'twice'?
4. Yes I do - but this is a rather handy feature of the chip - it is actually the only interrupt I use as the device is driven by zero-crossing logic to ensure timing integrity for triac control. All other interrupts are masked and polled instead.
5. This seems to be a general comment - but a good one!

I've stumbled across a discussion on EDABoard

Its very important to place decoupling capacitors at the supply pins of the PIC

You could but whats the point? Your flourescent tube needs 1500v to strike the remnant of that spike goes everywhere, line filters are just about mandatory.

No, it's not a snubber it's a low pass filter. A resistor in series with the 5v line followed by a cap to 0v. The bigger the resistor the better it works but you get a bigger volt drop on the line so you need to minimise the PIC vcc current. You can put it upsteam of the regulator but it's not as effective.

It seems like a good way to do it but it isn't. Firstly you will get multiple interrupts at the crossing point, any stay noise pulse will cause you false triggering and the there is the mains "drop out" problem to contend with. A better way is to have the timer interrupt running at say 50uS, poll the crossing point input each time through the loop, and do some "debouncing" on the result to give you a reliable trigger. You can also generate your time delays and trigger pule steams at the same time.

Ok - will do.

Point taken - and I should. But could this actually be causing a reset?

Ok - sounds good - will definitely try that. Just need to determine the PIC current req's - shouldn't be too high.

Ok - guess you have good reasons for doing this.

Don't get multiple interrupts - logic debounces through required processing during interrupt (admittedly

I would add a small capacitor between MCLR and the ground pin of the PIC.

Id this what you are referring to as 2803?

This sort of driver represents a connection between a relay driving supply and the logic supply. You should arrange the two supply connections at pin 9 to minimize the length of conductor that carries both logic signal returns and relay currents. When there are more than one of these chips, a true single point connection between the two supply negatives is not possible, but you can still keep this bus as short and heavy as possible (connecting all the pin 9s) and then tie both supplies to that bus.

What is pin 10 connected to? I am assuming it is the relay supply positive. There should be a bypass capacitor between the relay supply positive and the pin 9 bus, one at each 2803 (pin 9 to pin 10) with the relay power line going out from that point, instead of from somewhere else along the positive supply line. That way, when a switch pens, the current through the loop out to the relay detours from the switch to pin 9 path to the bypass capacitor to pin 9 path. So the transient is contained outside the logic supply.

Relay contacts bouncing while driving inductive loads like the lamp ballasts will generate lots of sparks and high voltage spikes and also take little bites out of the contact material. An RC snubber across the contacts or even an MOV will absorb lots of that energy.

There are several circuits in the data sheets for the pin, or you can just tie it to the vcc.

Yep - sounds like a good idea - and actually what Microchip suggest (now that I've read the datasheet properly!). Just got to extricate the board from the application and modify - argh.

Yep - although in this application, I only have one array - suffices for the number of relays that need powered.

I do try to keep the relay power lines fairly independent (in wiring terms) from the logic supply - however, they do both use the regulated +5v, so there are limits!

Pin 10 is not directly the supply +ve to the relays, but as above, they are both derived from the 7805. More bypass capacitors sound like a good idea though. What size for this purpose? Not sure what formula is proposed here. 0.1uF again?

Yes - sparking is a concern but the MOV's in place should help. Clearly more filtering to protect the logic line is the way to go, though. I suspect, though have yet to prove, the main problem is the weak pull-up on MCLR and will address that with a lower resister and addition of the cap.

Thanks again,

Richard.