Post by thread:[EE] PIC interference problems caused poor PCB de

I had posted some problems regarding PIC interference problems 1 month + ago. Thanks to everyone's help and suggestions regarding this. They sure helped me to improve the project and reduced the error from occurring frequently. Just to repost the 'problem' I am using PIC 16F877 to switch some relays to control inductive such as solenoid valves and contactors(230V AC). loads. ULN2803A is used to drive the relays. A LCD is used to do some parameters display. Problems occur are LCD display some addition "rubbish" and pic resets or hangs (probably program counter points at unknown address) after switching the inductive loads. I managed to trace the problem as follows: 1) Driving relays without inductive loads, 5V supply is very "clean" no transient. However driving relays with inductive loads, transient happens on 5V supply. Therefore it seems that the transient were caused by EMI from the inductive loads driven by the relays. 2) Thinking that PIC reset problem could be solved by placing a RC filter to the mclr. I placed a 39K resistor through a 1n4148 then to the mclr pin. Capacitors such as 0.1u and 0.01u were placed after the resistor and mclr pin. Transient could still be seen on the mclr pin seems that EMI is still the main cause. 3) 2 PCBs were constructed for the project. They were almost similar except the most tracks width were widen on the 2nd pcb. A ground plane was placed under the pic. Not directly grounded but through a 1M resistor to ground. Directly to ground seems worse. Unused areas on the PCB (mostly the 4 corners were left like 'island'. Grounding them doesn't seems good. Should the "islands" be removed? The use of ULN2803A was based on a comercial "barebone" PLC (Only double side PCB no casing). Would a double side PCB improve the situation? Any tips on good PCB design practice? Actually I am reluctant to change my current driving technique to triac with zero crossing switching as the commercial PLC (Unknown microcontroller) could do it why I can't do it with a PIC? Thanks to those reading my long "boring story" and give me a few tip to improve my poor interfacing technique and lousy pcb design. Regards, Richard -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> The use of ULN2803A was based on a comercial "barebone" PLC (Only > double side PCB no casing). > Would a double side PCB improve the situation? > Any tips on good PCB design practice? > Actually I am reluctant to change my current driving technique to > triac with zero crossing switching as the commercial PLC (Unknown > microcontroller) could do it why I can't do it with a PIC? I read through your email but I see no mention of bypass caps. I recently finished a project that had the absolute worst power supply issues I have ever experienced. I took several drastic measures but the most effective were: isolate the PIC power supply from the rest of the circuit (I powered it from a different higher voltage available from the power supply and put that through a linear regulator, while not the best power wise it resulted in very clean power to the PIC alone), and put bypass caps RIGHT ON the PIC pins, I soldered the bypass caps directly onto the PIC's socket pins, one for each pair. I've done alot of PIC designs without having any power issues but this one caught me by surprise. I had a bypass cap for the PIC but supposedly it wasn't electrically close enough. In my case the PIC didn't reset or hang, the CCP was just capturing at the wrong times. TTYL -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

On Saturday 29 March 2003 08:36 am, Richard wrote: > I had posted some problems regarding PIC interference problems 1 > month + ago. Thanks to everyone's help and suggestions regarding > this. They sure helped me to improve the project and reduced the > error from occurring frequently. > > Just to repost the 'problem' I am using PIC 16F877 to switch some > relays to control inductive such as solenoid valves and > contactors(230V AC). loads. ULN2803A is used to drive the relays. A > LCD is used to do some parameters display. You should have: * separate relay supply and logic supply * supplies' grounds connected only at A SINGLE POINT * return (GND) of ULN2803A goes to that SINGLE POINT, NOT TO DIGITAL GROUND * Digital ground goes separately to that SINGLE POINT * COM from ULN2803A goes to relay supply + * no cap on MCLR * short wire between MCLR and its pullup resistor -- Ned Konz http://bike-nomad.com GPG key ID: BEEA7EFE -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics