Hi All
This is the first PIC project I've attempted.
It is a timer which turns on and off various devices (up to 15A) which run from a car supply. The PIC is powered from the same supply using a 78M05.
It worked fine on the breadboard and the PCB worked fine too. Unfortunately when I connected the output relays - conventional automotive relays - I encountered a problem which is that when the relay turns off the PIC resets -presumably due to the back emf as the current in the coil decays.
I have a couple of ideas:
1. put a diode on each relay - I've found a reference that suggests the back emf could be as much as 200V - but presumably the diode should stop it getting anywhere near that so that with an 80 ohm relay coil an IN4001 should be adequate?
2. Do something to the power supply to smooth out the spikes. I've been playing with capacitor values on the input of the 78M05 (up to 2.2uF)(datasheet recommends 0.33uF) but this doesn't seem to make any difference. I haven't got much space on the PCB. Could I use an electrolytic as during spikes it will be the wrong polarity?
I'd be grateful for any suggestions or ideas.
Thanks for reading.
Russell
This is the first PIC project I've attempted.
It is a timer which turns on and off various devices (up to 15A) which run from a car supply. The PIC is powered from the same supply using a 78M05.
It worked fine on the breadboard and the PCB worked fine too. Unfortunately when I connected the output relays - conventional automotive relays - I encountered a problem which is that when the relay turns off the PIC resets -presumably due to the back emf as the current in the coil decays.
I have a couple of ideas:
1. put a diode on each relay - I've found a reference that suggests the back emf could be as much as 200V - but presumably the diode should stop it getting anywhere near that so that with an 80 ohm relay coil an IN4001 should be adequate?
2. Do something to the power supply to smooth out the spikes. I've been playing with capacitor values on the input of the 78M05 (up to 2.2uF)(datasheet recommends 0.33uF) but this doesn't seem to make any difference. I haven't got much space on the PCB. Could I use an electrolytic as during spikes it will be the wrong polarity?
I'd be grateful for any suggestions or ideas.
Thanks for reading.
Russell