I have a ULN2803 darlington array switching 6 small relays (5V, 72ohm coil) via a PIC microcontroller. Befre I knew about the induction voltage spike caused by the relay coil the PIC would get reset every time a relay switched open. I got some advice to put a diode in backwards around the coil of each relay. I did that and it helped. However, occasionally the PIC is still being reset when a relay switches. Not every time, just every now and then. Do I need a larger diode or what? I thought the ULN2803 was supposed to provide some protection against voltage spikes?

 

By the way, I have a 1N4001 diode around each relay coil.

 

The reversed bias diode is only good for induction current produced by the field collapse on the coil. There can be other induced transients caused by arcing of the contacts. You don't specify load which is important. High voltage loads ( 110ac< ) can produce major arcing which can feed back ( control line close to load line ) especially on inductive loads like a motor. The same arcing can cause minute spikes in the VCC that the PIC uses.

If the switched V from the relays is on the same power as the PIC feed check your power supply filtering and make sure you have enough bypass caps as close to the PIC as possible. If the load is a hi V on a different line buy a "quencharc" for $5 a pop or make one for each load from a .01 cap , like a Sprague orange drop and a 47Ohm 1/2 watt resistor ( in series ) . The V rating on the cap must be at least 2x the supply to load Ex. at 110VAC the cap should be rated @ 220 V or better. It connects directly in parallel with the load.

 

I mean, this is a supply problem. I never use same supply for PIC and another circuits. Apply an independent 78L05 for PIC.

 

ULN2803 has built-in diodes for this purpose. Just connect pin 10 to the +5v that powers your relays.

 

MOSFETs have intrinsic diodes inside of them which protect against the induced reverse voltage, in addition to being better switches.

 

The intrinsic diode will not protect a MOSFET from the flyback voltage that is generated when an inductor's current is interrupted. The diode must be across the inductor. A diode from drain to source of the MOSFET provides no protection.

 

I redesigned the circuit and seperated the PIC power from the relay driver power, I'll let you know how it does when the board comes back from the proto shop. Also, I took out the 1N4001 diode and went with the straight ULN2803.