switching high inductance loads

[kitcat]

Hi there.
I have been trying to solve my resetting pic problem for too long now.
I am using porta.0 of a pic16f872 to drive the coil of a dpdt relay (via transistor of course!). The relay is to switch polarity to some solenoids before they are operated using some ln2803s. So the pic sends a pulse to the coil. the coil switches the polarity to the solenoids and then back again ready for them to be driven by the ln2803. As soon as I connect a solenoid the pic resets when the pulse occurs. Even with the solenoid driven from a seperate power supply. Without a load it usually works fine and the relay is clicking over as expected. I suppose it is some sort of voltage flying around. Sometimes when the pic resets the outputs come on high which causes the relay to switch over and the thing starts chattering. The pic then gets hot. I have lost a pic through this problem and I don't want to lose another.
Any ideas?

[NavMicroSystems]

Could you post your schematic?

switching high inductive loads is always a challenge.

Some things that may help:

- Check for Ground Loops
- reverse diodes accross all coils.
- RC filters across the relays contacts
- RC Filters at the PICs and Drivers I/O Pins
- Many large Ground Planes
- Relays as far as possible away from the PIC
- opto-isolators between MCU and drivers
- If switching AC, toggle the relays at zero crossing !

etc...

[kitcat]

Here is a rough schematic. The relevent parts anyway. Sorry it is so bad!

The problem occurs with just one tiny solenoid connected and without the uln2803 plugged in. So just sending a pulse to one test solenoid causes the problem. Are you saying that I should remove the relay from the board and just send the feed back?
Thanks

[kitcat]

Hmmmnn having trouble uploading !

[kitcat]

Another try.

[Dave]

kitcat, I assume from the schematic you are using one of the Allegro driver chips but I don't see pin 10 connected directly to the solenoid + supply. The reverse diode in series with this pin makes any connections to it useless.
I have used quite a few of the Allegro (formerly Sprague) drivers and I have never experenced any problems with them reseting any processors I have connected to them. One thing to keep in mind when using these multiple sinking or sourcing drivers is the current return path in the case of sinking drivers, and the supply path in the case of sourcing drivers should be as direct to the current supply as possible. Therefore in your application the pin 9 of your selected driver should have a seprate high current path to the current source "return" and the pin 10 should be going to the the closest point of the "loads" current source.
Dave Purola,