MOSFETS and Solenoids

[Tobias]

Thanks for pointing that out. Theoretically, if the board is completely isolated like the attached schematic I will be immune to electrical noise like markedwards is talking about or am I just minimizing it?

markedwards...I am pretty sure the PIC isn't resetting because the first stage stays engaged. When the PIC powercycles it goes through a start up sequence thats about ten seconds long including turning off all outputs. If its electrical noise its definately only at higher rpm. Testing in the shop everything looks fine.

I am getting a 4 channel analogue module for our data acq system. I am going to make some voltage dividers to log the voltage output of the MOSFETS with the 0-5v inputs.

Look at your opto isolators. You have tied the Led cathode to the same ground as the opto transistor ground. That is what I wish you could break apart and keep apart.

[mackrackit]

Thanks for pointing that out. Theoretically, if the board is completely isolated like the attached schematic I will be immune to electrical noise like markedwards is talking about or am I just minimizing it?

If noise is found to be the problem and it is really bad then a grounded metal enclosure might also help. Think Faraday Cage...

[markedwards]

I didn't see any large capacitor for the battery + connection to ground.
With high frequency noise there is a significant impeadance between the battery and your device.
A scope will show you high frequency noise that your aquisition system
will likely average (filter) out. The aquisition system is similar to a DMM with a slower response. It will not show high frequency spikes or voltage dips.
Ignition and similar electrical noise can be conducted and re-radiated
inside the metal enclosure.

What is the solenoid's dc current draw?

[Tobias]

There is a capacitor on the chassis. Its sold by the ignition company. Its pretty big, about half the size of your fist.

The solenoids draw 26 amps per stage. Some cars will have four stages. The fourth stage requires 14 amps.

Our data acq system only samples 50 times a second on the logger voltage. I did hook up my scope to VIN with the cap disconnected and its pretty scary. The cap really helped alot.

[Jerson]

Thanks for pointing that out. Theoretically, if the board is completely isolated like the attached schematic I will be immune to electrical noise like markedwards is talking about or am I just minimizing it?

The circuit now looks good. However, I have not checked it thoroughly, just a casual glance over. By isolating the 2 grounds and the 2 supplies, what you have done is eliminate the electrical intermingling of currents. The radiation aspect is what you now need to control. For that, everything you read in the rule book for HF design and high current design should apply.

I am willing to wager on you solving your problem by just separating out the 2 supplies and grounds like you have in the new schematic.

You need to ensure that the PIC does not brown out during the time one/maybe more MOSFETS are on and the engine is revving. This is the time you will see brownouts if your 5V power is not well regulated. Like I said earlier, a simple 1N4007 diode followed by a reservoir capacitor feeding your 5V regulator should keep the PIC happy. The best way to check would be to ensure that the circuit stays on for at least half a second when you remove power. Now, come whatever spikes due to the engine rev, you will still have the PIC running safely.