MOSFETS and Solenoids

[Jerson]

I'm not sure you're getting the full advantage an isolator has to offer when you tie the output side ground to the input side.

Ideally, you should have a supply for the output side (26Amps) that is isolated from the input side (5V side) via the isolators. Indirectly, it also means, you should ideally isolate the 2 supplies. Otherwise, the effects of the ground currents running into the low voltage side and disturbing the PIC will still continue to haunt you.

If it is the supply noise due to RPMs increasing, a simple half wave rectifier at the input of your 5V regulator should keep the circuit happy. I've seen this kind of problems when the alternator starts spiking the battery voltage at high RPMs.

Cheers

[timmers]

Try lowering the value of the gate drive resistor. Mosfets have a very high imput impedance and the gate-source charge needs to be thought of as a small capacitor. I have seen values less than 1k used here for mid frequency applications.
Also I do not see any flyback catch diodes. They release the back EMF energy of the solenoid as current/heat to ground. A high voltage spike on the drain will certainly make life for the MOSFET difficult as the built in flyback diode is configured for low side drive not high side as in your case.
If I was designing a driver I would put battery + to one side of the solenoid and put the N channel MOSFET from the solenoid to ground. The built in protection diode will then act as your protector diode and catch the back EMF spike. Alternatively a more efficient method is to recycle the energy through a snubber network.

Just my thoughts...
Tim.

[Tobias]

I went with the P channel because thats the way the teams want it. Its more of a build what customers ask for deal. Right now the teams turn on and off the + side with relays.
I will give the lower gate resistance a go and see what happens.

Thanks to everyone. Once I get it sorted out I will let everyone know the results.

[Tobias]

I am not too sure what you mean by input and output grounds tied together. Can you give me a location on my schematic?
How about a suggested part number for the half-wave rectifier?
Thanks
Toby

I'm not sure you're getting the full advantage an isolator has to offer when you tie the output side ground to the input side.

Ideally, you should have a supply for the output side (26Amps) that is isolated from the input side (5V side) via the isolators. Indirectly, it also means, you should ideally isolate the 2 supplies. Otherwise, the effects of the ground currents running into the low voltage side and disturbing the PIC will still continue to haunt you.

If it is the supply noise due to RPMs increasing, a simple half wave rectifier at the input of your 5V regulator should keep the circuit happy. I've seen this kind of problems when the alternator starts spiking the battery voltage at high RPMs.

Cheers

[Jerson]

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.

[markedwards]

Do you have access to a scope? Check your gate to source voltage.
Does the drain voltage drop after the 1st solenoid energizes?
Ignitions generate incredible electrical interference. A high frequency lightning storm. The spark plug firing voltage can vary with engine load.
The interference could affect your mosfets and or PIC.
The electrical interference is both conducted and radiated into your circuits.
Proper Layout, shielding, twisted pair wiring, etc... may be needed.
Tires can also generate electrostatic electrical interference.
Try diagnostic LEDs to make sure your program is executing the code.

Good Luck

[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.