MOSFETS and Solenoids

[mackrackit]

For what it is worth I agree with all the above comments, but I have a question or two that might help.

When the car is on the track are you using an alternator? In my day we did not. We put a couple deep cycle batteries in the back to help with traction and would run everything frm that. I am getting at two things. Maybe the battery is just low enough not to satuarate the FET or if you are using an alternator by the time the second stage kicks in it is giving "dirty" power.

If it works on the bench or sittng in the garage, like Tim was getting at, also look for heat or mechanical stresses.

[spcw1234]

One other thing I didn't see mentioned is the solenoids have to work harder when there is 900PSI of pressure behind it. This would require more current to open the solenoids. When you tested in the garage there was no pressure in the line. I don't know if you log battery voltage either, but with the load of fuel pumps and ignition amps under heavy load will drop the battery voltage regardless of using an alternator, since you said 16V I doubt you use an alternator.

[Tobias]

Attached are two screen shots from our data acq system.

The first is labeled OctoShop.pdf.
There are two channels visible, engine rpm and battery voltage. Voltage is in green. Notice how smooth is it and the two steps down are the two stages activating.

The other is labeled OctoRun.pdf.
There are four channels, engine rpm, battery voltage, N20Fuel 1 and 2. The time is on the x axis. Right at 0 seconds the transbrake is released and the engine accelerates. Immediately stage one nitrous is activated and the solenoids open. This is validated by N20Fuel1 psi dropping and also battery voltage dropping. Two tenths later the second stage is activated but the solenoids do not open. This is validated by the battery voltage not dropping further and the N20Fuel2 not dropping.

Look at right before one second, the battery voltage drops and N20Fuel2 drops. This is the second stage finally opening. It doesn't stay open though. This happens two more times going down the track.

The differences between the OctoRun and OctoShop are the transbrake wasn't activated, the engine rpm is lower, and the car is not moving.

No alternator, just a pretty large 16v battery..around 45lbs.

I am not a solenoid expert but I was told that solenoids draw the same current no matter how hard they have to 'work' The current draw is a function of coil resistance and the input voltage?

The MOSFETS Source is a copper busbar 0.040" thick and 0.500" wide.

Tonite the team is going to start the engine and activate my box in the same manner its activated on the track with the transbrake release. I am going to have them do that with the engine at low rpm and then at high rpm. Its going to be either a problem with the transbrake and/or electrical noise from higher engine rpm. I will let you know what I hear.

I am going to draw up a board tonite that opto-isolates all inputs and outputs and uses an isolated DC/DC converter to help with the electrical noise side. I will post the schematic tonite.
Thanks for the input.

[Tobias]

Here is the schematic using opto isolated input and outputs plus an isolated dc/dc converter. I used a 5v regulator in from of the converter because the 16v gets to around 20v with the charger.

[Tobias]

Isolator and DC to DC (MEV3S0505SC) spec sheets

[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 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