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I have designed and created two systems that are currently present in a small race-car.

The first one is a circuit to control the fans of the car. The topology of the system is the following: Battery -> Master switch ->Fuse box -> Relay -> 3-way switch When using the switch, the first mode is to let the ECU (DTA s80) control the fans and the other is to turn the fans on constantly (through the relay).

The second circuit is the electro-pneumatic system that controls the two double 3/2 pneumatic valves responsible for the gear changes. There are two parts in this system. First there is there is the pneumatic part. I am using two valves (one for upshift/downshift and one for the clutch). These valves are controlled by the second part of the system which is the electrical. They are connected via individual cables to a PCB that has as an input two buttons coming from paddles from the steering wheel.The principal of operation is that both pads to whom the cables are connected have always +12V and when an input is detected, the micro-controller (STM32F103) outputs a signal to an analog circuit that grounds one of two pads so that the valve can "see" 12V. Between the fuse box and the PCB there is an on-off switch.

Now there comes a problem. Both circuits work fine except one critical moment. When I have the switch for the PCB to the on position and the 3-way one is either on mode 1 or 2 and I turn the fans off, the upshift valve fires to the piston. Note: These two system are not crossing each other in no point (in fact there are on separate sides of the vehicle).

So far I have disconnected the steering wheel (still problematic) and I have external harness in order to see if there is a wire cut somewhere (still problematic) as well as disconnecting some or all the fans to not have any power draw from them (guess what, the problem is still there).

Does anyone have any idea why there is such a weird behavior ?

  • \$\begingroup\$ Welcome to EE.SE. A schematic is better than words. You can add one in using the CircuitLab button on the editor toolbar. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. Note that when you use the CircuitLab button on the editor toolbar an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. \$\endgroup\$ – Transistor May 6 '19 at 10:27
  • \$\begingroup\$ But they are connected to the same supply.. So is there a surge on disconnecting the supply to the fan? \$\endgroup\$ – Solar Mike May 6 '19 at 10:39
  • \$\begingroup\$ @SolarMike Do you have any idea why I keep having this problem ? Is it possible I have a bad ground somewhere else in another circuit of the car and that causes my problem ? \$\endgroup\$ – Christos Xygkos May 6 '19 at 12:44
  • \$\begingroup\$ On turning off the fans, you'll likely see a voltage spike from the relay coil and from the fan motors (since the current flowing through an inductor can't instantly stop). Some things to try: put a 10uF 50V capacitor across the relay coil (that is, wired in parallel with it) and another across each fan motor. That's a common method to absorb the inductive kick, in cases where you don't need rapid switching. You may also want to check that you have proper power supply filter capacitors on the circuit that's having the glitch. \$\endgroup\$ – Luke May 6 '19 at 16:20
  • \$\begingroup\$ Anomolous behavior in systems containing motors and MCUs can often be fixed by adding flyback diodes and EMI caps everywhere. \$\endgroup\$ – Drew May 6 '19 at 17:51

I actually found the source of the problem and the solution that worked perfectly fine for my case. The problem was inductive kickback. The solution was to replace my fan relay with a relay with an integrated diode.

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  • \$\begingroup\$ So you've solved the problem of inductive kick-back of the relay coil - great. Just bear in mind that you may get similar kick-back from the fans - with nowhere else for the current to go it might try to jump the gap of the relay contacts, leading to contact welding. Also, have a quick think about what happens when the fans start acting as generators! \$\endgroup\$ – srl100 Jun 14 '19 at 11:31

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