Hi Everyone,
I'm back already... I have another circuit that I'm working on for an engine swap. This one's pretty long... Sorry about that.
The Storey:
Most cars have a safety feature which shuts the fuel pump off in case of a crash or engine stall. On the original engine this was a physical switch located in the Vane Air Flow Meter. (AFM) As long as the engine is sucking in air, the AFM stays open and the switch stays closed. The moment the engine stalls, the AFM closes and the switch opens. The switch in the AFM supplies ground to the coil in the "Circuit Open Relay", (COR) which runs the fuel pump as long as it sees ground.
The vehicle the new engine came from used a Fuel Pump Control Unit to control the fuel pump. I don't have the FPU and it's expensive to buy... besides, I don't want to rewire the whole fuel pump circuit when I think it will be fairly simple to adapt the existing wiring.
Others have solved this by simply hardwiring the COR, thereby bypassing the safety feature... this is not an acceptable (nor intelligent) solution; if I get in a crash, I definitely want the fuel pump to shut off.
There is a 5v PWM signal that the Engine Control Module (ECU) sends to the Igniter Module to tell it when to fire the Ignition coil based on camshaft and crankshaft sensor input. If the engine stops turning, the ignition signal also stops. (It might not technically be PWM, but that's what I read. The signal remains high until it's time to fire the coil, at which point it momentarily pulls low before returning to a high state.)
The Circuit:
The circuit diagram is attached I'm sorry it's unconventional. I like to draw the components out to help me assemble the circuit correctly, and I don't know enough nor have the appropriate tools to produce conventional circuit diagrams. I'm (clearly) doing this all by hand... MS Paint, to be exact... yes, I am the last person alive who still uses MS paint.
To monitor the the ignition signal and provide voltage to the MOSFET gate, I have selected the ADM805LAN IC with watchdog timer. (datasheet attached) I did the math and the maximum pulse duration, based on idle RPM of the engine and the number if cylinders, is easily short enough that the timer will not timeout between pulses.
To supply power to the IC, I'm using an L78S05CV 12v 5v 2A common ground positive voltage regulator (datasheet attached) with a 0.33uf capacitor on the 12v side and a 0.1uf Capacitor on the 5v side. I've selected a much higher current regulator than necessary in hopes that I can use it reliably without a heatsink.
To provide ground to the relay, I'll use an IRF610 MOSFET N-CH 200V 9A. A 10k resistor will be used to pull the gate low when there is no voltage there. Again, with higher current handling than necessary for use without a heatsink. (An automotive relay coil apparently uses less than 250 milliamps.)
For flyback, I have selected a Schottky and Zenner combination of diodes. The schottky for low latency, and the Zenner to limit the current passed to avoid damage to the relay contacts upon field collapse. (so I've read...)
The Questions:
Well, most importantly, does it work? I came up with this by combining 3 separate circuit diagram fragments and again, I really don't know what I'm doing, so there's a strong possibility I'm missing something.
On the last circuit I designed, it was explained to me that a resistor should be placed on the signal input for protection; is that the case here? If necessary, what value resistor should I use?
Do I need a resistor to pull the the drain high when the MOSFET is off, or is that not a thing? Or perhaps, does the connection to power through the relay coil do that already?
The MOSFET has a diode built into it, but I don't see a path to complete the circuit thought the MOSFET, so I think I need the flyback diode; have I got that right?
I have added the Zenner diode to limit the flyback current in an effort to protect the relay contacts; has anyone heard of this and does it make sense? I don't understand this, as I dont think the relay contacts would not be exposed to that current, would they? Is this because the coil and one of the poles on the relay share a common connection and could therefore provide a path to complete the circuit through the relay switch?
I'm back already... I have another circuit that I'm working on for an engine swap. This one's pretty long... Sorry about that.
The Storey:
Most cars have a safety feature which shuts the fuel pump off in case of a crash or engine stall. On the original engine this was a physical switch located in the Vane Air Flow Meter. (AFM) As long as the engine is sucking in air, the AFM stays open and the switch stays closed. The moment the engine stalls, the AFM closes and the switch opens. The switch in the AFM supplies ground to the coil in the "Circuit Open Relay", (COR) which runs the fuel pump as long as it sees ground.
The vehicle the new engine came from used a Fuel Pump Control Unit to control the fuel pump. I don't have the FPU and it's expensive to buy... besides, I don't want to rewire the whole fuel pump circuit when I think it will be fairly simple to adapt the existing wiring.
Others have solved this by simply hardwiring the COR, thereby bypassing the safety feature... this is not an acceptable (nor intelligent) solution; if I get in a crash, I definitely want the fuel pump to shut off.
There is a 5v PWM signal that the Engine Control Module (ECU) sends to the Igniter Module to tell it when to fire the Ignition coil based on camshaft and crankshaft sensor input. If the engine stops turning, the ignition signal also stops. (It might not technically be PWM, but that's what I read. The signal remains high until it's time to fire the coil, at which point it momentarily pulls low before returning to a high state.)
The Circuit:
The circuit diagram is attached I'm sorry it's unconventional. I like to draw the components out to help me assemble the circuit correctly, and I don't know enough nor have the appropriate tools to produce conventional circuit diagrams. I'm (clearly) doing this all by hand... MS Paint, to be exact... yes, I am the last person alive who still uses MS paint.
To monitor the the ignition signal and provide voltage to the MOSFET gate, I have selected the ADM805LAN IC with watchdog timer. (datasheet attached) I did the math and the maximum pulse duration, based on idle RPM of the engine and the number if cylinders, is easily short enough that the timer will not timeout between pulses.
To supply power to the IC, I'm using an L78S05CV 12v 5v 2A common ground positive voltage regulator (datasheet attached) with a 0.33uf capacitor on the 12v side and a 0.1uf Capacitor on the 5v side. I've selected a much higher current regulator than necessary in hopes that I can use it reliably without a heatsink.
To provide ground to the relay, I'll use an IRF610 MOSFET N-CH 200V 9A. A 10k resistor will be used to pull the gate low when there is no voltage there. Again, with higher current handling than necessary for use without a heatsink. (An automotive relay coil apparently uses less than 250 milliamps.)
For flyback, I have selected a Schottky and Zenner combination of diodes. The schottky for low latency, and the Zenner to limit the current passed to avoid damage to the relay contacts upon field collapse. (so I've read...)
The Questions:
Well, most importantly, does it work? I came up with this by combining 3 separate circuit diagram fragments and again, I really don't know what I'm doing, so there's a strong possibility I'm missing something.
On the last circuit I designed, it was explained to me that a resistor should be placed on the signal input for protection; is that the case here? If necessary, what value resistor should I use?
Do I need a resistor to pull the the drain high when the MOSFET is off, or is that not a thing? Or perhaps, does the connection to power through the relay coil do that already?
The MOSFET has a diode built into it, but I don't see a path to complete the circuit thought the MOSFET, so I think I need the flyback diode; have I got that right?
I have added the Zenner diode to limit the flyback current in an effort to protect the relay contacts; has anyone heard of this and does it make sense? I don't understand this, as I dont think the relay contacts would not be exposed to that current, would they? Is this because the coil and one of the poles on the relay share a common connection and could therefore provide a path to complete the circuit through the relay switch?
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