Beginner question - would this circuit work? - Page 1
Forum section: electronics / beginners

author: archbish
author_rank: Newbie
timestamp: July 02, 2020, 10:43:43 am
content: Hi,I'm just installing a standalone ECU into my, but have hit a problem with the air conditioning circuit. The old ECU used to control the A/C but the new one doesn't.So I'm planning to make a simple circuit to which takes the A/C switch from the dashboard and earths the A/C relay when the A/C switch is closed. This would sit inside the ecu case and use the existing A/C wiring.I've attached a diagram showing how I'm planning to do this - Would the transistor arrangement work or am I over complicating things by using 2 transistors?I have a rough idea how transistors work, but I'm new to all this - any feedback would be appreciated.

author: garethw
author_rank: Regular Contributor
timestamp: July 02, 2020, 12:27:39 pm
content: That circuit, in theory, would work but it would need to freewheeling diodes to protect against the inductive spike from the relay.My main concern is that most A/C systems in cars use compressor cycling to regulate the system. The compressor clutch is engaged and disengaged to keep the system pressures and temperatures within limits. If you simply engage the compressor and leave it on the whole system could, err, fail!

author: orolo
author_rank: Frequent Contributor
timestamp: July 02, 2020, 02:08:30 pm
content: If your lower transistor is PNP, I don't think your circuit will work. The upper transistor acts as a switchable current source, and the base of the lower PNP is also a current source. You should use a NPN as the lower transistor, so the current sourced by the collector of the upper PNP will be sunk into the NPN's base, turning it on.I think it's also good practice to shunt the base of the lower transistor with a resistor to ground, to sink any leakage current without turning it on. It also helps turning it off.

author: archbish
author_rank: Newbie
timestamp: July 03, 2020, 08:53:44 am
content: Thank you for you answers, that's really helpful feedback.I've rechecked the car's wiring diagrams and it looks like the A/C pressure switch is wired in series before the dashboard switch (I still need to make sure the A/C condenser fan gets activated though) .I just have one more question; do I still need a flyback diode, if I'm only grounding the negative side of the relay in the circuit? (the positive feed to the relay comes from the fuse box and battery).Thanks again

author: garethw
author_rank: Regular Contributor
timestamp: July 03, 2020, 04:33:09 pm
content: There will be freewheeling diodes somewhere in the system, whether that's in the engine management ecu or possibly built into the relay itself.If you are making and breaking a circuit with a solenoid you basically have a large inductor storing energy in a magnetic field. Inductors try to keep the current flowing through them constant and will try (and often succeed) to keep that current flowing by increasing the voltage. Although they quickly run out of available energy, this voltage spike can still reach hundreds of volts and will destroy your switch or transistors quickly. This very principle is how the ignition coil turns 12V into 10,000+VIf you are unsure, always add a diode. This way, your voltage spike will not go above one diode drop (0.6V, or 0.2 if using a Schottky diode)