I'm holding sales of my electronic fused fan fail unit. Using it with OEM fans that draw high start current can cause the "fuse" to trip. I have been prototyping new circuitry for a few months and now have a prototype unit I'm happy with. But one of the new IC's I'm using has not been released in production quantities. It is expected in 26 weeks. I'm using an alternate part in my prototype but it has less accuracy than the one not available yet.

I have designed a new current sensor used to monitor each fan current. I was using a hall effect IC rated for 20 amps continuous and 100 amps for 100 ms. But I have had a few of those components fail with customer units. My new design uses a one watt 0.001 ohm resistor and an op-amp to give me current reading up to 50 amps and should be able to run continuous at 100 amps without damage.

So I needed to be able to read more current and the software now does the following. I tested a 20 amp fuse in my car with the fan connector shorted and peak current was 120 amps for 41 ms then the fuse popped. A 30 amp fuse held for 134 ms before it popped.

1) When a fan is turned on no trip current is checked for 40 ms.
2) Then the fan trip current is set to 45 amps for 200 ms.
3) Then the fan trip current is set to 40 amps for the next 400 ms.
4) Then the fan trip current is set to 30 amps for the next 800 ms.
5) Then the fan trip current is set to the final current of 20 amps.

I chose those currents which should keep OEM fans (even older high current fans) from tripping the fuse. The whole point of fusing the fans is to prevent damage or fire to the wiring in the harness and I feel those short overload currents of the fan starting current will not overheat any wiring.

Another added benefit to my new circuit is less current draw by the new current sensing which helps the unit running cooler.

I also monitor the input voltage (power from the fan relay) and will shut off the fans if that voltage drops below 8.0 volts. If the voltage rises above 9.0 volts it will start up the fans with the same current trip setting as a normal power up. When the voltage has dropped to 8.0 volts the fan fail light will stay on continuous however the voltage driving the fan fail light will be that 8 volts or less..

I still keep the flashing fan fail warning if either fan draws less than 4 amps or greater than 18 amps. The second fan turn on delay is now 2.44 seconds.