Help with automotive fan controller - Page 1
Forum section: electronics / projects

author: NegativeResistor
author_rank: Newbie
timestamp: August 15, 2020, 10:05:37 pm
content: Hi Everyone. Sorry this is going to be a bit long winded, but I've not designed any circuits in almost a decade and I've forgotten almost everything. I'd like some extra eyes to check over some diagrams to make sure I've not done something really idiotic or just missed the obvious. Apologies also in advance if the layout of my schematics is horrible.Anyway, I've got a car which due to its large engine in a small bay, is prone to issues with heat soak. It runs cool enough, but once switched off the tempreture of the cylinder head rises as there is no air flow, causing problems. To add insult, it already has a secondary electric fan just sitting there doing squat. If that fan were to run for a while after shutdown, that would help the issue quite a bit.There are off the shelf fan controllers for this but they aren't programmable and are stupidly expensive. I've got an Arduino nano clone sat around doing nothing, so I thought I'd put it to use and make it into a programmable fan controller which will turn the fan on based on both time and tempreture sensors. I've decided that it must be able to kill its own power supply through a relay to physically disconnect it so that it cannot become a battery drain or catch fire due to fault or something.I need it to do these things: On ignition switching on, Hold its relay closed to keep itself powered. Then wait for the ignition to switch off, Trigger the existing fan relay if needed, then kill power to itself once no longer needed. To power the arduino I'll hunt down a good quality USB converter. My diagrams show a 7805 as a placeholder for this.Since we need to inject battery voltage before the existing relay feed, and we need to make sure that the circuit comes on with the ignition, we need two high side switches. So I came up with this circuit, which uses a NPN bjt to pull down the gate of a P channel mosfet.I've tested it in Multisim (hence the switches and dummy wire) and it seems to work, but then I realised I could use a driver IC instead of rolling my own, and came up with this one. I can't simulate this and check it for problems; does it look correct? Which of the two is the better design? Safety is the most important issue here, I need it to be able to handle all the noise generated by the alternator and not burst into flames no matter what happens. I like using the discrete components because I can just use through hole, but if using the IC is way better then so be it.Thanks.

author: Benta
author_rank: Super Contributor
timestamp: August 15, 2020, 10:52:11 pm
content: Seems total way-over-the-top overkill to me.The normal way of solving this is a thermoswitch on the engine which controls the fan. The fan and switch are on a fused "constant +12 V" circuit.You'll often experience people parking their car somewhere, but the fan keeps running, totally standard procedure. You can add a timeout circuit if you want, of course. Not really necessary IMHO, but OK.KISS rules.

author: NegativeResistor
author_rank: Newbie
timestamp: August 16, 2020, 12:13:22 am
content: It already has that, but it doesn't appear to work, at least under the normal conditions I've experienced with it so far. I'm not sure if it is triggering at the correct tempreture.Sadly it also has some pretty bad design flaws on this car. For a start the thermoswitch is mounted to the coolest part of the engine! It's threaded into the coolant on the water pump body so theres no easy way for me to test it without draining the system. If I use thermocouples instead of a coolant switch, I can probe engine bay tempretures and place them based on wherever is hottest.There's also the business of it having some black box module that decides whether or not to send power over to the switch, and the whole thing can be tricked if you cycle the ignition (Perfectly normal, if you want to adjust the windows after stopping). I could bypass that, but if the switch sticks closed then dead battery.At a later date, I plan to replace the main engine driven fan with an electric one (due to parts supply issues and noise, among other reasons), and having a system like this already there will allow me to control the main fan more easily when I get around to it.On an ordinary car I wouldn't bother, but ordinary cars don't have V12s in them. Its very clear what it does with all that extra fuel it drinks. It's not the tempreture of the heat so much as the quantity of it. After driving it the entire front end of the car is radiating and you can feel it on your face when you get out the door.

author: RJSV
author_rank: Super Contributor
timestamp: August 16, 2020, 02:23:37 am
content: Here is my seriously presented answer, for you truly got special case need. OK, seeing dollar signs burning!What if you 'dedicate' a smaller BATT for fan only...Now, that stays on, suppose from 7 pm, or suppose you've placed a timer, switching off, after 6 hours, regardless. (Also see 'alarm' battery lead-acid)Still going to be reliability issues: What are u doing now, for the overheat, in the 'barn' after?