Delay-Off timer for car fan

[Samuel43]

Hi,
My brother has tasked me with building him a 5 minute delay-off timer that controls a certain cooling fan in his vehicle that he recently purchased.  The previous owner already wired the fan to a switch just under the dash, so I just need to replace the switch with a relay and control circuit.  I came up with a basic design and would like some feedback.  The circuit starts up when the ignition switch is in the 2 position (IG2) and charges up C1, and keeps it charged until the car is shutoff.  At this point, the capacitor discharges through R1 until it goes below VGSth of the FET, opening the FET and relay, turning off the fan.  D1 keeps the capacitor from feeding back into the ignition switch once the car is turned off and D2 is a back EMF suppressor for the relay coil.  The ZVN2110A was used because I had it already from a university electronics kit, it's smallish, and I knew it wasn't going to be used in another future project because of it's relatively high RDSon and would probably be fine for driving a relay coil.

I also wanted to build something that won't draw any current from the battery when the car is off which is why I went with this basic-ish RC circuit instead of a microcontroller or 555 based circuit.

Would a basic electrolytic capacitor be okay or should I look into an aluminum polymer cap or something else?  This car is being driven in Canada where temps can reach -40C or lower but can also get quite warm in the summer.  This circuit would be tucked in the dash somewhere, not in the engine bay.

The relay I have on hand is rated for 40A but only 14Vdc.  Is that acceptable for automotive applications?

Thanks

[SteveThackery]

Could you show us the schematic?

[Samuel43]

Sorry. Forgot to attach picture, should be added to original post now

[inse]

The circuit should kind of work, but wouldn’t it be better to mount a thermostat switch to the component to be cooled?
Idk whether the fan needs to run in winter as long as in summer.

[Kim Christensen]

Circuit is simple and will do the job. If the capacitor is a bit leaky, then the run time would be shortened a bit. Time will also vary with temperature, but then does it really matter if the circuit runs for 4 or 6 minutes rather than 5? Electrolytics also have wide tolerance ranges so you'd need to trim the R1 value by experimentation.
You could make a microcontroller version that has virtually zero standby current. Either via a sleep function, or where the MCU cuts it's own power source when finished.

[DavidAlfa]

If it's the radiator fan, it won't have any effect in the engine as the pump won't be running.

R1 can get way higher, like 1M, so a much smaller capacitor will be required, around 220-330uF assuming the mosfet cutting out at 3V.
Just buy a 50A automotive relay, they're cheap and meant for this.

Think something else so it doesn't always trigger, imagine the fan starting everytime you simply put the contact for a second, not only it'll be annoying, it'll deplete the battery quickly.
I wouldn't trigger the fan unless the engine has been running for at least 20 minutes!

A MCU won't necessarily deplete the battery. It only needs to turn on with the switch contact, coun time (Or even better, read some emperature sensor and decide wether to run the fan or not).
In any case, it only needs to run for those 5 minutes. After that, it can release a transistor so the whole thing becomes unpowered.

[Benta]

Fixing the car's cooling system is a better idea.

[Samuel43]

Thanks for all your suggestions.  Yeah I didn't think of the thermostat idea, and that would probably actually be better than a timer.  I told my brother this and he did say that that's what the shop that did the safety inspection on his car suggested that he do, though they had no issue with the switch, so tentatively he may just look into finding an automotive thermostat for what he wants to do.

In response to Benta.  From what he explained, there isn't technically anything wrong with the cooling system as it is.  I suspect that the fan is for an intercooler or turbo (though not exactly sure, I'm not very knowledgeable about cars) and that it is more optimal to run the fan for a few minutes after turning the car off than just having the fan shut off with car.

I guess I should say that the car is a 1994 Audi S4

[Randy222]

Hi,
My brother has tasked me with building him a 5 minute delay-off timer that controls a certain cooling fan in his vehicle that he recently purchased. 
Thanks

Your original idea with 4 small pasive parts, diode cap resistor and a fet, is all that is needed. You'll just need to experiment with the cap and bleed-down resistor sizes to get your 5min run time, the time it takes for the relay coil to unlatch.

The only issue with using relay is when the contacts are held loose as the coil mag field dies down, this usually causes the contact to go south faster. Instead of mechanical relay just use an opto SSR or just a hefty FET to control the fan load. Just know as the gate voltage drops down the FET will be running in the linear region where FET's get hot.

More sophisticated setups are plentiful.

[Gyro]

Depending on how 'keen' your brother is. He might want to investigate fitting an auxiliary electric oil pump (although much more expensive). These are becoming more common, particularly in big hybrids, but they have been around for years as retrofit items for high performance engines. I think they are also used in very cold climates to bring up oil pressure in the engine bearings before starting.

The problem isn't heat in the turbo casing / impellers themselves, but coking of the stationary oil in the spindle bearings, it's the circulating engine oil that keeps them cool. Keeping the oil circulating for a few minutes prevents this, in addition to allowing the bearing to be lubricated before it spins up.

Just a thought.

[CaptDon]

I have a thermostatic switch screwed right into the water jacket of the intake manifold near the normal 180 degree engine thermostat. It's a 350 chevy engine. The fan automatically 'runs on' after engine shutdown until the water jacket is below 180 degrees. I also have an override switch to force the fan to run. One of the big advantages of fan 'run on' is the reduction of latent under hood heat after engine shutdown when there would normally be no air movement. Latent under hood heat is responsible for a lot of garage fires and parking lot car fires. Hoses, belts, plug wires, everything benefits from the heat being pushed away. My fan is a puller on the back side of the radiator. Some guys prefer the pusher style mounted to the front of the radiator. Our locomotive systems run a 1 minute prelube sequence but no pumps run on after engine shutdown. That can be a bad thing if an engine is suddenly shut down after a long 'Run 8' power setting. Probably can be proven to shorten turbo life but for the most part these loco turbos are very robust and stand up to foolish abuse. (The old ones made in Chekoslovakia tended to blow up in a spectacular fashion!!!)
 

[Randy222]

Depending on how 'keen' your brother is. He might want to investigate fitting an auxiliary electric oil pump (although much more expensive). These are becoming more common, particularly in big hybrids, but they have been around for years as retrofit items for high performance engines. I think they are also used in very cold climates to bring up oil pressure in the engine bearings before starting.

The problem isn't heat in the turbo casing / impellers themselves, but coking of the stationary oil in the spindle bearings, it's the circulating engine oil that keeps them cool. Keeping the oil circulating for a few minutes prevents this, in addition to allowing the bearing to be lubricated before it spins up.

Just a thought.

Another method that's better for "circulating" is to swap the turbo with one that has water jacket around the bearing section, you then do as you say but you do it with antifreeze and a small radiator with fan.
But noted, quality synthetic motor oil should have way less coking, if any at all.

As for the timer, since the vehicle is off there's no lectric being generated, so one key goal is to minimize batt use. Passive components seem ideal vs MC.

[Sacodepatatas]

Fixing the car's cooling system is a better idea.

In my case, it implied replacing the BSI of my 23yo car with an used unit that might be prone to fail soon. Instead of that, when mi engine failed 10 years back because my ex missed checking the high temperatures while driving in hot summer, i put a simple thermostat made  of two comparators (because my Xsara uses two fan speeds), and since then, my engine hasn't gone over 95°C at most. Of course, the OBD II still complains about failing of turning on the cooler relays, but i don't mind either.

[Sacodepatatas]

Duplicate post

[MathWizard]

I'm picturing older engines, with a fan driven off the engine, but how many these days have an electric fan on the radiator ? Or do a lot of cars have both ? (maybe I just never noticed)

Yeah if I ever owned a sports/muscle/etc car, I'd want some fan to run after turnoff as well.

[CaptDon]

Seems that most cars and small trucks under 200Hp use electric fans. Some have two fans that are staged with engine temperature and I noticed in many cars the second fan is in a 'full run' mode when the air conditioning is in the cooling mode. The huge engine driven fans have constant drag but many have gone to fan clutches. Some of the clutches are electric and some seem to have a fluid clutch. The high horsepower class of engines (O.T.R. trucks, etc.) seem to have large engine driven fans with electric clutches and some have over ride dash mounted switches to force the fan to run. Also on the large O.T.R. trucks there is a circuit tied in the the Jacobs Brake selector switch that will turn on the air brake compressor and the engine fan when the 'jakes' are activated to achieve additional braking action. I always wondered why 4 cycle locomotive engines never tried Jake Brakes in conjunction with dynamic brake resistors? Detroit diesel gave a 500,000 mile warranty without Jakes but only 250,000 miles with a Jake equipped engine.