i like this and the minimal contact concept.

 

For anyone interested in buying relays complete with sockets, these are the ones i mentioned earlier in this thread.
They're general purpose 12 Volt automotive style relays which can be used for the electric fan over ride, fog lights, driving lights or any 12 Volt electrical device rated up to 40 Amps.

I even used one as an exact replacement for my 2000 heater blower motor relay.
They come 5 in a pack and have a pin-out/spec sheet included.

 

From a college that gave me a piece of paper to go with it, not from something I read on the web. I've been engineering for more than 3 decades, much of that time (including currently) with a major company that is known to everyone on this board.

There are good reasons for not back-feeding circuits like you are doing.


A lot of people tailgate all their lives, and never have an accident. Doesn't mean it's a good idea.

 

where is this circuit backfeeding
?.
i dont see it .
it looks isolated from oem relay coil completly.

oops, is it an oem relay? or pcm, solid state, backup!


found this. it is a relay.

C (199° F) or below. Then, the cycle fan operation
will resume. When the fan is scheduled to be on, the
powertrain control module (PCM) provides a ground
path for the fan relay. This ground is provided to the
cooling fan relay through pin C2 of PCM connector
C3. Battery voltage is then applied to the fan
through the relay. When the fan is scheduled to be
off, the PCM opens the ground path to the relay. This
will prevent the cooling fan from being energized

 

Mark,
First lets get this out of the way.
EE degree from a prestigious University.
30+ years in uProcessor, telemetry/industrial control and other circuit design.
Also wrote code for automated control systems, automated manufacturing testing and automated environmental/reliability testing using '4-corner' parametric variables.

There is no backfeeding, take a close look at the circuit again.
Once again the relay contacts are electrically isolated from their coils in a mechanical relay.

Even if the oem relay were a solid state relay, the (+) power terminal of the relay anode is fed by the B+ rail same as the (+) power terminal of the aux. relay.
However in the case of a solid state relay, relay specs. and design precautions would have to be taken because of the instantaneous surge current when the motor is turned on.
But more critical, is the high voltage spike due to the inductance of the motor windings when the motor turns off.
One of the reasons, mechanical relays are the first choice in the harsh environment in vehicles and more reliable than solid state relays.

If there were any kind of a relay contact back feed problems, then every single relay used in the vehicle with their 'contacts' fed by the B+ rail would be an issue... which it isn't.

This Summer will be 2 yrs. with my relay and delay circuit with no issues.
After shutting my engine off, I'll press my momentary push button switch installed on my lower dash to energize the electric fan for 4 minutes.
Every single time after shutting the engine off.
Thats a lot of manually turning the fan on and off.
Got rid of my heat soak problem once and for all.
I also turn the fan on when i see the temp creeping up stopped in traffic.

Your circuit as is will work for the intended purpose same as mine.
Lets just agree to disagree on the best one.

 

i agree but i doubt it. not here!! too much argueing with anger

 

That is correct. The PCM fan control provides a ground to energize the relatively low current relay coil.
The energized coil produces a magnetic field which pulls in the high current armature/contacts. The coil windings are 'electrically isolated' from the contacts.
-------------
Don't know how to make this and the fact that there cannot be no current flow from two or the same conductor(s) that are at the same EMF potential as in the case of the B+ rail....any clearer.

If the contacts were not electrically isolated from its coil there would be no purpose for a relay in the first place.
15+ amps of fan current would be going through a PCM ground!....not good.

15+ Amps of fan current among all the other device wattages would need to be dissipated by the PCM in addition to a very large PCB and components needed to conduct all that amperage inside the PCM.
The PCM would need humungus cooling fins with a cooling fan if no relays were used in vehicles and would be the size of a suit case.

Think of all the relays used in vehicles; fan, fuel, blower motor, horn, lights, etc. some of which have their high current contacts connected to B+ via a fuse of course.

Far as solid state relays where low current control isolation is needed from its high current switching output transistors, MosFETs, etc., an opto-isolator or similar is typically used to provide isolated power to turn on the solid state output stage which prevents among other things, the infamous 'back feeding' to the control circuitry.

One example where solid state opto-isolated relays are used is in land line telephones.
There is a government requirement that the high ring voltage generated from the Telco Central Office is to be electrically isolated from the customer's telephone system.
Once the telephone is taken off hook, the voice data is typically further isolated from the Central Office via an impedance matching transformer.

I doubt it too!