KISS,
Actually yes, I would like variable speed. Just need to fathom the basics "on/off" first. I'm sure it seems uneccesary but I'm a slow yet thorough learner, that way.
Ideally a vairable speed range from the lo-speed fan motor winding would be desireable. This fan motor has two basically identical windings each drawing 15 Amps at 12 Volts. Hi-speed is achieved by supplying power to both windings simultaneously. Splice the two positve leads (one from each winding) together and conect them to 12 Volts then ground the negative lead and you have hi-speed. Lo-speed is by connecting either (but only) one of the windings by itself, to 12 Volts.
MrAl,
All good points and well taken. Here is some more background that may give a better understanding of what I am looking for regarding an electric fan controller for this Triumph TR6 automobile.
I have done testing with a mechanical dial type "coolant" thermometer made specifically for this task. I removed the radiator cap and inserted the thermometer through the opening directly and down into the coolant that is in the header space above the tops of the tubes. It's a downflow type radiator. There is clearly a steady flow of coolant past the thermometer when the engine is fully warmed up and idling. The coolant flow turns into a rush when I rev the engine, as expected. The thermostat is rated at 180 degrees F and the thermometer verifies the operating temperature of the engine to be 180 degrees F regardless how much AC powered fan I put in front of the radiator. There is a slight fluctuation of only a degree or two +-.
The design of the relatively very large grill opening, duct work directing the airflow into the radiator and the heat rejection capability of the radiator has me convinced there is no need for the fan to turn on at any roadspeeds above approximately 5 - 10 MPH. The coolant gauge drops rapidly as soon as I get the car moving when it happens that the engine gets hotter than "Normal" while idling at a standstill. It drops somewhat if I increase the RPMs at a standstill but only for a while. Without getting into too much detail it has to do with the radiator capacity and volume of coolant in the engine. It also has to do in large part with the airflow through the radiator.
My target operating temperature for the fan controller would be adjustable (with a knob accesible from under the hood) from a low of 160 degrees F and a high of 200 degrees F. Reason being, there are times when it's beneficial to use a thermostat that is rated at a different temperature than 180F. There are various thermostats available that fall within this range. With the 180F thermostat I would like to achieve a target fan control temperature of 185-187 degrees F, if possible. I'd rather the fan stays off entirely until the coolant reaches at least several degrees above the 180F operating temperature.
I realize it might be impossible to achieve such a narrow band of control. Working for me in this regard is the diameter of the fan blade which covers nearly 85% of the radiator fins. It is a modern "reverse S blade" design which I believe should be more than adeqaute to deliver enough CFM to do the job. Also the motor is comparatively a MONSTER as far as electric radiator fan motars are concerned. This fan is the largest I could find that will physically fit in the avaiable space and has the most powerful motor avaiable.
Working against me is my weak knowledge of electronic circuit design. If I found a controller already built to do what I am looking for, I would have bought it several months ago. After extensive research, I do not belive such a controller exists. I would have had to buy one and some of them are hundreds of $$$ and then modify it or break into the probably glued together case to figure out how/if I could make it do what I want. The way I see it, if I have to design or modify a current design then, I may as well just start from scratch. The deeper I looked into how to design the circuit, the more I realized I was in very deep and obviously getting in over my head, which is what brought me to this forum.
I got started on the PWM route when reading into stock fan controllers that are already used on various, more modern cars. In particular I owned a Dodge Neon that used a PWN driven fan controller. I didn't realize it at the time but there is was, a rugged solid state relay that was PWM controlled via the ECM. I had a problem with the radiator fans not turning on and of course the engine would overheat. I found the problem was a broken wire in the fan control circuit, deep within the wiring harness. The journey I took to fixing this had me on a Dodge forum asking about where the fan relay was. Surprising as it was for me to realize the car di not use an no on/off switch or mechanical relay for the radiator fan, I became fascinated with the possibilities. When This TR6 electric fan project came along, I could not ignore the modern fan control circuits that are already in use, as a good starting point.
There are many aftermarket fan controllers available with all kinds of features. From the most basic non-adjustable mechnical "on at 195-off at 180" with a capillary tube connecting to a sensing probe you stick into the fins of the radiator. The next step which is basically the same but adjustable albeit keeping the 15 degree range. This is what has been in the TR6 for the past 10 years and it works but is barely adequate. The electric fan is noisy and runs almost all the time when the car is at a standstill. Mostly because of the low turn-off temperature. It's time for a better fan setup. They get better and better all the way up to some very sophisticated, programmable controllers. One thing every fan controller I've found lacks though, is the inclusion of of every design element listed here :
1) the ability to adjust the target temperature
2) the ability to have a tight range of control that has the fan turn OFF very quickly (or immediately) as soon as the coolant temperature drops back down to the target temperature
3) soft starting
4) variable speed
5) dual motor winding control
6) sensing the voltage by sharing the same temperature sending unit that the temperature gauge does, without effecting the gauge reading
5) remote mountable on or close to the fan motor
I'm going to be looking at window comparators and related circuits to see if I can get a better understanding why they are such a popular choice. In my mind I had decided that two controllers are needed.,One for each motor winding. Besides the small signal level buffer transistor at the temperature sending unit, it would be only two more transistors. One for each winding. The lo-speed controller would be PWM. Once the coolant gets hot enough that the PWM controlled transistor is at 100% duty cycle, then the other transistor would come into play. It would control a relay that is wired to both windings. Maybe it would not be too difficult to use PWM on both transistors and have variable speed all the way up to full 30 amps of draw, maximum motor speed. That's beyond me at present but I'm trying to learn.
I'm not sure why but I have the basic feeling that using transistors instead of ics, would end up being a more "tolerant" circuit as well as more rugged in the high vibration and generally abusive environment of an automobile engine compartment.
One overriding factor to keep in mind is that the alternator is capable of only a measly 36 Amps ! There will probably be times when battery reserve will be called on for brief moments especially if a relay is used. Before anyone wants to tell me to put in a higher capacity alternator, let me add that it's going to stay small/as-is, in order to NOT add parasitic losses to the power output of the drivetrain. This particular TR6 is highly modified already and the engine SCREAMS ! I defintley do not want to detract from that aspect of the car. Not even in any slight amount, if possible and still get good electric fan control.