Simple Voltage Controlled Fan Switch

[fatdaddy]

.................Well simple it may be to some (hopefully you
but not to me

For background I can handle soldering etc but can't specify the components or have any idea what they actually do. I understand what the finished article does and it is what I am looking for but I don't want to order a kit from Aus.
I'm guessing that the circuit is simple enough to those who understand these things and hope that someone can give me a diagram and list of parts (preferably from Maplin for easy access )

http://autospeed.com/cms/title_Electronic-Radiator-Fan-Switch/A_2477/article.html

I want to use it to control an electric fan on my car.

thanks in advance

 

[BobK]

Unless you are equipped to make a PC board, you would probably do better to buy the kit. How much are they asking?

Bob

 

[fatdaddy]

it's not just the asking cost but the postage and time as well..................
just had a look to check and find it's not listed any more anyway!

 

[davenn]

as said for a beginner, making a PCB would be a big hurdle
failing a readily available PCB for the project ( speaking of which..... you should contact silicon chip they may have the PCB artwork that you could obtain and get some one to make it for you) you could assemble it on vero board. Would still be a major effort for a beginner, bt with a bit of help you would get there.

You are going to need a circuit diag of the unit with all the parts values. Silicon chip may be able to provide that themselves or you may find a copy of that issue at your local library. Without the circuit info no one is going to be able to help.

cheers
Dave

 

[davenn]

further searching......

Jaycar is still selling the kit AU$32.00
temp controller

dave

 

[Resqueline]

Here is a suggestion for a circuit diagram somewhat similar to that kit - if you can't find a suitable kit, and so wish to build it from scratch.
I have not calculated all of the resistor values (just approximate guesstimates) so some tweaking might be neccessary. The temp set pot is a multi-turn type.
The only suitable single op-amp from Maplin seems to be the CA3140, as it has a common-mode voltage range that includes the negative supply rail.
The LM358 is also suitable but it is a dual op-amp (so there would be one unused/spare section) and the pinout is different from the diagram.
The LM393 is a dual voltage comparator and could also be used, but it needs an extra pull-up resistor on its output pin.
The unused sections in the dual packages could nevertheless be used as an overtemperature alarm, in case the fan fails.
D1 & D2 is one of the 1N4000 series. You can use a BC635 instead of the BC337. Not much else of the component values/choises is critical. The LED is optional.
You can even do without the TL/TS 78L05 but then you'll have to find & tap into the 5V sensor supply in the car wiring.
I assume you have an electronic Engine Control Unit, and wish to use an existing temperature sensor?

 

[fatdaddy]

Hi thanks for that.
I also found this
http://www.isthatagnu.net/gallery/displayimage.php?album=16&pos=38

Does this do anything different to your circuit ?

 

[Resqueline]

That's a really bad design...

 

[fatdaddy]

well, I'm glad I showed it you then

Can you (keeping it VERY simple ! ) tell me what it does badly compared to your design.

Yes I have ECU and a standard temp sensor. I will check the cold and normal temp voltage from the sensor over the weekend.

 

[fatdaddy]

anyone know if Resqueline is away at the moment ?
I have a suitable fan now and would love to start this project soon as I can

 

[davenn]

anyone know if Resqueline is away at the moment ?
I have a suitable fan now and would love to start this project soon as I can

he will be back when he gets a chance we all lead other lives too, not glued to the puter 24/7

Have you got all the parts together that are shown in Res's circuit ?
you could build it up on a breadboard initially to try it out before going to the final soldered up version

cheers
Dave

 

[fatdaddy]

I run an automotive forum so I appreciate what you say, I wasn't trying to push Res

I know I will need all the help I can get with this and think a simple description of the difference in the two circuits might help me understand how they work..............

 

[Resqueline]

I'm sorry for being absent but I was unusually busy last week and didn't have the surplus energy for anything but kicking a few spammers out of here.

The KW0D circuit has a basic resemblance to my circuit but it lacks several design points. While it may work (with a 6V relay) it has the following issues:
1: It has a pot for both op-amp inputs. You only need one to have full adjustability, two will only allow you to adjust the circuit onto a non-functional state.
2: It has no input filtering. There's bound to be noise in the wiring of a car and this can upset the circuit (see point 3 & 7).
3: It has no positive feedback (hysteresis) so the output will amplify any noise and can thus assume an intermediate non-digital (linear) state. Erratic on/off.
4: It uses a 741 which is meant to work with a dual polarity power supply. Thus its inputs (& output) are not specified to approach the power supply rails.
5: The 741 is supplied with only 6.8V - which has consequences further down the line (see point 7). It uses no decoupling capacitors.
6: It uses a big 2N3055 to drive the relay. Driven right that transistor is almost capable of driving the fan all by itself and is so a complete overkill & waste of space.
7: It uses the (low-gain) 2N3055 in an emitter-follower configuration. This allows for a low (& even variable) relay drive voltage (less than 5V). Wears out relay.
8: The two diodes are not needed for protection when driving a relay in an emitter-follower fashion (and only one is needed in a common-emitter config anyway).

My circuit has the following design points:
1: Input filtering (47k * 330nF = 15.5ms = 10Hz) removing any noise.
2: A 5V reference voltage, equal to the 5V used by the ECU for all its sensors. The regulator is decoupled to ensure stability.
3: A single setpoint pot for full, quick & simple temperature adjustability.
4: Uses op-amps (or comparators - with a small mod) made for single supplies, thus guaranteed to work with inputs around 0V.
5: Runs the op-amp at almost full battery voltage, allowing the relay transistor to be replaced by a MOSFET, making it possible even to dispense with the relay.
6: Positive feedback (hysteresis) ensures a fast & clean switching action. A pot allows the temperature difference to be set for suitable/wanted on/off time intervals.
7: The relay driver transistor is small but has high gain and is sufficient to drive a relay.
8: It has adequate & applicable circuit protection.

I hope this clears it up somewhat, but please ask if there's anything more you wonder about.

 

[fatdaddy]

thanks for that, i follow most of it !

(and there is no need to apologise )

 

[fatdaddy]

I have a dual temp sensor (one for ECU, one for gauge). I have monitored the gauge wire.
I monitored the temp from an OBD device.
Having been parked overnight (ambient 9c) voltage was 3.89
normal running temp (99-106c tho the gauge shows no visual change) voltage 0.59
113c voltage 0.49
117c voltage 0.46
119c voltage 0.45
120c voltage 0.44
125c voltage 0.36

125c triggers a Temperature warning.
119c is where I would want a fan to cut in.

 

[fatdaddy]

I'm going to mount the fan and wire it to a straight switch for a start.

I have a motorway journey coming up and don't want to risk traffic jams without a fan.

Am I going to have any problem with the values at the switching end of the temp range being of such small increments ?
(I note one of your points being accurate control at around 0v !! )

Having done the temp checks I realise that my car (BMW) runs at higher normal temp than most so probably the standard Jaycar circuit wouldn't work well anyway.

 

[Resqueline]

Those are quite high temp's, and consequently quite low sensor voltages. It would be advisable to ground the switching circuit on the engine to ensure correct sensing.
As far as I could deduce from the pic's of the 2475 kit it's a proper & sound design. I have no doubts it would handle the low BMW voltages w/o problems.
If the Jaycar temperature switch goes up that high I don't know. Do you really want to have the fan cut in that high anyway? It's very close to overheating.
I would check the actual temperatures with a thermometer. Sensors (as well as thermostats) can age/wear and give false readings.
It would also be interesting to see if the OBD sensor gives different voltage readings than the gauge sensor.

 

[fatdaddy]

The BMW V8s do run at higher temps than most cars.
Looking at the gauge the needle runs at 12o'clock at normal temp. Looking at the OBD readout the temp varies from 99 to 106 without the needle appearing to move. If i let the car sit running on a high tickover the temp rises slowly but the needle doesnt appear to move until 113. 119 is where (looking at the gauge ) I would think "I dont want it higher than that". The warning message appears on the screen at 25 OBD readout when the gauge needle is almost in the red.
The model is supposed to run at 100 -106 under normal conditions so I think my readings are correct.

I did read the voltage on the ECU sensor wire as well. At normal temps they are are around 0.05 lower.

I want a manual override anyway so will go ahead with that and fit the switch circuit when I have it ready.

I like making things from scratch BUT with my very limited experience of electronics do you think I should go for a Jaycar kit rather than attempt to use your design ?

 

[Resqueline]

Surprisingly high operating temp's but ok if the manuals says so. The 0.05V difference might indicate a 7-8 degree difference which is also ok considering tolerances.
Temperature control and measurement is actually a very difficult discipline when tight accuracy is required.
In this project I'd consider the build quality as important as the design aspect due to the wanted reliability.
You don't want a broken track catch you by surprise and make the engine overheat before you have the time to react in a "heated" situation (pun intended).
Only you can be the judge if you have the skills to make a reliable contruction.
I've gotten lazy with the years and would go for a good quality kit like Jaycar's seems to be, though I know I could make it just as good and even more compact myself.
There's one thing with that kit though; the PCB tracks from the relay are not likely wide enough to stand up to the 10A the relay is rated for.
So you'll most likely end up needing an external relay anyway, adding unneccessarily to the cost. I'd make a relay-less PCB and use an external 30A car relay.
How much current does the fan(s) draw?

 

[fatdaddy]

I was planning on using an external relay anyway just for the "belt and braces" factor.
Sticker on the fan states 12v and 120W............... so thats 10amps ?
Bearing in mind what you said about build quality I dont know. If I get the Jaycar kit it still depends on my build quality but the advantage I see is that I have the board pre-made.
I know that I could handle the practical side of your circuit OK with guidance and fancy the challenge BUT I guess i should go with the kit for now.

What i would like is to do that and maybe get some more data then see if you have time to come up with any improvement so I can have a go at "my own"