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Miniature thermostat switch in car environment? 6

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knowlittle

knowlittle

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Jul 26, 2007
192
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I would like to use a bimetal switch for cooling fan control. It comes with different open/close temperatures at 5 degree C interval. The tip is about 1/4" in thickness. I am concerned about the mechanical stability of the bimetal contact in the automotive environment. Please advise. Thank you. I will use it with a relay/diode.
 
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Is it a snap action switch? If so then that will help prevent mechanical bouncing.
Otherwise you will need a lot of delay in the electronics to prevent rapid cycling from mechanical effects.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube
 
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I don't know if it is snap action or not. Since it is not the disc type and small, it may not be. I will look into disc type. Thank you.
 
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Amazon description says it's snap action, but I'd look up a mfgr. data sheet to be sure.

I don't know of any snap-action bimetal switches that don't wear out (the temperature setpoint starts to drift fairly significantly) within a few thousand cycles or so, it is the drawback to their simplicity.
 
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From a data sheet, :

Active temperature :90+/-5C, Reset temperature:70+/-10C

Structure and application: KSD9700 series product is a kind of thermostat which adopt the bimetal disc as the temperature sensing element. The bimetal disc is in free state and the contacts are closed when the electrical appliance is working in normal condition. When the ambient temperature raises to the preset operating temperature, the contacts open as the bimetal disc deforms to jump when being heated, then the circuit is cut off to control the temperature .Then the contacts will close automatically to cut on the circuit when the electrical appliance is cooled down to the reset temperature. This product is widely used in home appliance’s motor and electrical equipment, such as air conditioner motor, transformer, thermal appliances, etc.

It looks like an over-temperature protection switch not a temperature regulation switch.
 
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Isn't this backwards from what you would want for turning ON a fan when the temperature is too warm? The description states "the contacts open as the bimetal disc deforms to jump when being heated, then the circuit is cut off to control the temperature ."

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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Thank you for the notes. The description is for Normally Closed, but Normally Open is also available, which is what I am going to use. I didn't know these switches drift away from set point with age. I thought about Arduino logic board + Arduino relay board. But electronic control seems to have the disadvantage of resetting upon battery disconnect and constant current draw during engine off.
 
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"But electronic control seems to have the disadvantage of resetting upon battery disconnect and constant current draw during engine off."

Neither needs to be true. A normal Arduino processor would have some amount of non-volatile store, such as the memory required to store the actual program, which retains its information when power is off. Otherwise, you'd need to connect a PC to an Arduino every time you turned the power off. Since the Arduino has non-volatile memory, there is no need to maintain power when the engine is off.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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I didn't know - as always - Arduino does not lose program on power-off. I am now interested in Arduino-based radiator fan control. Please critique my plan:

Parts list: Arduino board, relay board (to turn on fan relays), LM35 temperature sensor, automotive fan relays

I will tape LM35 temperature sensors to radiator outlet and AC condenser outlet. If either exceeds preset temperature, turn on low fan. If radiator outlet exceeds a higher preset, turn on high fan. Also a question. Can Arduino board be powered directly from battery? Or via USB?

LionelH,
I need to use AC condenser outlet temperature for fan control. That's why I cannot use a commercial controller.



 
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High temperatures will cause the LM35 to age pretty quickly, though as long as you aren't putting it in the oil or coolant stream it might do ok. An RTD might hold up better.
 
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btrue,
Both AC condenser outlet and radiator outlet reach 60, maybe 70 deg C. Would you still recommend RTD? I plan to tape LM35 to the outlet. It will not see the fluid directly.
 
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I would think using pressure would be better suited to control the fans based on the AC operation.
 
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The LM35 is rated for operation up to 150°C junction temperature

Per datasheet: its 1000-hr stability at 100°C junction temperature is ±0.08°C. While 1000 hr isn't particularly long, for 2 hr operation per day, it represents over a year of normal operation, and it'll be many years before the stability error exceeds its ±1°C error at 150°C max operating temperature.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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LionelH,
When vehicle is moving, AC condenser cools to near air temperature at the outlet. According to my measurement, not even 10 deg C above air temperature. Tying fan operation to vehicle speed would be ideal but I don't know where vehicle speed wire is or what format signal is available. That's why I am going to use AC condenser outlet temperature.

Thank you, IRStuff, for the LM35 data interpretation.
 
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What does the AC system pressure have to do with vehicle speed? It's typically recommended by the aftermarket AC guys to use a trinary pressure switch to control the cooling fans.

Temperature sensors on both sides of the condenser to measure the differential would likely work OK.

You need to run the fans at a minimum speed all the time with the AC on unless the vehicle is moving something like 25mph or more. If you have 2 speed fans then the low speed would be appropriate for this.

Also, since you're going Arduino, might as well do PWM control of the fans as well.

You should be able to find the info for speed signal if you search forums related to the car. People often install aftermarket stuff that needs the speed signal.
 
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Thank you, LH, for the note. I think doing PWM control on my own is above my level. I understand PWM but don't have the working knowledge. I googled PWM radiator fan control but found nothing. My 2008 Chevy has 2 fans with 3 relays for series (low speed) and parallel (high speed). I think I can handle this method on my project car.
 
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Any suitable DC PWM circuit would work. Lots of examples on the internet.
 
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Your encouragement revived my interest in PWM radiator fan control. Honestly, I attempted PWM control once before. I got a circuit from internet and started testing on a breadboard. Day 1 it worked well. Day 2 it stopped working. I must have done something to kill the IC but didn't know how I killed it. That's when I gave up. Now Arduino generates PWM output. PWM fan controller modules are available on internet. I think these modules can generate 20 to 30A. AFAIK many PWM fan modules run on LOW signal. Higher fan speed on longer LOW signal. Since Arduino cannot sink current, I will need something to sink the current from PWM module. Can I use the attached transistor switch circuit? If yes, which transistor and what R values? Thank you.

Another approach I have seen on internet is Arduino + SSR. This looks very simple.

EDIT: The PWM module has 4 terminals, Ground, PWM input, PWM output to motor and +12V. The only circuit I have to build is the switch to reverse the signal.
 
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