Some time back I found an IC I'm trying to build a circuit around. The IC is somewhat specialized and perfect for my application however I've run into a heck of a problem coming up with a circuit to supply the proper input voltages to it under certain conditions. I haven't felt well the past few days so my mind just isn't working very well, figured I'd ask for some ideas from the crowd here.
Several limitations apply as to what I can use as this has to end up as rather small, inexpensive and capable of being done with simple components. The sensor itself must occupy a rather small area as well as being subject to some pretty extreme weather and vibrational conditions.
Here's the IC involved:
http://ww1.microchip.com/downloads/en/DeviceDoc/21756b.pdf
which you can see needs an input range of 1.2V - 2.6V to change from 0% - 100% duty cycle. Some amount of linearity would be nice but not essential.
A common NTC thermistor such as this will fit into the area I need to monitor, it has to be potted into a rather small adapter nipple with some thermally conductive epoxy.
http://www.mouser.com/Search/Produc...irtualkey64800000virtualkey81-NTSD0XH103FE1B0
http://search.murata.co.jp/Ceramy/image/img/w_hinm/S0425E.pdf
I have found this chart that shows typical values for NTC thermistors with the B curve, however I'm totally unfamiliar with the various "Constant 3350-3399K" part of the specs involved in thermistors.
http://www.alphatechnicsonline.com/b_curve_main.php
What I need is to generate an output for the IC as follows from a regulated +5V supply:
82*C (180*F) = 1.2V
99*C (210*F) = 2.6V
This will be used to control an electric fan on a motorcycle and obviously I'm trying to maintain a temperature of around 180*F HOWEVER it would be nice to have a pot in the circuit such that the low end starting point or entire range could be raised by 5 or 10*F.
I may have an area in which I could mount a slighly larger sensor but won't know until I go look closely at the bike this afternoon, it's about 1-1/2 hours out of town undergoing some final restoration and considering the way I feel it isn't going to be a fun drive.
Any and all ideas would be appreciated but remember that I live in the world of op-amps. While I'm sure one of your fancy computer boards could make this a breeze I have no way of developing such a circuit, much less programming or understanding it. The advantages to utilizing the controller chip I found are that when you hit the starting temperature it bump starts the fan with a higher PWM signal to get the DC motor moving before settling down to full temperature control. It also incorporates a /FAULT output which goes low when the PWM output hits 100%.
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Why do I need the circuit? These rather old bikes originally came with a manual fan mounted directly to the camshaft. This may have been fine at the time but several problems in that design exist.
1) The original fans are made of plastic and due to their age they eventually develop cracks in the plastic, come apart and usually take the radiator out with them.
2) While replacement fans can still be found, even if they're unused old stock the plastic is already 30 years old and in people's experience they don't last long until they too start coming apart.
3) At cruising speed sufficient airspeed often exists that they're not even necessary on a cool day, this has been proven by running one without a fan so it becomes nothing more than a drain on the engine's ouput.
4) In heavy traffic or at a long stoplight they don't create enough airflow to keep the engine cool unless you manually hold your idle speed higher.
Many have already done away with the stock fan and mounted an electric one, however their best attempts at any sort of control has been to mount a bi-metal sensor on the radiator. This ends up as a fully on or off situation and due to the inherent differential in these bi-metal sensors it has proven to be a far from ideal method of control.
There is already a sensor mounted early in the water stream to drive a temperature gauge, however measurements of these have proven them to be about as accurate as the fuel gauge on a Ford Pinto. They're also in the wrong place to utilize as a sensing circuit to drive the fan as they only look at the temperature of the coolant as it exits the engine instead of what the radiator is returning.
Several limitations apply as to what I can use as this has to end up as rather small, inexpensive and capable of being done with simple components. The sensor itself must occupy a rather small area as well as being subject to some pretty extreme weather and vibrational conditions.
Here's the IC involved:
http://ww1.microchip.com/downloads/en/DeviceDoc/21756b.pdf
which you can see needs an input range of 1.2V - 2.6V to change from 0% - 100% duty cycle. Some amount of linearity would be nice but not essential.
A common NTC thermistor such as this will fit into the area I need to monitor, it has to be potted into a rather small adapter nipple with some thermally conductive epoxy.
http://www.mouser.com/Search/Produc...irtualkey64800000virtualkey81-NTSD0XH103FE1B0
http://search.murata.co.jp/Ceramy/image/img/w_hinm/S0425E.pdf
I have found this chart that shows typical values for NTC thermistors with the B curve, however I'm totally unfamiliar with the various "Constant 3350-3399K" part of the specs involved in thermistors.
http://www.alphatechnicsonline.com/b_curve_main.php
What I need is to generate an output for the IC as follows from a regulated +5V supply:
82*C (180*F) = 1.2V
99*C (210*F) = 2.6V
This will be used to control an electric fan on a motorcycle and obviously I'm trying to maintain a temperature of around 180*F HOWEVER it would be nice to have a pot in the circuit such that the low end starting point or entire range could be raised by 5 or 10*F.
I may have an area in which I could mount a slighly larger sensor but won't know until I go look closely at the bike this afternoon, it's about 1-1/2 hours out of town undergoing some final restoration and considering the way I feel it isn't going to be a fun drive.
Any and all ideas would be appreciated but remember that I live in the world of op-amps. While I'm sure one of your fancy computer boards could make this a breeze I have no way of developing such a circuit, much less programming or understanding it. The advantages to utilizing the controller chip I found are that when you hit the starting temperature it bump starts the fan with a higher PWM signal to get the DC motor moving before settling down to full temperature control. It also incorporates a /FAULT output which goes low when the PWM output hits 100%.
-------------
Why do I need the circuit? These rather old bikes originally came with a manual fan mounted directly to the camshaft. This may have been fine at the time but several problems in that design exist.
1) The original fans are made of plastic and due to their age they eventually develop cracks in the plastic, come apart and usually take the radiator out with them.
2) While replacement fans can still be found, even if they're unused old stock the plastic is already 30 years old and in people's experience they don't last long until they too start coming apart.
3) At cruising speed sufficient airspeed often exists that they're not even necessary on a cool day, this has been proven by running one without a fan so it becomes nothing more than a drain on the engine's ouput.
4) In heavy traffic or at a long stoplight they don't create enough airflow to keep the engine cool unless you manually hold your idle speed higher.
Many have already done away with the stock fan and mounted an electric one, however their best attempts at any sort of control has been to mount a bi-metal sensor on the radiator. This ends up as a fully on or off situation and due to the inherent differential in these bi-metal sensors it has proven to be a far from ideal method of control.
There is already a sensor mounted early in the water stream to drive a temperature gauge, however measurements of these have proven them to be about as accurate as the fuel gauge on a Ford Pinto. They're also in the wrong place to utilize as a sensing circuit to drive the fan as they only look at the temperature of the coolant as it exits the engine instead of what the radiator is returning.
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