Working with MOSFET bias

Discussion in 'The Projects Forum' started by marshallf3, Aug 21, 2010.

  1. marshallf3

    Thread Starter Well-Known Member

    Jul 26, 2010
    2,358
    201
    I'm working on a circuit for a future project and I'm not familiar enough with the way a power MOSFET would respond in this situation.

    Here's the circuit: http://www.innoengr.com/examples/fan_speed_controller2.jpg

    What this is for is to change from the existing mechanical fan to an electric one on my motorcycle. The fan I found to fit draws 1.2A in free air on the bench when supplied with 12V.

    Obviously it's meant to start turning the fan on at a temperature set by VR2. It takes a good 6V to get the fan started but once turning will operate with less.

    I'm wanting it to start when the thermistor senses 180* F and be at full speed when and if it sees 210* F.

    Anyone care to suggest some components & values or if I need to alter the circuit to achieve this? As soon as I finish reading through the rest of the new forum posts I've got to move on to completing a project for work they're expecting to be 100% functional come Monday. It's a monster and complex enough that I had a PC board professionally etched. I've got about half the components soldered in but need to finish and test it, hopefully with no need to make any value changes.
     
  2. marshallf3

    Thread Starter Well-Known Member

    Jul 26, 2010
    2,358
    201
    Well, 33 reads and not a single comment.

    I fished an old STF20NF06 MOSFET out of a box of stray parts, used a 10K thermistor and a 500 ohm pot adjusted to around 370 ohms. Not exactly perfection but useable.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    In the schematic the mosfet is used in a linear region.
    The mosfet will get warm if not HOT.
    You better take a look at a PWM circuit.

    Bertus
     
  4. sage.radachowsky

    Member

    May 11, 2010
    241
    38
    Yah, I would agree with Bertus.

    I wonder, what is the resistance of the motor leads when measured when the motor is stopped? I bet it is a brush motor and is lower than 10 Ohms. (10 Ohms is the effective resistance when running on 12V according to your reported 1.2A so probably it is lower than 10 Ohms when the brushes are touching one coil not moving.)

    So, when the thing is in partial conduction, say it is 6V drain to source, and the motor is 6V, it is passing at least 0.6 A minimum. Then the MOSFET and the fan are each dissipating 3.6W and the fan is not turning. That is the minimum, it's probably more. I think it is not saving any power by doing the variable speed, unless you do a PWM like Bertus suggests, and it is generating heat as well.

    So, I would go with a simple "on or off" circuit with a Schmitt action, which would go on at 180F and off at 170F... something like that.

    When you turn off a newer car after driving it, one with an electric cooling fan, it then goes on and off several times during the cooling process. I think that is the same idea.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Sorry it takes so long to get a reply nowadays. I used to try to help everyone; I simply don't have that kind of time anymore.

    You really do need a PWM-type circuit rather than attempting to bias the MOSFET in the linear region. The higher the PRF (pulse repetition frequency), the lower the losses will be in the motor. However, switching at high speeds is a challenge, particularly if you are using discrete components.

    There are other issues to be concerned with. Automotive environments (which includes motorcycles, BTW) are amongst the most hostile on the planet. Temps can range from -40°C to 120°C, continuous vibrations over a wide range of frequencies, voltage transients to +60v, and exposure to corrosive/flammable fluids (battery acid, gasoline, oil, water/mud) tend to tear up circuits pretty quickly. Even if you use components that are rated for the environment, great care must be used in fabricating the board, using conformal coating, vibration isolation mountings, and proper wiring harness routing add up to be quite a challenge for even experienced people.

    With all of the disclaimer and other yammering out of the way, have a look at the attached. It's a slight modification to a fan temp speed control that I put together for a computer that I had which was overheating with the lousy stock controls and fans, but now stays nice and cool.

    Basically, I changed the diodes to higher breakdown rated equivalents, and changed the LT1171 to the HV model. That should satisfy the transient protection requirements. As shown, the circuit is better than 90% efficient even at the lowest output setting. That means low power dissipation in the regulator.

    If you're interested in pursuing this route, I'll have to change things around a bit, and will need at least 2 data points for your thermistor; one at 180° and one at 210°F.
     
    Last edited: Aug 22, 2010
  6. marshallf3

    Thread Starter Well-Known Member

    Jul 26, 2010
    2,358
    201
    It is a brush motor and this was just a test circuit to make sure that a variable speed fan would indeed provide adequate cooling. It worked fine, almost too well. When in place it will have a PWM controller, probably driven off a signal I can easily tap from the temp gauge.

    That's one of the few advances that weren't around or not really practical back when this bike was built, later years of production did go to an electic fan. In 1979 tubeless tires, mag wheels, water cooling and a driveshaft were rather rare as were a few other things they incorporated into the design. Story goes that at one point Motoguzzi (who had developed a sideways V) challenged Honda to best it and apparenttly Mr. Honda took it as a personal challenge. Without a doubt they're one of the most reliable and easiest to work on bikes ever made, easily exceeding 200,000 miles with only a few expected maintenance items, the cam chain and rotor being the two most time consuming to deal with but they rarely start failing until you're approaching 60K - 100K.

    Ever see a V-twin that sits sideways? They made several variations, some came fully dressed like the Gold Wing but were badged as a Silver Wing. The engine was later modified to make a 673 cc variant, they also made a few turbo versions of both the 500 and 650 but they're very rare and really too fast to ride because the turbo control circuitry was far from perfect - lag coming on, lag coming off and far too much power for a light bike.

    Pictures don't do these things justice, I have the Custom variant which is more a sport version considering the pulled back bars. Pic is representative only, when I'm done mine it will be metallic blue with a slightly smoked windsheld.
    http://shoc.hu/data/files/news-228/honda_cx500c.jpg
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Interesting. I barely recall the sideways V-Twins.

    When you find out what kind of signal range you get from the temp sensor and want to proceed, let us know. As I've already mentioned, it's going to take a good bit of planning to come up with a design that will be efficient AND survive the environment for a reasonable period of time. Failure of the circuit could cost you an engine.
     
  8. marshallf3

    Thread Starter Well-Known Member

    Jul 26, 2010
    2,358
    201
    When I get to that point I'll let you know what I come up with before I commit to building it.
     
Loading...