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PWM heater circuit — Parallax Forums

PWM heater circuit

dherdedherde Posts: 13
edited 2012-10-17 11:59 in Propeller 1
I will be controlling a 30 amp 12V heating element with a prop quickstart.

Any recommendations on the appropriate high power driver to use?

Also do I need to optically isolate the power circuit from the quickstart PWM output pin?
(Finally getting around to the grease car heating controls):smile:
TIA
David

Comments

  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-10-16 18:29
    I know an IRF3708 mosfet can be driven directly from the Prop. And I've made some PWM heaters using them.

    What's a grease car?

    http://forums.parallax.com/showthread.php?137035-MOSFETs-and-the-Propeller&p=1063408#post1063408
  • tonyp12tonyp12 Posts: 1,951
    edited 2012-10-16 18:30
    No optical isolation needed, as 12v is not high and also heater element should not create ground noices.
    Do you need just so-so temp control (temp peeks and valleys), like your run of the mil room space heater?
    If so use a temp sensor and a relay for on/off.

    If your pwm should run a mosfet, anyone of of these with a large heat sink should work. (you may need to use additional circuit to boost gate voltage to 5v with some)
    http://www.mouser.com/Semiconductors/Transistors/MOSFET/_/N-96mrkZscv7?P=1z0w5zkZ1z0ss6dZ1z0w5pkZ1z0vx5wZ1z0vz70Z1yzs1kiZ1z0w781Z1yzrmzgZ1yzvu4zZ1z0ssbxZ1z0w6myZ1z0w4onZ1yzulxwZ1z0w94qZ1yzf1dkZ1z0w5amZ1yzoeklZ1z0w94jZ1z0w7ikZ1z0jyyxZ1z0w95aZ1z0w00cZ1z0w55yZ1z0vy65Z1z0w00dZ1z0w96tZ1yzs1p9Z1z0jm5lZ1z0w5sjZ1z0s56yZ1yzoeksZ1z0w5x3Z1z0w789Z1z0vwruZ1z0w5t6Z1yzt1uvZ1z0w969Z1z0w6eeZ1yzrvnqZ1z0w6ogZ1z0wa9sZ1z0ssbzZ1z0w72tZ1z0y3zr&Keyword=mosfet&FS=True&Ns=Mounting%20Style|1
  • RDL2004RDL2004 Posts: 2,554
    edited 2012-10-16 18:37
    What's a grease car?
  • dherdedherde Posts: 13
    edited 2012-10-16 18:44
    A grease car is a diesel (Mercedes, BMW, GM, Dodge) which has been converted to run on waste french fry oil. You have to heat the injector lines (and grease flowing to the injectors ) to 180 degrees F before it enters the fuel injectors or you risk coking the engine.
    I promised a few months ago that I would do the controller with a prop. I am at the point of getting it done.
  • dherdedherde Posts: 13
    edited 2012-10-16 19:07
    Thanks for the great leads and links.
    Now if I can just figure out how to mark this thread solved........
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2012-10-16 19:14
    dherde wrote: »
    I will be controlling a 30 amp 12V heating element with a prop quickstart.

    Any recommendations on the appropriate high power driver to use?

    Also do I need to optically isolate the power circuit from the quickstart PWM output pin?
    (Finally getting around to the grease car heating controls):smile:
    TIA
    David
    One thing to remember with heating as opposed to lighting or motor control is that you only need a very low frequency PWM due to the thermal inertia of the heating element. Typically most of them are thermostat controlled and rely on the bang-bang method where there are either on or off for several seconds or more. You don't have to go that slow but going slower means that the MOSFET will not be getting warm from switching losses plus you can drive them directly from the Prop. Think of a frequency in the 10 to 100Hz range for example. You can use the big ones that are recommended on this thread or just to highlight how small you could go with a MOSFET in this application think 8-pin SMD!
  • kwinnkwinn Posts: 8,697
    edited 2012-10-16 21:19
    Peter Jakacki wrote: »
    ........... You can use the big ones that are recommended on this thread or just to highlight how small you could go with a MOSFET in this application think 8-pin SMD!

    Wow, that's impressive, thanks for the link Peter. Saved for future use/reference.
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2012-10-17 05:52
    Hi dherde;
    dherde wrote: »
    I will be controlling a 30 amp 12V heating element with a prop quickstart.
    I'm a fan of using the IRF3708 directly driven by the Prop for relatively low current applications with no heatsink.

    However, your application is for high current where a heat sink will be needed.
    3.3V gate drive is not recommended.
    A MOSFET gate driver running at 5V is recommended.
    A single IRF3708 will need a heat sink that dissipates about 12W.
    If you use 2 IRF3708s they will dissipate about 6W.
    MOSFET_IRF3708_30V_62A_12mO_Vth2.0_TO-220

    I might suggest you look to the BUK965R8 which has a lower ON resistance.
    This is a bit more expensive though.
    A single BUK965R8 would dissipate about 5W
    and a pair of them would dissipate about 2.5W.
    You may not need a heatsink as each BUK965R8 will only disipate about 1.25W.
    Maybe worth the extra cost!
    MOSFET_BUK965R8-100E_120A_5.8mO_Vth2.1_D2PAK

    Another method would be the use of DC SSRs, Solid State Relays.
    Isolation is built into these.

    I agree with Peter, since this is a heater application a fast PWM is not required.
    Slow PWM is better described as "Duty Cycle" control where the cycle is may be a few seconds.
    Much easier to do.

    I assume this heater is associated with fuel line heating. Is this true?
    If so, the control may be as simple as a simple set point temperature controller.
    Yes, there would be greater temperature swings, maybe up to 5 degrees F,
    but this is no a problem for a fuel line heater.

    Duane J
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2012-10-17 06:28
    Duane C. Johnson wrote: »
    Hi dherde;

    I'm a fan of using the IRF3708 directly driven by the Prop for relatively low current applications with no heatsink.

    Duane J

    Although the IRF3708 seems to be championed in this forum there is a huge variety to choose from. For the heavier loads it's important to get as low an RDSon as possible at logic levels. So I use the IRLB3036 which has 1.9mohm Rds although I would still drive it at 5V which I typically do using a single 74HCT gate for relatively slow speeds.
  • Mark_TMark_T Posts: 1,981
    edited 2012-10-17 07:29
    Peter Jakacki wrote: »
    Although the IRF3708 seems to be championed in this forum there is a huge variety to choose from. For the heavier loads it's important to get as low an RDSon as possible at logic levels. So I use the IRLB3036 which has 1.9mohm Rds although I would still drive it at 5V which I typically do using a single 74HCT gate for relatively slow speeds.

    That's device is available surface mount too: IRLS3036-7P: http://www.irf.com/product-info/datasheets/data/irls3036-7ppbf.pdf Am currently using one in a 12V lead-acid discharge-curve / capacity meter device I made - handles 15A continuous with just a little copper area as heatsinking!
  • dherdedherde Posts: 13
    edited 2012-10-17 09:02
    Peter Jakacki wrote: »
    One thing to remember with heating as opposed to lighting or motor control is that you only need a very low frequency PWM due to the thermal inertia of the heating element. Typically most of them are thermostat controlled and rely on the bang-bang method where there are either on or off for several seconds or more. You don't have to go that slow but going slower means that the MOSFET will not be getting warm from switching losses plus you can drive them directly from the Prop. Think of a frequency in the 10 to 100Hz range for example. You can use the big ones that are recommended on this thread or just to highlight how small you could go with a MOSFET in this application think 8-pin SMD!

    I figured I would start with 420 hz but i suppose that could be seriously dropped down into the 10 hz range for a start. I wanted to make sure that i didn't throw the alternator for a loop. I was thinking it would react better to somewhat higher frequency drain that it would see as a more constant drain than a continual low frequency pulsed current surge that it would try to compensate for.

    Thoughts?

    (I am not set up for any SMDs (Or so I currently think..) Might make the circuit board easier to construct, but I am not comfortable with that yet.)



    Thanks..
    David
  • dherdedherde Posts: 13
    edited 2012-10-17 10:29
    Thanks for your great advise.
    I think I am going to try to see about SMT and using the BUK9K6R2-40E driving it directly. It seems this will fit the bill nicely. Using both FETs in Parallel it should dissipate about 3 watts at 30 amps. That is a board heat sink. Worth overcoming SMT fears.

    Anybody know if there are board mounted fuse holders that hold a Ford or GM style plug-in fuse?

    (I am finding the search for parts to be the most annoying aspect of the project. Is there a good way to go about this?)

    Thanks bunches..
    David
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2012-10-17 11:59
    Hi Peter;
    Peter Jakacki wrote: »
    Although the IRF3708 seems to be championed in this forum there is a huge variety to choose from. For the heavier loads it's important to get as low an RDSon as possible at logic levels. So I use the IRLB3036 which has 1.9mohm Rds although I would still drive it at 5V which I typically do using a single 74HCT gate for relatively slow speeds.
    Yes, the IRLB3036 MOSFET is nice.
    However, its only rated at 16V. In automotive applications the 12V battery can have voltages considerably higher.
    In cold weather charging the voltage may exceed 16V, possibly as high as 18V, or spikes as high as 25V sometimes.
    I would stick to the BUK965R8 which has a voltage rating of 30V. Much safer.

    Duane J
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