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Drive 12V 15A load with PWM 18V-48V 2A

Discussion in 'General Electronics Discussion' started by app, Nov 13, 2014.

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  1. app

    app

    6
    0
    Nov 13, 2014
    Hallo,

    I'm new in the field of electronics.
    I'm working on a study project to control the heating power supplied by
    the PTC ceramic heater.

    The specifications of the ceramic heater are (12 V/DC 150 W, 15A ).
    Here is the link of the heating element

    http://www.conrad.de/ce/de/product/532894/Heizelem...

    As far as I understand basic circuits, the steady current of 15A must be
    provided to the load (Ceramic heater) all the time. But I can apply
    variable voltage input. This could help me to get variable heating
    power, e.g, from 0W, 45W, 75W, 150W by changing the input voltage to 0V,
    3V, 5V, 10V. I can measure both the voltage across the heating element
    as well as the current flowing through the circuit. The problem,
    however, I'm facing is that the module I got to interface with the
    heating element is either an analog output module with (-10V to 10V,
    20mA) output or a PWM module (18V to 48V, max 2A). Since, the modules do
    not match my loads , so I can't connect them directly.

    Does any of you know how can I realize an interface to connect either my
    PWM module of Analog I/O module, whichever possible, to the Ceramic
    heater?

    Many thanks for your suggestions.

    Regards,
     
  2. Arouse1973

    Arouse1973 Adam

    5,164
    1,087
    Dec 18, 2013
    I think your best option is to have a separate driver which can be controlled via PWM. This could be as simple as a MOSFET which is driven by your PWM signal. You may need to reduce the output voltage of the PWM so it's doesn't damage the gate of the MOSFET as you say it's lowest voltage is 18 Volts.

    I would also like to see some temperature and current monitoring which is fed back to the controller. This gives you an element of safety as far as the driver goes. You could look into PIDs but that's probably over complicated for what you want to do, a simple thermostat / temp probe maybe all you need.

    Adam
     
  3. app

    app

    6
    0
    Nov 13, 2014
    Thank you very much for your answer. As you mentioned, I've looked for some driver circuits and found one that can support the heater. But the PWM input range is 0-5V.
    I'm attaching a block diagram of interconnection.
    But I'm not sure how can I transform my PWM (18V-48V) to (0-5V)?
    Could it be some kind of voltage divider?
    Also do i need to take care of the current rating of the driver(20A) with the load (15A)?

    Regards
     

    Attached Files:

  4. Colin Mitchell

    Colin Mitchell

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    314
    Aug 31, 2014
    You have got a very nasty heating element that has an output of 21 watts but it will take a very high current during the time when it is heating-up.
    This heating element is only designed to be connected directly to a 12v car battery. You are wasting your time driving it in any other way.
     
  5. app

    app

    6
    0
    Nov 13, 2014
    Hi Collin,

    you get it wrong. The heating power is 150Watts not 21Watts. The project is to control heating power with the help of PWM signal.
     
  6. Colin Mitchell

    Colin Mitchell

    1,416
    314
    Aug 31, 2014
    No. You got it wrong.
    The ceramic heater heats up to a final temperature and releases it heat-energy to the surroundings and takes 21 watts.
    If you direct a fan onto the heater, the temperature of the element will drop slightly and the current will increase.
    If you blow harder, the temperature will drop further and the current will rise. At the limit, you can be blowing with such force that the heat extracted from the surface of the heater can be as much as 150watts.
    That's how it works.
    It's nothing to do with PWM and you must connect it to a very low impedance source to get it to work correctly.
    All you have to do is connect it to a car battery.
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    11,981
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    Nov 17, 2011
    Colin is right with respect to the power´. See last page of the datasheet, diagram middle right. This shows you the power vs. air speed.
    Colin is also right concerning the inrush current. This heater is a PTC, short for Positive Temperature Coefficient resistor. This means it will have a low resistance when cold and resistance will increase with temperature (diaghram top left in the datasheet).

    .
    You could do this, but this is not how the heater is meant to be operated. Typically you apply 12V and the current results from I=V/R where V=12V and R is the resistance of the heater (which, of course) will vary with the temperature of the heating element and the airflow).

    You can control such an element using PWM. However, I recommend that you do not use an operating voltage greater than 12V (not even with PWM) as this will increase the current proportionally and will damage the heater when the PWM's duty cycle nears 100%.
    Using PWM works like this:
    [​IMG]
    V1 is the 12V DC source.
    V2 is the PWM controller, here shown as a pulsed voltage source with 0V=off, 5V=on, Period=2ms, On-time=1ms (duty cycle = 50%), 1ns are rise time and fall time respectively.
    Q1 is a power transistor that must be able to carry the max. current for the heater (>15A). An N MOSFET can also be used, I used a bipolar NPN for convenience.
    R1 is the load (heater). It is shown here as a fixed resistor, the resistance will vary with the PTC's characteristic in practice.
    R2 limits the base current to Q1. Base current needs to be Ibase=Iload/gain(Q1).

    Note that load current Iload will increase with falling ducty cycle (ton/Tperiod) because the heater will not heat up to full power, therefore the resistance will be smaller than at full power and the current nears inrush current.
     

    Attached Files:

    • pwm.png
      pwm.png
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  8. app

    app

    6
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    Nov 13, 2014
    Hi Colin,

    Thanks for correcting me.
    So it means, the higher the air velocity, the higher the energy dissipation. Do you think that there is a linear relationship between the air velocity and the power dissipation?
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The relation is shown in the diagrams on the last page of the datasheet.
     
  10. app

    app

    6
    0
    Nov 13, 2014
    Thank you very much Harald and Colin for your help.:):)
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    A note on the side: As this is a ceramic element, it may (or may not) exhibit a more or less pronounced piezoelectric effect which may lead to a whistling noise at the frequency of the PWM. You may therefore want to choose a PWM frequency outside the human hearing range (<20Hz or > 20kHz).
     
  12. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    Let me take it further.
    If you put the heater in a box of asbestos, the heater will take a very high current and when it reaches its operating temperature the dissipation will be 21 watts. The amount of heat will heat up the asbestos and the temperature of the ceramic will get slightly hotter. This will cause the resistance of the ceramic to increase and the current will drop. Eventually the heater will get to a slightly higher temp and it may release as little as 2 watts to keep the ceramic at the elevated temperature.
    You will be able to keep the car battery "On-Charge" and deliver a voltage sightly higher than 12v as the ceramic will increase in temperature and this will increase the resistance so that a little more than 21 watts will be released.
    Providing PWM is pointless because the device is self-regulating.
    It delivers between 21 watts and 150 watts, depending on the air flow, and you simply reduce the air flow, rather than providing any PWM control.
     
  13. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    I don't think it is pointless. All the "regulation" you describe is correct. However, you cannot extract more energy than you input to the heater. By applying PWM you can reduce the input power and thus output power (assuming a constant airflow, of course).
    The relation between PWM duty cycle and available heating power will be highly non-linear as the resistance of the heater varies sharply with temperature.
     
  14. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    As soon as you provide PWM, you are taking away the ceramic heater feature and you might as well use a nichrome heater for $3.00. That's why it is POINTLESS.
     
  15. app

    app

    6
    0
    Nov 13, 2014
    I got another idea with your help.
    Instead of controlling the ceramic heater with a PWM signal, I'll keep supplying it 12V DC.
    I can vary the air velocity with a fan to get a variable heat dissipation as shown in the datasheet.
     
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