Electrical component question

Discussion in 'Workshop Forum' started by Dr.Jeff, Nov 11, 2019.

  1. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    Regarding the HVAC blower, heater fan, vent, AC box blower. Instead of the stock "toaster" mechanism (coil resistor box mounted in the front scuttle tray) that controls the three blower speeds, could a PWM control unit be used to adjust the blower speed? This would be replacing the existing control lever/switch and resistor box. The PWM control has simple connections; 12V in, and +/- feed wires out to the blower motor. They are a small package with a remotely mounted rotary control knob. Rated for 20 to 40 amps (depending on the unit chosen) for 12V use, they can be found very inexpensively online. Here is a pic of just one of many to choose from:
    HTB1aP13bN6I8KJjSszfq6yZVXXab.jpg
    I am planning to replace my AC heater box controls to eliminate the vacuum operated system. So the dash will be modified to install manual levers for the air flow 'flapper' doors and heater valve (also being replaced with a VW item). That makes it a good time to also replace the blower speed control to a rotary knob. In addition to making the non-vacuum conversion a little easier, it also eliminates that 'hot box' thing in the scuttle - which I do not care for. I considered utilizing a electronic control from a newer vehicle but there are just too many choices to know which is best. I guess one added benefit to using the PWM control is the blower will then be infinite speed instead of three. I also think the small rotary knob will look a bit cleaner on the new custom dash (the gauge cluster is being replaced with aftermarket dials).

    Any info on this from all of our electronic wiz kids? Thanks.
     
  2. beezee

    beezee True Classic

    I don't see why it wouldn't work.

    A couple of things to watch for: Make sure it is located where there is adequate air circulation to allow the heat sinks to do their job. Secondly, the heat sinks can be live, so make sure they don't touch any of the cars metalwork.

    Brian
     
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  3. It should definitely dissipate less power than the resistors do.
     
  4. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    Please help me understand what that means in terms of using it to control the blower motor speed. Thanks
     
  5. beezee

    beezee True Classic

    Resistors limit the fan motor speed by reducing the voltage at the fan terminals, thus reducing the current. The voltage drop across the resistor generates heat.

    A PWM (pulse width modulation) controller limits the current through the fan by rapidly turning the voltage on and off. The ratio of on vs. off time controls the fan speed.

    Brian
     
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  6. carl

    carl True Classic

    Location:
    Virginia
    You may be overthinking this. The non-AC X and all spiders use a simple three position rocker switch...off in the center, high one rocker position and low on the other. Three wires, 12v source, high speed wire and low speed wire which matches with the high and low speed wires (and ground) that come out of the non AC heater fan. Pretty much a basic mechanical set up.
     
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  7. Integrale

    Integrale Daily Driver

    Location:
    Davenport, IA
    The main argument I've heard against this method is the potential for electrical noise.
     
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  8. myronx19

    myronx19 True Classic

    Location:
    Toronto, ON Canada
    I use those PWM units for many projects (not in my X1/9). They're inexpensive, and they work well. Most of mine don't handle that much current, but they work great! I even use one to dim LED lighting for my koi pond :)
     
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  9. bpimm

    bpimm Brian Pimm

    Location:
    Washougal, WA
    As Integrale said it may cause electrical noise that could interfere with your sound system if you have one, the other possible problem I see is mechanical noise if the windings of the fan motor are not really tight, it could cause the motor to sing so make sure the operating frequency is above human hearing frequencies.
     
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  10. The advantage of a PWM system over a linear system is that the output devices dissipate less power because maximum output current is delivered to the load with very little voltage across the output devices (P=V*I).
     
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  11. The noise issue could be real depending on how good the design is, including shielding and filtering. In my car, I get so much noise on the AM band from my non resistor rotor and ignition wires that I'd probably never hear the PWM unit.
     
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  12. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    Thanks Brian, that is my understanding as well. Which is why I was a bit confused by the comment regarding the PWM controller dissipating less power. But the added explanations have helped.
     
  13. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    Interesting about noise. I did not know this could be a factor. With the PWM controllers being so inexpensive I'll get one and see what happens...now that everyone has helped me to understand that they should work for this application. Thanks.
     
  14. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    However I'm retaining the AC box and blower, which is a three speed motor - four position switch. That's why it has the hot toaster box in the scuttle. And as I stated the entire dash is being modified/customized, so I have no motivation to use any stock components. Especially if newer technology may offer benefits; such as an infinite speed blower, and elimination of the fire starter in the scuttle. So I do not see it as over thinking, but more as improving. And actually simplifying in some ways.
     
  15. beezee

    beezee True Classic

    FYI, the PWM controller in the picture looks a little primitive. Later technology units would not use through-hole components and should not require the size and number of heat sinks that this one has. I'm not saying it won't work but a later technology unit would be smaller, dissipate less heat, and this is a consideration in the X with so little space under the dash.

    Brian
     
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  16. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    I saw another one that might be more what you describe (pic below). The specs for it happened to be less (lower amperage capacity) so I was leaning toward the larger one. But if the one shown here is the better type then I can look for one like it but with greater capacity. I really have no experience with these devices so all input is appreciated, thanks.

    Tell me if this is what you are referring to. It is a 10A unit, but I'm sure they have larger ones:

    HTB15rewoBHH8KJjy0Fbq6AqlpXai.jpg
     
  17. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    About the 'noise' issue. Will there be any significant different depending on the type of PWM controller used? For example the fist one I pictured vs the last one I just showed in my last post?

    And about their capacity. I need to double check what the blower motor amperage rating is (anyone know off hand?). However I assume the stock circuit for it will be much higher (e.g. the fuse rating) due to it using that big heat resistor box thing? With the motor itself less, meaning a PWM controller need not be as robust as the old toaster circuit was?
     
  18. beezee

    beezee True Classic

    That looks like a more modern device; Much smaller and no heat sinks.

    Modern PWM controllers utilize switching frequencies well above the audible range, so you won't hear anything other than the normal fan noise (hopefully:)). I can't speak for radio interference.

    Brian
     
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  19. rachaeljf

    rachaeljf True Classic

    Location:
    Northampton UK
    I have fitted one to my Montecarlo, using a linear pot to replace the two speed switch. Noise is a concern but a car is a noisy environment anyway so I hope the radio copes with it, if I ever fit one and ever get the car on the road.

    Edit: I looked up my original thread on bookface. Another owner, Eric Weston fitted a PWM controller and said it was fine with his radio, though he did have a suppression choke on radio his power lead.
     

    Attached Files:

    Last edited: Nov 11, 2019
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  20. beezee

    beezee True Classic

    Oops. Spoke too soon. The device in your last pic uses a 555 controller, which is a little primitive. This controller will not likely have short circuit protection (in case you make a mistake) and the operating frequency may be in the audible range.
     
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