Low pass filter on PWM analog gauge control

Discussion in 'General Electronics Chat' started by Carbibbles, Feb 24, 2015.

  1. Carbibbles

    Thread Starter New Member

    Nov 14, 2011
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    Hey guys, im fairly new to electronics but im working on a project that involves controlling a 12v DC analog gauge. Normally the gauge would use a thermistor for the signal and as resistance increases the gauge decreases (ground state will max out the gauge). Ive set up a controller to PWM the gauge signal to ground instead of using a thermistor (100% duty cycle is full ground) running about 150hz. It actually works great with no extra circuits and I get gauge min at about 10% duty and gauge max at about 98% duty. The only issue is that I can hear it buzzing/humming very quietly and im a bit worried about any long term damage to the gauge internals. I did some research and found guys using a low pass filter so I tried a few cap/resistor combos and found that a 47ohm resistor tied into the PWM wire with a 1.0uF electrolytic cap (+ on resistor, - to ground) seems to cut down the humming the most (gauge output seems mostly unaffected). I just wanted to know if there is anything wrong with setting it up this way?


    Thanks

    Jason
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What is the resistance of the guage?
     
  3. Carbibbles

    Thread Starter New Member

    Nov 14, 2011
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    The gauge has 3 pins: 12v power, ground, and signal. 12v pin to ground pin is 195-ohms, 12v pin to signal pin is 150-ohms, and ground pin to signal pin is 218-ohms.
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    sounds like an automotive guage. that low of a cap shouldnt hole enough charge to cause problems if the guage were a fuel guage. you have to be carefull with fuel guages, too much capacitance might cause a spark in the gastank.
     
  5. Carbibbles

    Thread Starter New Member

    Nov 14, 2011
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    This is a temperature gauge so no worries about fuel or anything like that. It seems like most schematics show a non-polar cap being used for this job but I didn't have any that were higher than .1uF and those didn't seem to make a difference in the humming at all. Is there anything wrong with using an electrolytic?


    Also, id like to get a better understanding of what this circuit is doing. I understand that its a buffer of sorts but what is the electricity actually doing relative to the floating and ground pulses of the PWM?
     
  6. Carbibbles

    Thread Starter New Member

    Nov 14, 2011
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    Actually, im not even sure this qualifies as a low pass filter? Most circuits ive looked at have the current flowing through the resistor and out with the capacitor teed into the line after the resistor. The way I have it, the resistor and capacitor are in series and teed into the pwm line which has no resistor in it.
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Isn't the +12V to ground circuit powered constantly, and used to heat one end of a bimetal, while the +12V to signal-in circuit is a varying current?

    What is the amplitude of the pwm signal? Is it driven both high and low, or is it an open-collector type of drive that can only sink current?
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Could you post your schematic?
     
  9. Carbibbles

    Thread Starter New Member

    Nov 14, 2011
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    Im not sure how the gauge works internally but here are some examples of its behavior. Nothing connected = needle rests at lowest position; 12v and ground, signal not connected = needle comes slightly up from rest position to lowest gauge reading; 12v and ground, touch signal to ground = needle swings up to max; 12v with ground disconnected, touch signal to ground = needle swings up to max but with a little more force than with ground connected.

    The pwm is open-collector and only pulls the signal low, there is no pull-up.


    Here is a schematic drawing
    [​IMG]
     
  10. Carbibbles

    Thread Starter New Member

    Nov 14, 2011
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    Any input on this? Id like to understand how things are working in this electrical layout.

    I tried to set it up in LTSpice but it didn't show it affecting the square wave that much. It has to be though because there is a distinct reduction in the slight humming/buzzing.


    Thanks
     
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