PPTC Resetable fuses (polyswitch)

Discussion in 'The Projects Forum' started by HiProfile, May 22, 2009.

  1. HiProfile

    Thread Starter Member

    Mar 15, 2009
    23
    0
    I've come here again for advice. My basic project now requires a resettable fuse or breaker, and a polyswitch will work from what I gather. However, I'm confused how they rate the things. I can understand the hold current, but they rate them with trip current X amps (i.e. 30A), then rate the trip time at a much higher current (20sec @ 75A). Could this cause a safety issue running 14awg wire? Will it ever trip if the motor is used 1/2 second at a time?? The motor I'm using has a 15A fuse in it's original application, which leads to another confusing issue - how it can blow 30A fuses.

    My project uses a automotive wiper motor (high-torque gearhead) to pull a cable, which opens an exhaust cutout. It only has to move that cable ~1" back and forth, which is roughly 20 degrees of shaft movement, but several rpm's. The car it's from uses a 15A fuse, but in my application it can sometimes meld a 30A fuse! I think because it is cycled closed/open when I shift gears, and because it's rubber stoppers are fairly stiff, its a bad mix of inrush current, stalling, and fuse over-heating causing my problems. I have the diagram on paper, but all it really does is use a DPDT relay to reverse the polarity to the motor, with limit switches to kill either 'throw' of the relay. FYI they are set to limit before the bumpstops, since the inertia carries them well enough.


    All I could really find is the following thread, which kinda confirmed my suspsicious why its pulling so much current.
    http://forum.allaboutcircuits.com/showthread.php?t=6802&highlight=polyswitch

    Here are 2 short vids of my project in action:
    http://www.youtube.com/watch?v=1_2RlSZoplw
    http://www.youtube.com/watch?v=Cz27DVw7HD0
     
  2. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    I would imagine that you're drawing much more current than you expected because this application sounds like it's stalling the motor. The motor is not designed to do this. BTW, why would you want the exhaust cut out to open and close when you shift the vehicle? Back in my muscle car era I had manual dumps on my headers but they were opened before the race and remained that way until I left the strip.
     
    Last edited: May 23, 2009
  3. HiProfile

    Thread Starter Member

    Mar 15, 2009
    23
    0
    Its not that I want to, I'm just slightly limited by my Engine Management System. It will open up beyond 60% throttle possition automatically, and I've recently been thinking of making a delay circuit for closing it. I can also do it via switch, but IMO manual cables belong on carb'd cars with manual chokes, etc. :D Many people have a hard time understanding the effects of backpressure on a turbocharged car, and why its needed. With the exhaust closed, its quieter than stock, but opened I can get another 6psi from the turbo, which equates to 50 horsepower. That also doesn't include the reduced lag I get. [Don't scream rice rocket until you've driven it. ;)]

    The unfortunate thing is all other electric cutouts you can purchase use horribly slow gearhead motors right by the cutout, which means they burn out from heat and take up way too much space. Mine uses a cable for a remote install, but the return spring has to be rather stong -- this requires a powerful motor.
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Have you investigated using a powerful solenoid instead of a motor? There are also small pneumatic & hydraulic piston/cylinder packages available that can provide a much longer stroke.
     
  5. HiProfile

    Thread Starter Member

    Mar 15, 2009
    23
    0
    In fact I have, even servos. Maybe I just don't know where to look, but I've not found anything under $100 (let alone under the $10 I spent) -- be it air, solenoid, or servo-based. The big solenoids I did find barely overcame the cable's friction (new, braided go-kart type), let alone the return spring. Servos become very complicated for me, as well as pricey. Air works great, but it needs to open right where manifold pressure is 0 psig, and dont' want to rig up an air solenoid/vac chaimber.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Well if you're going to use the motor that you have I think it would be preferable to use it in a Lead Screw arrangement with limit switches at the end of the travel. This type of drive can produce maximum torque from a small motor and will make it easier to control. The Lead Screw drive has been used on lathes and mills for decades.
     
  7. HiProfile

    Thread Starter Member

    Mar 15, 2009
    23
    0
    Just thought I'd update this. Just hours before spending money on PPTC's and a nice lead screw arangement, I discovered the issue. Apparently Honda grounds the motor frame I'm using, assuming nobody would ever want to reverse the power. There was enough paint rubbed off in spots in my trunk that it was occationally shorting out! No wonder a it could blow a 30A fuse.

    Luckly it was just 1 wire to disconnect, and now it works great. Far more powerful & faster than any wimpy gearhead, solenoid, or lead screw I could have used - at least for the price. They're $10 at the bone-yard.
     
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