Is this a sign of a bad DPDT switch?

Discussion in 'The Projects Forum' started by roykirk, Feb 29, 2016.

  1. roykirk

    Thread Starter New Member

    Feb 29, 2016
    3
    1
    I'm trying to wire up a simple circuit to reverse a motor using a DPDT switch and a 12 VDC motor (see attached for wiring diagram I'm using). When I wire the motor directly to my bench power supply, it works just fine. If I manually change the polarity of the wires, the motor reverses like you would expect. However, when wiring up the DPDT switch, the motor only runs in one direction. Flip the switch to the other side of "OFF/CENTER" and...nothing. Multimeter doesn't so much as twitch. I've doubled checked all the wiring connections on the switch and everything seems fine. The switch is new and is rated 20A 125 VAC. I thought I'd check with the experts here to find out if there's anything else I should be checking, whether they also think the switch is bad, or perhaps it's not an appropriate switch for my application?

    Thanks! DPDT.jpg
     
  2. TheButtonThief

    Active Member

    Feb 26, 2011
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    38
    Use your multi metre to take a continuity readings of the contact, if your metre has no continuity test mode, use the mode that allows you to measure resistance. If there is no continuity (infinite resistance) between what you expect to be closed contacts, then the switch is defective. Also true if you have good continuity (low resistance) between what you expect to be open contacts. Also measure continuity between the two common contacts.
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    Well..
    Is the switch also rated to carry DC at the current level your motor needs?
    Technically, an AC rated switch is not suitable for a DC circuit unless it also carries a acceptable DC rating.
    Many do have multiple approval ratings...
    DC is harder to switch than AC due to its lack of a cycle (aka DC current is never "off" with each cycle as there are no cycles in DC)
    AC is always going to 0 current during each frequency cycle...
    I'm not saying that is the problem at all..just passing along the information..

    As stated above though.. A simple test with a multimeter for continuity will allow you to check the switch..
     
  4. roykirk

    Thread Starter New Member

    Feb 29, 2016
    3
    1
    Well, you just blew my mind, mcgyvr. :D Quite honestly, I've always used AC rated toggles for all my DC applications. I don't know that I've ever seen a DC rating on a switch, but I wasn't necessarily looking for it either. I just knew the AC switches always seemed to work for my projects, but this is admittedly the first time I've played around with a DC motor circuit. I'm at the day job right now, but when I get home tonight I'll use my meter to check for continuity and I'll also look closer at that switch to see if it has a DC rating on it.
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    Yep.. Its a big "no-no" for any safety agency/inspector..

    Many switches carry dual ratings.. however many do not..
    And just because it doesn't have the DC rating doesn't mean its going to "explode" or anything.. But there are also no guarantees it won't when its not rated for what you are using it on.

    Many companies just don't spend the time/money for a DC approval if all they are doing is a switch intended for only AC applications..

    And typically the datasheet for that particular switch is the best place to look.. Many times there isn't enough room on the products body to include all the ratings..
     
  6. TheButtonThief

    Active Member

    Feb 26, 2011
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    38
    The circuit diagram with a battery suggested to me that the supply voltage was rather low (≤12V) and so the motor would only pull a low amount of current.
     
  7. roykirk

    Thread Starter New Member

    Feb 29, 2016
    3
    1
    Well, the switch has no DC rating that I can find (on the switch itself or the packaging). But that seems to be a moot point because it fails continuity test when the switch is on that one side of center where the motor wouldn't reverse. I've already ordered a new switch, this time with a DC rating.
     
    mcgyvr likes this.
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