Killing Float Switches

Discussion in 'The Projects Forum' started by bowlingo, Dec 11, 2011.

  1. bowlingo

    Thread Starter Member

    Jun 29, 2011
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    Hi all,

    I have been trying to switch 2 x small 12V bilge pumps of which total 1.8A DC (as of using a mulitmeter)

    I have tried using 2 x float switches and both times the switches have stayed in closed position and it looks like I have burnt them out...I have tried putting a continuity tester across both switches and they are not opening and closing and are staying in the closed position although they are open.

    The switches are rated at Max 25VA, 240V ac/120V dc, 0.6A (as whats written on the packets)

    I have also tried using my last float switch and breaking the AC Neutral conductor of the supply before the transformer and again it seems to of burnt out the switch.

    When I initially connected the switches they worked one time only and energised the pumps when in the closed position but after that they wouldnt work.

    Are pump motors inductive?..maybe its something to do with that?

    I have just looked on the manufacturers website and they do the same switch but 100VA rather than 25VA...

    Thanks
     
    Last edited: Dec 11, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Yes, motors ARE inductive.
    Your bilge pumps are 12v 1.8A; you are trying to use switches that are rated for 0.6A, and are wondering why they are failing. You are trying to control 3x as much current as your switches are rated for.

    You will either need to use a more robust float switch, or you will need something to go between the pump motors and the switch that IS rated for the current.
     
  3. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    Thanks for the reply...

    I always get up muddled up with the amperage rating for AC and DC...in this case I thought they could take 0.6A at 230V AC but a lot more Amps when on 12V DC.

    I think I will have to put a 12V relay on the pumps maybe as I have already cut the hole for this exact float switch.

    Thanks
     
  4. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    I am also wondering why the last switch blew when I put it on the AC supply side? Would this not be a lot lower current?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    The current would be lower, but the VA would be higher.
     
  6. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    Thankyou :)
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
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    Voltage rating doesn't have much to do with it at the voltages you are working at. Higher voltages mean the contacts need to be further apart when off so there isn't an arc.

    The 1.8Amps is what you need to worry about. You need to be able to switch at least 2 Amps, preferably 6 to 10 amps if turning the motor off under load (it is an inductive load), which may go high enough to arc.

    What is blowing the switches is the current rating, as Wookie stated. 0.6A is 1.4A too small.

    The VA rating is simply Volts * Amps, so 25VA at 13V is about 2A, while 100VA is about 5A. Look for a switch that can handle the current first, then make sure it is rated for at least the voltage you are using.

    In other words, 1Amp at 100V will blow a 1A fuse just as fast as 1A at 12V would blow the fuse. Current is what makes wires hot and melt (fuses, switches, etc).
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    This is a job for a relay.
     
  9. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    I pretty much now understand the above...

    What I dont quite follow is if the switches are 25VA ...its written like this one the packet "n/o Max 25V A, 240V ac/120V dc, 0.6A"

    as youve said above 25VA at 12V is around 2 Amps although as of the spec on the packaging its saying 0.6A..if it could take 2A I am pretty sure the switch will be ok...apart from like you say its inductive and could draw a heavy load when under strain etc.

    Thanks
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Ignore the VA for what you are looking for. I put in bold the important part to focus on when looking at a switch.

    This should NOT power your pump directly, it should power a relay (usually 75mA or 250mA, or 0.075A-0.1A), and the relay can than switch many amps of load.

    When you look at relays, you will be bewildered.

    You'll see ratings like 12VDC50Ω5A125VACSPDT

    They sometimes don't space that stuff out.
    They put the coil voltage and current (or resistance) first: 12V 50Ω (draws 250mA roughly)
    Then they put in the current rating of the contact: 5A
    The max voltage the contacts can switch: 125VAC (125 Volts AC)
    Then the contact type: SPDT (Single Pole, Dual Throw)
    Other relays may use an entirely different order.

    You are looking for:
    12V coil
    50Ω or higher coil impedance
    500mA or lower coil current (works with impedance above)
    6 to 10 Amp contact ratings
    12-120Volts (DC or AC) Contact Rating, higher voltage just means bigger gaps in most cases
    SPST (One Pole (wire), one throw (on or off)

    You probably wont find a SPST, but stumble over the standard "Bosch" type relay (sometimes "Bosch 87 due to the pin numbering) instead, which is perfectly fine. It is a Single Pole dual throw and usually has a nifty little diagram on it somewhere. You'll need either a socket for that relay (Auto junkyard) or 5 female spade crimp connectors, heat shrink tubing, and a crimp tool.

    Below, I'm assuming you end up with a Bosch Relay, since they are everywhere:

    Go from 12V+ to the new and working reed switch on your float, out from it goes to the relay coil +, then relay coil - goes to ground.

    The "COM" of the relay goes to +12V, and the "NO" (Normally Open" side of the contacts goes to the pump motor.

    You will DEFINITELY want to put a crimp connector on the "NC" and seal it there with heat shrink tubing, since it is carrying a live 12V and could be flopping around.

    The relay is not waterproof, so put in a dry location, they dissipate very little heat, so airflow isn't a problem.

    Make sure you use at least 14 gauge wire if the runs for power and ground are over 10 feet (there and back). Otherwise, 16ga will do for 5A.

    If you get stuck, ask! Take a photo of your wiring, or attempt at wiring and we can guide you to a fully working pump again.
     
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