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Relay question for Dankoff slowpump

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  • Relay question for Dankoff slowpump

    Hi folks,
    This is my first post, so hello and thanks for being here. I am happy to have found this forum. I live in Panama and have a relatively large off-grid system mostly running on PV, with some microhydro and wind generation.

    I am installing a Dankoff 1408 24VDC slow-pump. Unfortunately Dankoff has dropped the ball on their pump controllers (their Linear Current Booster). Their newly designed DSP200 Pump Controller can't handle high sufficient amps for their pumps when dealing with a high head (which increases total amps being pulled).

    So I have to come up with another solution.

    We're looking at 280ft head, which according to the Dankoff 1408-24 info will require around 8 to 9 amps, at 24 VDC. So I want everything to handle at least 15 amps at a minimum (for safety and spikes, etc).

    The float switches Dankoff have are low amps. Not suitable. In fact most float switches I see are low amps. So I started to look at other options. I need something that will turn the pump OFF when the source tank is empty.

    One option is to buy a high amp bilge pump switch.
    One issue I see with this is that it means I have to run cable suitable for 24VDC at aprox 10 amps from the panel, to the switch (in the bottom of a tank) and out to the pump. Seems like heavy cable overkill to me.

    The switch I was considering getting for this application is this one:
    My concern is the wiring issue pointed out above, and also that in the manual it says not to run it on an AC/DC inverter. We were playing to have an AC/DC inverter in place for backup power and for potential emergency operation at night (if and when required). I've not been able to speak to anyone at SPX Johnson as of yet, to find out why running on an inverter would be an issue.

    The other option I see is that I could use a relay. But I don't really know enough about this to move forward. Aside from my interest in people's thoughts on the above points, this is what brought me here asking for help.

    So far I understand that if I use a relay I will need to also include a battery into the system. Which means I'll also need a charger that can keep the battery charged from the 300 watt panel we have for this pump.
    I understand the relay needs to be able to handle at least 24VDC/15 amps in the coil. And that it will use a lesser current for the switch.
    But that's as far as I got, at which point I am feeling a bit stuck.

    Any help on this would be greatly appreciated. If I've missed out any pertinent details, please ask.

    With my regards,


  • #2
    Originally posted by Sol-la-rae View Post
    I understand the relay needs to be able to handle at least 24VDC/15 amps in the coil. And that it will use a lesser current for the switch. ....
    you need a relay or Contactor, with CONTACTS rated for your pump. The coil needs to be rated for the power supply you run to the float switch. (I used the standard float switch to control my pump contactor, because the float is 600' from the pump control panel.
    120VAC contactor (relay) coil at a couple of watts, with beefy contacts to handle the 240VAC well pump. Contactors come in all sorts of combinations - DC coils, DC contacts, AC contacts.....

    [ a relay is cheaper than a contactor - generally. A contactor is used for a higher load. ]

    Grainger carries a bunch of them, Pump & Well stores have common ones, RE stores should have DC/AC ones .
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A



    • #3
      Thanks Mike.

      We might have to take a step back, to make sure I've understood the whole set-up correctly.
      Here's what I am thinking I need, which may or may not be correct:

      - 300W PV Panel (the overall power source)
      - A 12 volt car battery (power source for the switch)
      - A small MPPT charger of some kind, which will use the 36 volts from the panel to keep the battery charged
      - A relay that handles 12VDC (and low amps) on bilge pump switch side, and 24VDC (nominal) x 15 amps on the pump side

      Is that correct?

      I am thinking it's going to be most cost effective and simpler to find a decent bilge pump switch.

      It's late at night here... I'll give this some more thought in the morning.