liquid level float switch

Discussion in 'General Electronics Chat' started by Spogpean, Apr 30, 2008.

  1. Spogpean

    Thread Starter New Member

    Feb 5, 2008
    hi all,

    im using a liquid level float switch but i dont know what voltage to drive through it to get it to work. the electrical spec is as follows :

    Switching voltage va = 25

    Switching voltage ac = 240

    Switching voltage dc = 120

    Switching current max a. = 0.6

    Breakdown voltage dc = 800

    all ratings are for resistive load only.

    Can anyone shed any light on which voltage i should use? am i right in thinking that a float switch is a type of circuit breaker? any info much appreciated!
  2. DC_Kid

    Distinguished Member

    Feb 25, 2008
    is this a relay switch, or just a plain 'ol switch.

    the specs you list are contact ratings. i dont really know what you mean by "what voltage to drive through it to get it to work"
  3. SgtWookie


    Jul 17, 2007
    Float switches are usually SPST or SPDT.

    To get maximum life out of the contacts, you could use it to switch CMOS or TTL logic levels, or perhaps drive the base of a transistor or Darlington pair.

    Don't use it to supply power to a motor or a relay coil directly, as the contacts will rapidly burn/pit and need replacement.
  4. Spogpean

    Thread Starter New Member

    Feb 5, 2008
    thanks for the replys :) .

    its just a regular float switch, crydom rsf 70 series. there's 2 wires coming out of it and i cant find any info bout connections to make or anything on the net. id guess that its a SPST switch, seems fairly simple.

    so could someone tell me what to do with the 2 wires? im trying to read in thru a data acquisition card whether the switch is on or off :confused:
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    I belive the spec's you give apply to the switch contacts:

    Don't apply more than 240V AC thru the switch.
    Don't apply more than 120V DC thru the switch.
    Don't run a load of more than 600mA maximum.

    Anything below these numbers should be okay, but for longer lasting contacts the lower the better (to a certain extent).
  6. SgtWookie


    Jul 17, 2007
    Connect one of the switch leads to the data acquisition board's COM terminal (common aka ground)
    Connect the other switch lead to one of the data acquision board's signal input terminals.
    Additionally, connect a 10k Ohm resistor from that same signal input terminal to the V+ supply of the data acquisition board.

    If the switch is open, the signal input terminal will have V+ on it.
    If the switch is closed, the signal input terminal will have 0v on it.
  7. Spogpean

    Thread Starter New Member

    Feb 5, 2008
    cheers bud. :)

    just to be clear, there will be a voltage flowing when there is no water in the tank, and there will be 0 volts flowing when the tank is full?
  8. SgtWookie


    Jul 17, 2007
    Just to clear things up...
    Voltage is analogous to pressure. Voltage does not flow
    Current is analogous to a quantity of electrons. Electrons flow due to the pressure of voltage.

    You have not told us if it is a NC (Normally Closed) or NO (Normally Open) switch.
    You could find out by using a multimeter set to measure Ohms, and placing one test lead on each of the wires coming from the switch. With the float in one position, the contacts should measure infinite Ohms, in the other position, it should measure less than 1 Ohm.

    When connected as I have mentioned, when the switch is open, current will flow from V+ through the 10k Ohm resistor to the signal input terminal. Since the switch is open, there is no load, and the signal input terminal will measure very nearly the value of V+.

    When the switch is closed, it places a very low resistance between the signal input terminal and the common or ground terminal, so the signal input terminal will read very close to 0v.
  9. thingmaker3

    Retired Moderator

    May 16, 2005
    As SgtWookie has noted, current will flow when the switch is closed, but not when the switch is open. The question then becomes whether the switch is open or closed when the tank is full.

    If you have a meter, you can check the leads with the meter on the "Ohms" setting.

    If not: Hook it up. If it works backward, reverse the switch leads.