Minimum switch current

Discussion in 'General Electronics Chat' started by jpanhalt, Feb 22, 2008.

  1. jpanhalt

    Thread Starter AAC Fanatic!

    Jan 18, 2008
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    Is there a minimum switch current to ensure reliable switching for toggle and push button switches?

    I am thinking of using a switch in a delay circuit with a comparator. The more current I design for, the bigger the capacitor I will need. Since the target delay is 0.5 to 1 sec. the capacitor size is not trivial on an SMD PCB with a 1 mA current. Put another way, any opinions on whether a 100 uA current is too low for a mechanical switch.

    Some relay specifications give a minimum current, but I could not find such a specification for the toggle switches (CK, E-Switch, Tyco) I reviewed. My thinking is that switches have a different mechanical action that might lead to much lower minimum currents.

    Thanks. John
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    There is no minimum current that has to flow through the contacts on a switch. The significant figure is the contact resistance, typically in milliohms. If there is a difference in potential, current will pass through the switch.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    A low current or no current switch has gold-plated contacts. They have a max power rating of about 2W. They are called a DRY contact. they cost the same or slightly more than silver plated contacts. I use them as audio switches.

    Silver contacts tarnish and don't work at low or no currents. They rely on a high power spark to burn away the tarnish.
     
  4. jpanhalt

    Thread Starter AAC Fanatic!

    Jan 18, 2008
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    Thanks for the great advice. I will use a switch with gold contacts. What gave me pause was the datasheet for some Tyco relays:
    http://ecommas.tycoelectronics.com/...pdfEnglishENG_DS_1308242_RT-DC-PB_0106.pdf

    Contact Data
    Arrangements: 1 Form A (SPST-NO) Wiring Diagram Code 1, 2,3.
    2 Form A (DPST-NO) Wiring Diagram Code 5.
    1 Form C (SPDT) Wiring Diagram Code 1, 2, 3.
    2 Form C (DPDT) Wiring Diagram Code 5.
    Material: Silver-nickel 90/10.
    Minimum Load: 12V/100mA.
    Expected Mechanical Life: 10 million operations.
    Initial Contact Resistance: 100 milliohms max @ 1A 12VDC.

    When I saw that minimum load, I thought I would never be able to match that. So, it started me wondering if there was a similar minimum for switches.

    A 100K resistor at 12V into 10 uF will do just fine and still have some current. John
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Some switches have a wiping action which prevents build-up of oxidation. Have a look at this blurb from Carling. Search for "wiping".
     
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