Passing AC current through a 12 Volt DC Solenoid

Discussion in 'General Electronics Chat' started by Bikegoddess, Dec 19, 2016.

  1. Bikegoddess

    Thread Starter New Member

    Dec 19, 2016
    5
    0
    I understand using an AC voltage to control a DC solenoid will cause it to chatter or possibly not function at all. I read the following post:
    http://forum.allaboutcircuits.com/threads/ac-through-dc-solenoid.31610/

    What I want to do is use a car starter solenoid 12 volt DC (with an AC to DC converter) to control a current that remains AC.

    I'm making a battery tab welder like one of the many tutorials on You Tube. The thing is is that I want to have multiple safety switches to control it an ensure that things don't run away, or hurt me or somebody else, who happens upon it. I know I can buy one for $150 But I can build this with two solenoids for near nothing. What I'm worried about however is what a low ac voltage (5-10 volts) but high current (several if not many hundred amps) would do to a DC solenoid. I understand a little bit about magnetic fields and other stuff like that I learned in my engineering classes, but I didn't finish my degree and don't work with this stuff outside my work shop so can somebody explain if this idea won't work?

    One of the big reasons I want to use the solenoid is to put a center tap in the transformer so that I can have a high and a low setting on the welder, which will require two solenoids and a little extra wiring.
     
  2. crutschow

    Expert

    Mar 14, 2008
    22,029
    6,371
    There should be no problem with running AC through a solenoid that normally carries DC.
    Generally relay contacts are rated for more AC current then DC current since the arc upon the contact opening only lasts until the AC current crosses the zero point of the AC waveform.
     
  3. JUNELER

    Member

    Jul 13, 2015
    181
    25
    Hi,
    can you send the schematic drawing of your circuit, coz a lot of variety of circuit of such battery tab welder on the youtube. your drawing can help me to understand what you like to do.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    17,917
    5,477
    Automotive starter solenoids come in continuous rating and intermittent rated types.
    Generally 85amps continuous.
    If you only have a 12v AC source for the coil, just use a bridge rectifier to source them.
    Max.
     
  5. BR-549

    AAC Fanatic!

    Sep 22, 2013
    4,826
    1,315
    A relay is an electrical circuit that controls another electric circuit. Which one of these circuits are you taking about in relation to AC.

    Do you want to use ac to control a relay?

    Or do you want to pass ac with a dc relay?

    Draw a picture.
     
  6. BobTPH

    Senior Member

    Jun 5, 2013
    1,650
    405
    Using an AC voltage will, of course, result is far less current in your solenoid than the equivalent DC voltage.

    Bob
     
  7. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    7,767
    2,183
    A bit of confusion for some is that an automotive solenoid is what the rest of industry calls a contactor, which is a relay with contacts rated for relatively high current ans with a different mechanical arrangement. Whatever the name, all of these devices have two separate sets of specifications, one for the coil and one for the contacts. In your case, the coil must be energized by a DC source, while the contacts should have no problem switching a low voltage, high current AC source as noted above.

    ak
     
    mazhur, cmartinez and shortbus like this.
  8. WBahn

    Moderator

    Mar 31, 2012
    24,087
    7,476
    If you are going to have the solenoid energized for more than a short period of time, then be sure you have one rated for continuous duty. I learned about that the hard way when I rigged a dual electrical system, dual battery system in my Bronco back when I was eighteen and used four solenoids to control which battery was connected to which system. Went through solenoids very quickly. That's when I discovered RV battery isolators, which might be better suited for your application.
     
  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
    6,464
    3,788
    Like AK said if your using a Ford type starter "solenoid"(really called a starter relay), just apply 12V DC to the coil and the AC through the contacts. This is what a Ford starter relay/solenoid looks like. The mounting feet of it are the ground side of the coil.

    [​IMG]
     
  10. Bikegoddess

    Thread Starter New Member

    Dec 19, 2016
    5
    0
    This is what I have on mind.. Im sure some of my symbols are off and the 12VDC converter is a black box but. 20161219_100251.jpg
     
  11. Bikegoddess

    Thread Starter New Member

    Dec 19, 2016
    5
    0
    I think you all have answered my question. Using a solenoid is okay for this application.

    Thank you!
     
  12. Bikegoddess

    Thread Starter New Member

    Dec 19, 2016
    5
    0
    I forgot the LED and resistor in that is going on the circuit after the solenoids. I also put the LED and the resistor backwards where I food put it.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
    17,917
    5,477
    You will find that especially aftermarket types will specify continous or intermittent rating.
    Max.
     
  14. Bikegoddess

    Thread Starter New Member

    Dec 19, 2016
    5
    0
    Fixed. 20161219_102846.jpg
     
  15. shortbus

    AAC Fanatic!

    Sep 30, 2009
    6,464
    3,788
    Exactly. But the intermittent ones do work for fairly long periods, at least the OEM ones. Years ago I made a winch for a Jeep using a Ford Y block type starter. And four Ford solenoids in a Hbidge configuration to control direction. Used it many times pulling for over ten minutes at a time. The motor and solenoids never failed, and they were scrap yard items.
     
    mazhur and cmartinez like this.
Loading...