using capacitor to power relay for brief period (no startup delay)

Discussion in 'General Electronics Chat' started by rp_guy, Jan 21, 2011.

  1. rp_guy

    Thread Starter New Member

    Jan 21, 2011
    2
    0
    hi,

    i'm trying to build 2 12VDC circuits, both fairly easy i'm assuming.

    1. the first is to prevent my HID's from flashing on crank (as + is disconnected from the relay for 3-5s). i'm assuming a capacitor, wired correctly, could prevent this (by continuing to provide + across my relay for a brief period of time after + is disconnected).

    in this circuit, the time delay from when + is applied is not important. where would I place the capacitor in reference to my relay to do this? would wiring in series with 85 pole be sufficient?


    2. my second circuit would be providing positive to a rear view camera wire. the wire is supposed to see positive when i shift into reverse (to change the output on the screen to the camera). i want to provide + to this wire for approximately 5 seconds after i shift out of reverse (so that the screen stays on the camera output).

    the time delay is much more important, as I need the camera to switch as soon as i shift into reverse. is it possible to wire a capacitor in parallel with 85?


    thanks
     
  2. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    I would discourage messing around with the HID lamps on a vehicle; it is also probably illegal, and against forum rules.

    #2 seems okay though, would have to get moderator approval. I would initially suggest a 555 timer of some sort.
     
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    907
    If you do the calculation for the amount of time a capacitor can power anything of significance, you will find you need either a large capacitor or very small drain. A better way to "keep on" is to use a capacitor to power a very low drain switch. The switch (e.g., a mosfet) powers the load. A resistor and small capacitor controls the switch.

    John
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    HID Headlights draw a good deal of power, and are turned off to give that extra power to starting the engine.

    Modification of lighting is also a safety hazard.
     
  5. rp_guy

    Thread Starter New Member

    Jan 21, 2011
    2
    0
    HID headlights do not draw any more past startup than a halogen bulb (and in fact, 35W low beam HID's use less power than 55W halogen bulbs). the OEM system on my car stays lit, i am inquiring about a circuit that i built using an accessory wire on my car (which clears power during crank). the accessory wire runs to a switch, which then goes to the 85 pole on a relay. the relay powers the HID ballast for HID foglights. these are not for providing power to my PRIMARY lights (which the forum has rules against).

    regardless of this, for circuit 1, i want to know how to provide continuous power to a relay for a matter of 5 seconds after power is cut.

    circuit 2 being similar, i want to provide power to an input wire for a similar amount of time. however, i need this to happen without the "fill" delay of a capacitor.

    i'm intentionally avoiding 555 circuits because i want the simplicity of using only a capacitor.
     
    Last edited: Jan 21, 2011
  6. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    This should help you:

    http://www.electronics-tutorials.ws/rc/rc_2.html

    A Bosch style relay coil is 75 ohms. Because T = RC, coil resistance is important. Measure yours to get good results. You will also need to know its dropout voltage too. Most electrolytic capacitors have very loose tolerances. Maybe Tantalums are better but this could get pricey. Once again, measure yours to get good results.
     
    Last edited: Jan 21, 2011
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    907
    I thought the OP wanted to use only a capacitor, not a capacitor + relay. BTW, the relay will probably drop out before RC. That is why I suggested a mosfet. Same principle; much smaller capacitor.

    John
     
  8. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    Twice, OP refers to terminal 85 on 2 different relays. Terminal 85, one side of the coil, pretty much makes these Bosch style relays.

    No doubt, mosfets would make sense.
     
  9. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    Firstly with the HID, do you know what the operating voltage range of the HID is? Even if some control relay stayed on during crank, the HID may have a problem with say 8V from the battery?

    Relay drop out voltage is relatively low for a 12V relay. Say it was 40% of your initial capacitor voltge of approx 12V prior. A diode would isolate the capacitor and relay coil from + supply. Each second of capacitor ride-through duration would require about 13,000uF (16V rated cap) for a nominal 75R resistance.

    The same general design concept would apply to a reversing camera. The diode provides fast initial supply to the relay coil and capacitor, and isolates the coil-capacitor for a delay period after.

    A large capacitor increases the initial surge current through any upstream control switch (as well as the diode), which may reduce the service lifetime of that switch.

    Tim
     
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