DC relay question

Discussion in 'General Electronics Chat' started by rainyday101, Sep 24, 2009.

  1. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    Hi,
    I am building a rocket launch controller that is powered by a 9 volt battery. The control circuitry would energize a 9 volt relay and the contacts for this relay would provide the current to the rocket igniter. The problem I am having is with relay contact ratings. Not many relays give decent DC contact ratings. I know ohms law doesn't apply to contacts as DC is much harder on the contact. A typical Estes rocket igniter will draw 12-13 amps for about 150 milli seconds. Most of the designs I see on the internet for launchers are using no relay and switches rated at 2 to 2.5 amps. I question the longevity of these designs. I can't seem to find a 9vdc relay with decent ratings. Solid state relays are pricey. Does anyone have an idea?
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    What is the value of the DC voltage the igniter works?
     
  3. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    Typically 6-12 vdc. The igniter requires a minimum of 2A, but test show 12-13A is the typical draw. I am trying to fit this all into a small hand held control box with a 9 volt battery compartment. If I did 12vdc and a larger enclosure it would be alot easier to accomplish. Any solid state options?
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    You can buy 9V coil relay which can handle 20Amps at 250V AC. This will work fine because 250V AC relays work with 30V DC too.

    Another option is 12V coil car relay which can handle 20Amps. They are designed to break DC currents. Maybe 9V is enough to pull it in.
     
  5. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    Thanks for the info, I will look into both of these.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I'm afraid that a 9v "transistor" battery has so much internal resistance that it's output voltage would plummet when subjected to such a heavy load, as the igniter would act as nearly a dead short across the terminals. Those batteries are made from six (in some cases, seven) "AAAA" batteries, which are tiny.

    This would cause a semiconductor such as a MOSFET to turn off, or a relay's coil voltage to fall low enough that the relay dropped out.

    You might use a Darlington transistor like a TIP140-TIP142, but you'll automatically lose a volt due to the Vce.

    I suggest that you would be far better off using four to eight AA NiMH rechargeables or alkaline batteries.

    Don't forget to follow NAR guidelines. Their website is here: http://www.nar.org/

    Your launch controller must have a safety interlock; removeable is preferable - like a key switch. The person connecting the controller to the rocket carries the key with them.

    It should have a visual, and preferably also audible warning when the controller is armed. An LED w/resistor and a small piezo buzzer would take care of those items.

    It is a good idea to have two launch buttons that are separated by some distance, to help reduce the chance that a single button represents to be accidentally pressed.
     
  7. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    I am using the appropriate safety cicuit, a keyswitch in addition to the launch switch. Most of the small hand held controllers are using 9 vdc akaline batteries, including the Estes and Quest launch controllers. Yes it is a dead short when you hit the launch button, but only for about a 150 milli-seconds. I would use a 12 volt source, but I want to keep this thing in a nice pretty handheld control box with nice rubber molded sides. I see a few rocket sites selling there own controllers and they use 9v batteries or 4 AA batteries. They are small enough there can't be a relay in them and the switches are nothing more than Radio Shack 2 amp switches. I am looking at some automotive small 12 vdc relays and am wondering if 9 vdc will pull them in. The solid state solution is not out of the question! Which darlington pair would I use.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    That is good to know.

    Interesting. I bought an Estes launch controller back in 1966. It was in kit form, designed for use with a 12v automotive battery, however I usually used a large 6v lantern battery. Always had plenty of "juice" to light off the rockets. The launch switch was just a piece of springy brass. There was a screw-in light bulb to indicate continuity through the igniter, and as a warning that the controller was armed. A pull-out pin shaped like a screw-eye was the safety interlock.

    OK, but keep in mind that when that dead short happens, the relay will drop right out. It will probably buzz.
    It might, but it would immediately drop out as soon as the contacts closed. You would be better off to use a 6v relay.
    A TIP141 or TIP142 could be used, BUT you would have to make certain that the battery could not be connected backwards. The TIP141 and TIP142 have an integral body diode that will pass current.

    On 2nd thought, at 10A, the Vce will be 3v. That'll take about 1/3 of the power from the igniter. Not so good.

    Have a look at this switch:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=EG4400-ND
    15A @ 250v, has a green lens with a 12v incandescent bulb in it, under $4. Pretty small.
     
  9. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    Yea, now that I think about it, you're right the relay would drop out as soon as the contacts closed because the same power source is pulling in the relay. I checked out the switch and am going to order a couple. I never searched digi-key, just Newark and Allied. Okay so the switch will handle the current, now even if the keyswitch is rated at say 5A, would that still be okay. My thought being- the contacts on the keyswitch are not switching the load, they will be closed already when the launch button is pressed?
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Digikey is hobbyist-friendly in that they have no minimum order requirement, and they will ship SMALL orders via USPS 1st class mail. The lower shipping charges will save you money.

    I think you'll be fine using the 5A key switch. The load will be there for such a brief time, the contacts won't have time to heat up.
     
  11. rainyday101

    Thread Starter Member

    Sep 24, 2009
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    Sgt. Wookie,

    Thanks for the help, it's been a pleasure.

    Semper Fi!
     
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