339 to a 25A Relay?

Discussion in 'General Electronics Chat' started by pntrbl, May 8, 2008.

  1. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    Now that I have a 339 comparator working I'd like to use it to actuate a 25 amp relay. Being as the 339 has a 20 mA max output sink and the relay will need 200mA, I'm thinking a transistor in the middle, but I honestly don't have enough experience or knowledge to know if this is feasible or not.

    But, if so, can anyone recommend a transistor that can handle the 200mA?

    The other thing I'd like to ask about is, having recently realized the 339 is a low power comparator, is there a high powered comparator out there that could handle the 200mA? Eliminate the middleman?

    Firmly believing in the power of the internet and realizing Google is my friend, I did some poking around for such a device. What I found seemed to application specific tho. Audio this. Auto that. Not sure if I'm barking up the wrong tree or what.

    All advice welcome. I'll believe anything you tell me ..... lol.

    SP
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well, there may be some specialized comparator out there I'm not aware of - but it's probably not worth your time to track it down.

    Instead, get a TIP120. It's a Darlington transistor in a TO-220 case. It'll handle the current of your relay with 7 or so Amperes to spare. If you're in a hurry, you can pick one up at Radio Shack.

    Connect the collector of the TIP120 to the ground side of your relay's coil, the emitter to ground, and the base to the output of your LM339 using a 1K resistor. Don't forget, you'll still need a 1.2k resistor from your LM339 output to your 12v supply.

    Rule of thumb for an LM339 output pull-up resistor: 100 Ohms per Volt. 100 x 12v = 1200.
    Transistors always need a current limiting resistor on their base. Darlington transistors have extremely high hFE (gain) as the gain of the two internal transistors gets multiplied. That's the reason you use them.
     
  3. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    Thanx again Sgt. Consider yourself saluted!

    I just downloaded a TIP120 datasheet from MOSPEC and when I leave out all the gibberish I don't understand, :confused:, what's left looks more than adequate. I notice it has a metal mount that I'm sure is for a heatsink, but I'm also thinking 200 mils will hardly warm it up. Should I bother with bolting it down?

    SP
     
  4. DC_Kid

    Distinguished Member

    Feb 25, 2008
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    there has to be thousands of darlingtons in smaller cases that can handle 200mA. search Digikey.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    No, you wouldn't need a heat sink with that tiny load on it.

    Sure, there are lots of other Darlingtons that could be used. I simply mentioned the TIP120 since it could be picked up at any Radio Shack if time were an issue. Even a 2N6039 would be overkill, but odds are the OP would have to order it. Shipping costs are certainly an issue nowadays, particularly if you only need a single item.
     
  6. DC_Kid

    Distinguished Member

    Feb 25, 2008
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    i think rad shack also has some to92 darlingtons. aside from getting one fast, i was just noting that there are smaller ones that may be desired if space for the device is limited, etc..... a trip to rad shack may cost 50cents in gas today!! certainly less than a shipping charge, but 50cents just to run to rad shack, can you believe that..... gas prices.., dont get me started....
     
  7. anne

    Active Member

    Apr 20, 2008
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    I just filled my six gallon can with 2cycle mix for the summer. With daily use of the venerable ol' 50cc Puch two speed grocery/whatever getter I'll probably fill it again in the middle of August.

    Highly recommended.
     
  8. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    The Radio Shack TIP120 is wired up and working. Got a 20mA LED on it at the moment so I wouldn't say it's working too hard, lol, but I'll have to order and wait for the 25 amp relay. Even if it's only a little light I needed it to do something ....

    Thanx again to all who responded.

    SP
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Cool beans :)

    Actually, I took a look on the RS website before I'd posted earlier, and the TIP120 was the least expensive Darlington they had. Didn't see any Darlingtons in a TO92 case.

    I could've suggested that he pick up the 15-piece NPN transistor assortment, and then find a 2n3904 and 2n2222 or the like to build one - but why complicate the issue? Besides, that TIP120 is going to be pretty hard to kill ;)
     
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