Which SMT transistors to use?

Discussion in 'General Electronics Chat' started by nevermind1281, Mar 10, 2008.

  1. nevermind1281

    Thread Starter Member

    Dec 4, 2007
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    I need SMT versions of 2n2905 and 2n3053 transistors. Would 2n2222 work for 2n2905? And would both 2n3904 and 2n3906 work as well as 2n3053? Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    All you have to do is satisfy the parameters of the load each transistor drives. If the transistor will handle the voltage and current, then many substitutes are possible. I do seem to recall that a 2N2905 is PNP, so a 2N2222 would not sub.

    Look for the current carefully. The 2N2905 and the 2N3053 are in TO-5 (now TO-39) cans. They are rated for quite a bit more power than 2N2222's and 2N3906's. If they are used as saturated switches, though, this is not as large a problem as it would be if they were operating in the linear region.

    Zetex makes quite a number of TO-92 transistors that have impressive current specs. The ZTX651 and ZTX751 might substitute for your TO-5 transistors. If you clip and bend TO-92 leads, you can stick them on a set of smt pads. Just remember to keep the lead functions straight, so you don't swap emitter with base.
     
  3. SgtWookie

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    Jul 17, 2007
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    2N2905 = PNP, Vce=40, Ic=600mA, hFE > 75
    2N3053 = NPN, Vce=40, Ic=700mA, hFE > 50

    2N2222 = NPN, Vce=30, Ic=800mA, hFE > 50
    2N3904 = NPN, Vce=40, Ic=200mA, hFE > 30 (works better with small signals)
    2N3906 = PNP, Vce=40, Ic=200mA, hFE > 30 (works better with small signals)

    You'll have to look at the datasheets to be certain.
    You might go on Mouser.com and try their transistor search function. It can be very handy for finding substitutes. You can select whatever parameters are most important to you.

    Fairchild Semiconductor has a similar search engine.
     
  4. nevermind1281

    Thread Starter Member

    Dec 4, 2007
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    Could the 2n3904 replace the 2n3053? And could 2n3906 replace 2n2905? I came across an ebay auction for 100 2n3904 and 100 2n3906 in SOT-23. I beleive I have used them both before for the circuit I am trying to make, but I just want to double check that they will work. Heres the circuit they will be for. Thanks:)
     
  5. SgtWookie

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    It's difficult to tell whether or not they will work by that schematic, because there is absolutely no intel about the current requirements of the load for T1. The 1k8 (1.8K) for T2 suggests that if it's a reasonably low-voltage logic level, the 2N3906 would probably work OK for substitution for the 2N2905.

    What you might consider doing is laying out your board so that you could put three or four of the T1's in parallel, with fairly low-value resistors in series with the collectors and bases. That way, if a single 2N3904 proves inadequate (with jumpers across it's base & collector current limiting resistor pads) you could remove the jumpers and parallel several of them in it's place. Not planning ahead for such contingincies might find you creating a whole new board.
     
  6. nevermind1281

    Thread Starter Member

    Dec 4, 2007
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    I will try it with the 2n3904. When I made the circuit with radioshack transistors, their 15 pack is listed as 2n2222, but when I first looked at the transistors there were several different kinds, one of which was 2n3904. Their 15 pack of PNP transistors is labeled as 2n3906, but they all have different codes written on them.

    If the SMT 2n3904's are inadequate I will post back. Thanks for the help!
     
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    It looks like RS has fallen even another step. The 2N2222 clearly says 2N2222, not "like" or "similar to" 2N2222. The PNP link says,

    "Your source for transistors.
    PNP small-signal type amplifying transistor, similar to 2N3904. 350mW dissipation."

    That's a little confusing. Bottom line, at this point, I don't know that I would trust the RS product to be what it is labeled.

    John
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, I've purchased some NPN and PNP "transistor assortments" from RS.

    You generally get a selection of various transistors, and considering the overhead the store has, they're actually a decent bargain. I've never had to pay a shipping charge from RS, only "schlepping charge"; aka the cost of getting there and back ;)

    One has to inspect the transistors in the assortment, look up the datasheets, and test them. That's part of the fun.

    Some of the transistors might actually BE PN2222 transistors. I've also found 4401's, etc in the assortment. It's sort of like opening up a Christmas present for nerds ;)

    I DO have some favorable things to say about RS resistors.
    I have a bunch of NOS (new old stock) RS resistors in my collection, mixed in with some supposed mil-spec 1% resistors. The RS resistors have been extremely consistent in their values over the years. The mil-spec 1% tolerance resistors have varied considerably.

    The RS resistors have proven to me to be highly stable in their values.

    I have YET to find an RS-purchased NOS resistor to be out-of-tolerance, even though it may be 20+ years old.

    This does not include pots. I found a 10k pot that I had in my collection that was around 30% below spec. Even though I had owned it for over 20 years, my local RS store exchanged it for a new one, no questions asked - it was obvious that it had not been installed. Expect resistance (sic) if you have soldered leads to it.
     
  9. nevermind1281

    Thread Starter Member

    Dec 4, 2007
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    I received the transistors and made a circuit board. I used the toner method and it worked prefect. When using the circuit the right trigger stopped working but the left trigger was fine. I took the battery out of my controller and it worked again for a few minutes and stopped. Whats wrong?
     
  10. jpanhalt

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    It looks to me like the E and C connections to the right trigger may be reversed in the schematic. Try hooking up the right just like you have the left hooked up. John
     
  11. nevermind1281

    Thread Starter Member

    Dec 4, 2007
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    The transistors I have are sot-23, I have the correct wires going to the correct places on the transistors (base, collector, emitter). The schematic works, but after a few minutes it stops working but works when power is disconnected then reconnected. I made one mod with both triggers, then one that only used T2, then one that only used T1. Could it be the capacitors? The 1uF is 35v and the 0.1uF is 25v.
     
  12. jpanhalt

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    That's quite a conundrum. If the schematic is correct and all the wiring is correct, then why doesn't it work?

    Since the two transistors are wired differently to their respective controller triggers, I was simply questioning whether there was a typo in the schematic. Since the left one works with the emitter to the trigger, I applied Sutton's Law and proposed that perhaps the emitter on the right transistor should also go to the trigger. I have no idea whether the triggers are wired in series or parallel, which could make a difference. I am assuming they are basically paralleled inputs to the 555.

    John
     
  13. SgtWookie

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    If you look closely, you'll notice that T1 is PNP, T2 is NPN. If they were both hooked up the same way, one of them would not work.
     
  14. SgtWookie

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    OK, what is the EXACT model of 555 that you're using?

    With the base resistors being T1=100 and T2=220 Ohms, you'll be getting somewhere in the range of 40mA max source/sink from the 555. This would be OK for a bipolar 555, but perhaps not for a CMOS version, and particularly not the TLC555 (it can sink up to 100mA, but sources only 10mA). You might try increasing R4 to 390 and R5 to 510 Ohms. You DID use two diodes in series for the two D1's, right? Each should be dropping about 0.6-0.7V across themselves.

    Did you use electrolytic caps for C1 and/or C2? If so, do you have the (-) terminals going towards the 0v from the controller? Are they rated for at least 10v?
     
  15. jpanhalt

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    Thanks, SgtWookie. I didn't look that closely at the parts list to notice the difference. John
     
  16. nevermind1281

    Thread Starter Member

    Dec 4, 2007
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    I am using 555 timers form Maxim. They are ICM7555, heres their page on them. I will try the alternate resistors next time I can get to a radioshack.
     
  17. SgtWookie

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    Jul 17, 2007
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    No, replace the ICM7555 with an LM555.

    Link:
    http://www.radioshack.com/product/i...&cp=2032058.2032230.2032279&parentPage=family

    There is nothing wrong with the ICM7555, but it is not the correct timer IC to be using in your application. It is only rated to source or sink 1mA, which is nowhere near the 40mA your circuit requires. It's overheating and shutting down. When you disconnect the trigger from power, it gives the circuit a few moments to cool down before you plug it back in; that's why it starts working again.

    If you increased the base resistors high enough to be within the ICM7555's limits, the transistors wouldn't have enough output to trigger your game.

    A standard LM555 is a bipolar device which can source or sink 200mA. You won't have to change anything else for that IC to keep your circuit working.

    If you're careful when you unsolder it, you can use it in a different project.
     
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