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Simple transistor question

Discussion in 'General Electronics Discussion' started by ross135, Apr 12, 2011.

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  1. ross135


    Apr 12, 2011
    Hi everyone,

    Very new to electronics and just need some once off help, I think. I am a software developer but I require some indication lights for my Windows based software. I have picked up a thing or two and programmed a pic12f683 to receive commands serially and turn pins high based on the command. Everything is working, but I obviously cannot add a real load to one of these pins since they are only 25mA and 5v.

    I have 2 Strobe Beacons that I want to connect to each pin, they are 12v DC and 800mA each so I need an NPN Transistor I just have no idea which one! I dont think a relay is required, since it is only lets say 1Amp (800mA so 1Amp to be safe?) I know how to wire up the transistor and such, I just don't know which transistor and what resistor to use with it.

    Can anyone be so kind as to advice me on a part number suitable for my needs?

    Thanks in advance,
    Ross Steytler.
  2. tip120


    Sep 24, 2010
    The easiest thing to find would probably be something like a 2N3055, way overpowered for this application but you can pick them up almost anywhere, including radio shack, and they're cheap.
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    2N3055 will probably require far too much base current for the PIC to supply.

    I would probably look for a logic level mosfet (e.g. 2SK401) or a darlington transistor (e.g. TIP110), or a medium power device like a BD139.

    If you choose a bipolar device you will need a base resistor (chosen to supply the correct base current -- use 180R or higher for the darlington). If you use a MOSFET then it is also typical to have a gate resistor (and 220R would be appropriate)
  4. tip120


    Sep 24, 2010
    That thought actually crossed my mind, and I tried to check the datasheet to see but I wasn't really sure what to look for. Sorry about the bad advice!
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    There's a number of things to look for, and a calculation to make.

    First the calculation. The OP wants 800mA collector current, and has around 25mA of current available from the uC. This means a gain of 800/25, or 32 is an absolute minimum requirement. However with just this gain, the transistor will not be saturated, so the voltage across it (Vce) will be higher, and thus so will the power dissipated. Additionally this means that the load will not see anything near the full supply voltage.

    Now the specs. We look for HFE in excess of 32 (lets say better than 50 would be nice) AND with it specified for a collector current close to 800mA.

    A 2N3055 has the following specification: Ic = 4A, Vce=4V, HFE between 20 and 70. The problem here is that the load currents are expected to be far higher, and the specs don't give us something we can use at lower currents. Whilst a gain of 70 seems fine, the minimum is 20.

    Now it just so happens that I found a 2N3055 datasheet that gives graphical information for Ic = 1A (fig 4 on page 3). Contrary to expectation, we can see that with 20mA of base current and 1A of collector current, we can expect a Vce of under 0.4V.

    If we look at my recommendation, a BD139, we can see from Figure 1 on page 2 that we should typically see a gain of around 60 at Ic of 800mA and Vce = 2V. In this case there is no graph of Vce at various Ic (they give Ic vs Vbe at Ic=20 Ib -- which is not what we can achieve. (incidentally it shows about 0.3V for 800mA Ic, so we will not get that far...)

    If we look at a darlington, then gain is not going to be an issue, however the Vce is unlikely to be below 1V. The datasheet for the TIP110 shows a gain of well over 2000 at Ic = 800mA at the sort of temperatures you're likely to see (Fig 8, page 4). However Figure 9 on the same page shows you that you would be lucky to see a Vce under 0.8V.

    The 2SK4017 is a mosfet, so we look at different things. Looking at Id Vs Vds (first graph on page 3), all the lines ar pretty close together at Id = 1A, but for Vgs = 4V (we will have 5) the Vds is around 0.1V.

    The surprise is that the 2N3055 is actually a possibility, and may be better than a BD139. The real take-home fact is that the mosfet will be way better than any other option in this case.

    Also please note that I selected these transistors from a very cursory look at some broad specs provided by Digikey, and selected mostly based on price. You could do better, one hopes, by doing a more thorough examination of the available devices.
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