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Need help calculating correct resistor…

Discussion in 'General Electronics Discussion' started by SaltyDog, Apr 11, 2012.

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  1. SaltyDog


    Apr 11, 2012
    Hi all,

    It’s been over ten years since I last did any kind of electronics math. But I’m going to be using a 2N2222 Transistor that has a current rating of 800mA.I’d like to know what size resistor i’m going to need to drop the current to 700mA. The voltage used would be 12v.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    R=V/I but that alone is not very useful without knowing the circuit. If the full voltage (12V) is acros the resistor, that equation can be used directly. However, if there are any other circuit elements in the current path their current-voltage characteristic has to be taken into account.
    If you use the 2N2222 just as a switch, then the voltage drop between collector and emitter is ~1 V as long as the transistor is operated in saturation (base current >= Ic/40). This voltage drop is not available at the resistor, so the voltage across the resistor is only 11 V.

    Beware of the power loss in the transistor. At Vce=1V and Ice=0.7A, the total power converted to heat in the transistor is at least 0.7W (not counting the contribution by the base current). As the 2N2222 in TO-92 case has a thermal resistance of 200 °C/W (Junction ambient), this power will heat up the junction to 0.7*200 °C = 140 °C above ambient temperature. Add 25 °C for ambient temperature and the junction will be hot at 165 °C. Max. junction temperature is 150 °C, so you will need a heatsink.
    Calculate backwards:
    150 ° (max) - 25 ° (ambient) = 125 ° max. temperature drop across the thermal resistance junction-ambient. Since the case has an irreducible thermal resistance of ~84 °/W junction-case, there is a temperature gradient of 84 °/W * 0.7 W = 59 °C acros the case. So for the heatsink the remaining temperature difference is 125 ° (from above) - 84 ° = 41 °. SO the heatsink's thermal resistance must be <= 41 °/0.7W = 59 °/W.

    Note all temperatures in ° C!

  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    It all depends on your load. Will it draw more than 700mA?

    Are you sure the transistor will be able to withstand this current for any period of time? At 700mA, even assuming the Vcd(sayt) at 500mA, you get a dissipation of 700mW which exceeds the 625mW dissipation limit for this transistor. This ignores the additional dissipation caused by the increased Vce(sat) and that due to Ibe.

    So it's not going to work (without the risk of smoke).

    I'd recommend either a darlington in (say) a to-220 package, or a mosfet.

    However it all depends on what the load is.
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