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Let's say I have a transistor which is rated for maximum 800mA current at 50V. Is it possible to have it running at 1000mA & 5V for example?

Edit : please provide me some details in your answer by using the BC337 as sample. Here is its datasheet https://www.onsemi.com/pub/Collateral/BC337-D.PDF

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    \$\begingroup\$ Lookup the term "SOAR", Safe Operating Area. \$\endgroup\$ – Sredni Vashtar Jul 22 '17 at 8:08
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    \$\begingroup\$ First of all, it is not rated for 50V 800 mA. It is rated for 50V when it is off, and 800 mA when it is on. But not for both at the same time. Second of all, it is probably not rated for 800 mA under real world operating conditions. Please edit your question to include the manufacturer and part number for the transistor. A link to the datasheet might also be convenient. After you do that, someone may right a much better answer explaining everything in more detail. \$\endgroup\$ – mkeith Jul 22 '17 at 20:19
  • \$\begingroup\$ "Write." I meant to type "write." \$\endgroup\$ – mkeith Jul 22 '17 at 22:12
  • \$\begingroup\$ @mkeith : Editted, please take a look and help me clear my confusion about transistors. FYI, my knowledge is basically zero at this electric field. \$\endgroup\$ – Huy.PhamNhu Jul 23 '17 at 3:45
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Based on the edited question, now that we are talking about the BC337, I will make comments specifically about that part.

First of all, the section at the beginning labeled "Maximum Ratings" should not be used for design. In other words, if Vceo maximum is given as 45V, you should avoid exposing the IC to 45V. Maybe stay at 40V or below.

Also, note that you should avoid all of the maximums. You have noted the voltage limit and the current limit, but you should also note the power limit. In this case, there are two power limits, which give different conditions. One applies when Ta (ambient temperature) is 25C, and the other applies when Tc (case temperature) is 25C. For our purposes, we can ignore the Tc limit because it is essentially impossible to keep the case at 25C when the transistor is dissipating a lot of power.

The second thing to note about the Ta limit is that it only applies when ambient is 25C. See the note which says the limit must be derated by 5mW/C. This means that for every degree above 25C, you must subtract 5mW from the limit. For example, if the ambient temperature is 45C, that is 20 degrees above 25C. So the limit will be 625mW - (20 * 5mW) = 525 mW.

The power dissipation is Ic * Vce + Ib * Vbe. So you will have to calculate that with your specific operating conditions and see if it works out.

But if you actually need something like 800mA, I suggest you look at using a MOSFET. The DC gate current on a MOSFET (analogous to Ib) is negligible, and the voltage drop from drain to source (analogous to Vceo) is typically much smaller, so the MOSFET doesn't need to dissipate as much power.

Hope this helps.

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    \$\begingroup\$ Very insightful answer for a noob like me, thank you :) \$\endgroup\$ – Huy.PhamNhu Jul 27 '17 at 19:00
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Not only can't you do that, but very likely you can't run it with 800 mA and 5 V at the same time either.

Typically the voltage rating is the maximum voltage the transistor is guaranteed to block in the cut-off state.

And the current rating is the maximum current in a fully saturated state (BJTs) or in the triode region (MOSFETs).

When operating in the forward active region (BJTs) or saturation region (MOSFETs), the maximum power limit is likely to give a lower limit on current & voltage than either of those limits.

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  • \$\begingroup\$ may I know what will happend with that rated 800mA transistor when use in a circuit which have a power source can deliver 2000mA, a load is requesting 1000mA. Will the transistor blown up or it just lettting near 800mA go through? \$\endgroup\$ – Huy.PhamNhu Jul 22 '17 at 5:15
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    \$\begingroup\$ Like the other answer said, if you don't run it near the maximum rated temperature, it's likely to work, at least for a while. But there's no guarantees, and it could fail at any time. \$\endgroup\$ – The Photon Jul 22 '17 at 5:17
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If it's rated for 800mA, that's at any voltage (within the rated voltage limits). There is of course a chance that any given 800mA-rated transistor may work for a while, or possibly even forever, at 1000mA, but you can't count on it. The specifications are the conditions under which the manufacturer guarantees that it will work.

If you're willing to exceed the specifications, having a better heat sink is going to do more for allowing the transistor to run at higher current without damage than a lower voltage will.

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