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Using three TIP3055 transistors for RGB LED switching. Collectors are tied to cases. They need to be heat sunk and everything's already mounted and assembled and I don't have time to get rubber isolators.

Surprisingly it seems to work if I tie the collectors to ground and put the LED loads between the emitters and +12v. Running just a few LEDs right now to test the Arduino circuit, but I'll have around 4-5 amps of current through each transistor when finished. Is this safe or am I going to have a meltdown when I ramp up the current load?

Chris Stratton
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John Meiers
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    Schematic where? – Ignacio Vazquez-Abrams Aug 14 '14 at 19:28
  • A biploar transistor with emitter and collector swapped is still a transistor in terms of the junctions it has... but not a very *good* one as the geometric optimization is backwards. If it works without underperforming/overheating, it probably means your chosen part was overspec'd to begin with. – Chris Stratton Aug 14 '14 at 19:36
  • @IgnacioVazquez-Abrams it scarcely matters. The question really hinges on device properties, the arrangement is trivial from the wording. – Chris Stratton Aug 14 '14 at 19:36
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    @ChrisStratton: Train them early, train them often. – Ignacio Vazquez-Abrams Aug 14 '14 at 19:40
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    @IgnacioVazquez-Abrams - no. What has general utility about the question is the fundamental 'can I put it in backwards' part - circuit diagrams would only distract from that. But a diagram of how a transistor is actually *fabricated* could be a great part of an answer. – Chris Stratton Aug 14 '14 at 19:43
  • http://electronics.stackexchange.com/questions/33303/what-is-the-difference-between-emitter-and-collector-for-bjts and especially http://electronics.stackexchange.com/a/29757/7348 - for engineering purposes the latter is a duplicate, but this question probably needs some explanation to make the applicability of that answer fully apparent. – Chris Stratton Aug 14 '14 at 19:56

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If you have the following:

schematic

simulate this circuit – Schematic created using CircuitLab

Yes it will work. This operating region of your BJT is called "Reverse active region". Now is it a good idea? Well... it will work but you will get crappy performance, since the current gain will be much lower. Also, the component may overheat (that region is rarely used and your device is not designed to be operated in that region). It is more a side effect of BJT's internal than a real expected usage. Usually, it is used only as failsafe in some circuits so you don't have to put diodes to discharge another part of the circuit or to do very clever circuits with very little component count. There is no point in you doing this, really.

Mishyoshi
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    While technically equivalent, this is a really confusing way to draw the circuit. It would be much better if you could talk not about the data sheet property, but *how* the collector and emitter differ. – Chris Stratton Aug 14 '14 at 19:45
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    I'd like to, really. But I'm no semiconductor specialist at all. I understand how it works and side effects, but I'm very likely to provide a wrong explanation, so I prefer providing none to providing a bad one :) Sorry. But feel free to add another more detailed answer or complete mine if you feel like it. – Mishyoshi Aug 14 '14 at 19:51
  • Its good you put up what the operating region is called though. It is a good lead for anybody to go research should an answer explaining what Chris stated not come about. – Funkyguy Aug 14 '14 at 19:54
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    Actually, this is well outside the specs for the transistor. According to the datasheet, the maximum emitter-base voltage is just 7.0 volts. – Dave Tweed Aug 14 '14 at 20:20
  • But wouldn't emitter-base voltage be quite low? If there is a resistor, most of the voltage will be undertaken by the resistor, not the transistor, isn't? If there is no resistor than the diode may just fry... Arduino runs of 5V (gpio) and Vbe is a voltage drop of 0.7V typical, so we are still in safe zone. – Mishyoshi Aug 14 '14 at 20:26
  • I have it wired exactly as shown, Mishyoshi (thanks!) although the LED resistors are slightly different based on the color of each LED, and there's a 1k ohm resistor between the base and the Arduino output. It seems to drive the LEDs at full brightness but there are only 9 LEDs hooked up in the test circuit. I'm going to try it tonight with all LEDs hooked up (450 or so) and see what happens! – John Meiers Aug 14 '14 at 23:40
  • correction - 150 LEDs on each transistor, 450 total. – John Meiers Aug 14 '14 at 23:45
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    @Mishyoshi - when you deactivate the transistor all the supply voltage will be reverse biasing the emitter-base region and then it will likely fail. This technique works for low voltages and low powers. – Andy aka Aug 15 '14 at 08:34
  • @Andy aka. Yes you are right didn't think thoroughly enough, my bad. – Mishyoshi Aug 15 '14 at 15:58