I'm going to use a 2n3773 (http://www.onsemi.com/pub_link/Collateral/2N3773-D.PDF) to drive a 10A load but the problem is that at 10A the base current required is around 1A (pretty awful current gain) which would likely provoke the magic smoke to come out of my 555 timer if driven directly. My initial thought was to use another NPN transistor to drive the 2n3773 (see image) but I'd need the resistance of R2 and R3 combined to be less that 5 ohms in order to get 1A to the base which means that I'd have a <= 5 ohm short to ground when the driving transistor is on, which of course isn't good. I guess the solution would be use high side switching so that R2 would be between the transistor and GND and could be any value. Unfortunately this means I would have to use a PNP driving transistor (or could a get away with an NPN?) and PNP's have a habit of blowing up when I try to use them. Anyone have a solution?
Yes the large base current does cause an issue - either you wire as a Darlington pair (and increase the saturation voltage for the main device), or have to dissipate a lot of power in the driver transistor. Having the driver transistor powered from a lower voltage than the load helps a lot.
In new designs such a transistor is never used, MOSFETs are superior in every department (well IGBTs for high voltage) and that 3773 is probably only manufactured for legacy hardware.
Incidentally your driver circuit is a poor choice - the driver transistor should be an emitter-follower, not a common-emitter stage (very power inefficient). Typically the driver transistor would need to be mounted on the heatsink alongside the power device.
wire as a Darlington pair
Wow I should get some sleep, I guess I'm completely brain dead. Can't believe I didn't think of a darlington pair. I would usually use MOSFET's but I don't have any on hand that are powerful enough.
Well as I said Darliington's have the issue of very large saturation voltage (2V or more is not uncommon for instance). That's 20W to get rid of at 10A...
Well, it's certainly better than my original idea. If you want to share an emitter-follower driver circuit, it would be appreciated. If my understanding is correct, you mean something like in the picture? Again, PNP's never work for me so I'm sure about this.
Not So... With a Darlington the base drive would be on the order off 10 mA X 1000 (Hfe) = 10A. A 330 ohm base resistor and a TIP140 is a 10A NPN Darlington with a Min Hfe of 1000. The real issue is that it requires 1.4 volts base to emitter to fully turn it on. A single device solution with one 1/4 watt 330 ohm resistor. If the Emitter of the transistor is grounded... I would run a small wire from the 12V ground to the transistor emitter to your Arduino and keep the other wiring to the transistor and load separate form the Arduino. You really don't want the ground current for the high current load to flow in the Arduino ground path. A wire from the Arduino ground to the emitter of the power transistor (return) and a wire from the switched port to the resistor connecting to the base of the Darlington transistor (control).
Doc
bobthebanana:
Well, it's certainly better than my original idea. If you want to share an emitter-follower driver circuit, it would be appreciated. If my understanding is correct, you mean something like in the picture? Again, PNP's never work for me so I'm sure about this.
No, look up emitter follower on wikipedia - an NPN emitter follower sources current, just what is needed.
Ah, I see now. So it's kind of like what I was getting at in the OP, where I would reverse the circuit and put the resistor on the bottom so that it could be any value. I just tried it and it seems to work fine.
Good!
BTW I find it useful to have a few n-channel logic-level MOSFETs "in stock" as it were - handy to have to hand.
A handful of BS170's, 2N7000's, or VN2222's are great too.
Doc