Hey folks, First time poster. I'll try to be as specific as possible. We have been working on our senior design project for one semester and we have a semester remaining. We came to a snag after being very successful with our design. Essentially, we're using an arduino deumilanove micro controller. We have it outputting a pulse-width modulation (PWM) signal at 5V with proportional control (will elaborate more if needed). It does a PWM at about 500hz. The problem is that we need to amplify this signal to power some TEC's, which take like 3A each (we might need about 4 for our purposes). We originally bought an H-bridge (for anyone unfamiliar, it essentially takes a small signal and amplifies it from some other source), and it worked alright at low currents, but whenever we got up to about 1A, it would fry. Its datasheet says it should support at least 3A. I was talking to an experienced EE and he laughed when I told him we were using an h-bridge, saying, "good luck!" I came up with the idea to use relays. You know, the ice cube relays (not solid state) since they have complete isolation from the leads and the coil that switches the leads. This seemed like a good fix since the leads can take something like 10A and it doesn't take much current to power the coil. By taking the signal from the arduino to power the coil of the relay, I could hook the relay leads to a different source and connect one output of the relay to the TEC and leave the other output open. (Does this make sense?) This would end up doing the job that the h-bridge should have been doing. It turns out that the arduino can only output 40mA, and the relays I used need at least 80mA to power the coil. I then thought that I could use one of those super common transistors (p2n2222a or whatever) as a switch to then be able to make the relay work on a really low signal from the microcontroller. I got that all to work with an example circuit (replacing the uC with a waveform generator so I can change amplitude and frequency) with much excitement (my partners love me right now). FINALLY, the last snag. Like I said, the arduino is outputting this signal at around 500hz. These ice cube relays can't keep up with that quick of switching. THey work fine to around 50Hz and then the unforgiving laws of physics take over and the thing just stops switching. I then figured, well... lets scrap the relay idea and just use a really high current transistor. My question is this: is this possible? I ordered two transistors from digikey a couple days ago: 2N6284: http://www.st.com/stonline/books/pdf/docs/5236.pdf and TIP142T: http://www.fairchildsemi.com/ds/TI/TIP142T.pdf to test out this idea. Unfortunately, it didn't work. I think I'm overlooking something. My partner thinks that it has to do with the high voltage ratings on the transistor. Okay I'll outline what I want to do in the following and someone tell me if this is possible, and if so, is it possible with either of those transistors I listed above. I want a low signal going into the base (5V, <40mA, square wave) to use as a switch, so that every time the square wave is on /at 5V, the high collector current flows through the emitter, and when the square wave is off or at 0V, the circuit is open, with no current flowing through the collector. I need a really high current through the collector, like 10A. The problem I think is that if I connect 4 TEC's in parallel (they are about 1 ohm each, so the eq R is about 0.25 ohms), that means I would need like 3VDC at the the collector side to get 12A and 3A through each. Is this possible? So my set up right now is I have 12VDC to an 8ohm resistor (20W resistor) that goes to the collector of the TIP142T NPN BJT. This would mean that is about 1.5 A going through the collector. The datasheet for this transistor says that the current gain (hfe) is 1000 @ 5A, so I used that. That would mean I_B = 1.5A/1000 = 1.5mA. The datasheet also says that Vbe to turn on is 3V (but at a higher current) So then I have a 5V square wave @ 1kHz from a function generator connected to a R_b ([5V-3V]/1.5mA = 1.33kOhm) connected to the base pin. I connected an oscilloscope across the 8 ohm resistor, but once I turned on the 12V supply and the waveform generator, the resistor voltage reads 12V, which means it's not working (probably fried the transistor). I have another one and the 2n6284, but I don't want to try the same thing and fry those too. So, am I doing it wrong? I'm still an undergrad, and I know there's so much I still don't know, but I was hoping that the brilliance of this forum could help me out or lead me in the direction or let me know that my design will be fruitless. Any help would be greatly appreciated. Thanks, I know this was a long one. PS I had a really bad electronics professor. I had to learn how to use a transistor on my own (most kids at my level still don't know either), and I still really only know how to use it as a switch. Like I said, I got the p2n2222a to work as a switch, so I know the calculations it takes to get the right resistors, currents, etc, to make it work, but my nomenclature might be off since I never learned it formally. Maybe it's just that this darlington transistor has much more obscure properties that I don't understand. PSS, I just thought, what if I put in a delay in the code for the PWM to make it around 50 Hz? Would that be an easy fix, and I could keep using the relays? EDIT: Schematic Attached in post #10 (sorry, I'm not good with all the forum stuff yet), and thanks to everyone for their help. For science!