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So I have recently started my first group project, which is to design an autonomous hovercraft. We decided to drive the propulsion fans (which are rated at ~7 V and our battery runs at 12 V) using a TIP120 transistor.

We found the design online and it seemed to match our situation, so we used it. The problem is that when we connect the battery, the fans do not turn on. I made a quick picture of our circuit: enter image description here

The original circuit that based ours on is found here.

Is there a problem with the wiring, or did we pick out the wrong transistor?

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Your flyback diode should be across the inductive load (fan), not across the transistor. – m.Alin Nov 21 '12 at 18:20
Place the diode as M.Alin suggests. Your circuit shows a 2N2222, which is not a good idea. A TIP120 is OK. I hope ~ 7V means approximately 7V, not 7V AC? – Wouter van Ooijen Nov 21 '12 at 18:25
yes, ~7V means about 7V, and the 2N2222A is the TIP120, as I couldn't find it in the multisim library. Also, would the diode even be necessary? Our propulsion fan can run in both directions. – Tim Nov 21 '12 at 18:27
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@Tim Yes, the flyback diode is necessary. It protects the transistor from the fan's inductive kick. – m.Alin Nov 21 '12 at 18:35
I understand it needs to protect the transistor, but as of right now the fan doesn't even turn on, and we've checked all of the connections. Would moving the diode fix that? Also should the ringed end of the diode go towards the positive or negative end of the diode? – Tim Nov 21 '12 at 18:42

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Are you sure you have it wired like the schematic. If I'm reading the schematic correctly as soon as you connect the battery current should flow through the Fan then through the diode to the negative terminal of the battery. So your fan should be running continuously. Except that you are going to be putting more like 10 volts across it which it may not be happy with if it's rated at 7. It seems that your transistor and diode need to be reversed. But if it's not even running for you then you may have bigger problems do you have a scope? If so what do you see on the 9 pin?

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Yea, I'm sorry, I accidentally drew the schematic incorrectly, I fixed it now. We used a multimeter to make sure that current was flowing where it was supposed to (between the collector and emitter, through the diode, and the gate). We made sure that the fans weren't busted by connecting it directly to another power supply at 7V 2A. – Tim Nov 21 '12 at 18:20
OK so are you getting current leaving the fan? How much? What is the voltage across the fan ? How about across the transistor? – Perfect Disturbance Nov 21 '12 at 19:15
We only did used the multimeter to make sure there was current, as we were not sure if the transistor was wired correctly or not, and didnt want to leave the power on too long as it could have fried a component. Unfortunately we don't have access to the project until monday, so we cannot collect data on the voltage/current drops – Tim Nov 21 '12 at 20:05
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Hmmm. This should be simple. What is your software doing with pin 9?

Also, be really sure that your transistor hasn't been blown with your experimentation. Double check that it still gives proper results with a basic vom test.

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