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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
@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

There are several problems with your circuit:

  1. 2N2222A is inappropriate for this application. You are trying to control power, although you didn't say what current this fan takes at 7 V. A hovercraft fan sounds like it would take some power, so let's say 1 A to pick something (since you didn't specify anything). That's more than the little 2N2222A can handle.

  2. Your circuit doesn't have enough gain. You didn't say what the digital logic level of this arduino pin 9 is, but let's say 5 V when high. Figure 700 mV for the B-E drop, which leaves 4.3 V accross the 1 kΩ resistor. That will only provide 4.3 mA base drive. To support 1 A collector current, the transistor would have to have a gain of 233. Maybe some parts in the bin will do that, but it's well beyond the guarantee.

    This problem is even worse at 3.3 V logic or with a proper power transistor. You mentioned TIP120, but then are using 2N2222A for some reason. That makes no sense.

  3. The flyback diode is in the wrong place. The problem is that the fan acts like a inductive load. You can't shut off current thru a inductor instantaneously. When you try, like when your transistor switches off, the inductor will make whatever voltage is required to keep the current flowing, at least in the short term. In your case that means raising the collector voltage to the point the transistor conducts anyway, which is bad for the transistor.

    Put the diode in reverse accross the fan. In normal operation it doesn't conduct. When the fan is abruptly turned off, it gives the inductor current a safe place to go until it dies down naturally.

  4. Use a Schottky diode instead of the ordinary silicon diode you show. Schottky diodes have very fast reverse recovery time, essentially intantaneous for your application. That means the switch won't get beat up and you won't waste current when it is first turned on but the flyback current is still circulating. This can easily be the case when a motor is driven with PWM.

To fix this, replace the 2N2222A with a IRLML2502 FET and connect the PWM output directly to its gate. There is no need for a resistor between he PWM output and the gate. These are great little FETs as long as the voltage doesn't exceed 20 V, which it doesn't in your case. Don't forget to fix the diode before you change the transistor, else you'll be changing it again.

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I have trouble dealing with surface mount items in the SOT-23 package because of their small size. Would there be a recommended through-hole alternative to the IRLML2502? – mikeY Sep 20 '13 at 14:49
@mike: SOT-23 is not small. You're not going to find most parts in archaic and klunky thru-hole packages. There is no demand for them, except for parts like connectors where the extra mechanical strength matters. Think about it. Why would someone design in a TO-92 instead of a SOT-23? Nobody wants the old stuff, so they largely aren't made anymore. Soldering a SOT-23 is easier and cheaper than soldering a TO-92, and they're cheaper to produce too. You don't have to flip the board over and clip the leads, for one thing. – Olin Lathrop Sep 20 '13 at 17:23
Whatever. I was asking a question. I know you don't think it is important, so kindly ignore it. Surface mount items are small and more difficult to work with for many people, regardless of what you think. I doubt anybody else will respond at this point with a recommendation for an appropriate FET, so we'll just consider it an opportunity lost and move on. – mikeY Sep 23 '13 at 15:28
@mike: Usually those that don't want to use SMD devices haven't tried them. There is a reason this is how it's been done for the last 20 years. You're going to find less and less parts available in thru hole as time goes on. Most already aren't. Fix your process instead of trying to hold back the tide. Worst case, solder a short wire to each of the three pins, then use a SOT23 as a thru hole part. At least that way you have access to modern parts. – Olin Lathrop Sep 23 '13 at 16:16
"solder a short wire to each of the three pins, then use a SOT23 as a thru hole part". Gosh, I would have NEVER thought of that. Gee whiz. – mikeY Sep 23 '13 at 17:20

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

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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