How do I stop my motor from spinning and spinning?

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teehowe

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Hello,

I am using a TIP120 to power up my vacuum motor that runs runs at 3530rpm with a voltage rating at 12V. The transistor's base has 5V input and a 10k resistor. The motor is connected to the collector and a 12V Lead Acid Battery. The emitter is grounded.

From what I know, when initially ON, the amount of current that flows into the motor is about as much as I expected. However, when I disconnected the base, the motor slows down and then the speed climbs and climbs until the TIP120 becomes extremely hot and the motor spins even much wilder. I believe the TIP120 has an internal diode on its own, but why does it not work the way I wanted? Hope someone can give me a pointer. Thanks!
 
It seems "back-emf spikes" are getting into the transistor and causing a breakdown. Put a diode across the motor.
 
According to the specification sheets, it seems it has its own diode. Any specific diode that I can use? I tried a 1N4148 but it gets extremely hot in less than a second. And I do not really know in which direction does the emf flow. If I'm not mistaken, should the negative of the diode be connected to the TIP's collector?
 
The diode should be external and a diode does not have a "negative" or "positive" end. It has a cathode and anode.
The cathode of a 1N4004 should be connected so that the cathode is connected to the supply-rail. This is called "reverse connection" as the current flows from the supply rail to the 0v rail and thus it does not flow through the diode. But the back-emf is a voltage and is generated in a direction that is opposite to the supply. Thus the diode is "forward-biased" with respect to this voltage and allows current to flow and produces a voltage drop of about 0.7v. This is now the back emf (voltage) and will not damage the transistor.
 
You should also add a resistor between the base and emitter of the TIP120 to remove any stray current. 10 k would be fine.
 
I'm trying that now with a 1N4002 that I digged out from an old stock. Hope this works. Thanks a lot for the knowledge.
 
I used a 10K resistor from B-E and a 1N4002 from E (Anode) - C (Cathode) but strangely, the problems still persist. It's a new TIP120 I bought this morning and yet it still smells like deep fried silicon after the base is disconnected. As usual, the rotation gets wilder, but it seems to build up speed slower at this time.
 
This is how I exactly connect the components. As far as I know, from left to right, pin 1 is Base, pin 2 is Collector and pin 3 is Emitter.

p.s: I missed out the wire from the anode of the diode to ground, but in reality I did connect them

 
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The diode wants to go directly across the motor - just slide it straight up and connect across the motor.

I would also suggest dropping the valure of R1 a LOT - say to 100 - 200 ohms.

What kind of heatsink do you have the transistor on?, darlingtons have a quite high voltage drop across them, so it could just be overheating (and be toast by now).
 
From left to right, with the transistor marking faced to you.

Your diode is missplaced.

Replace the transistor and connect it between 1 and 2.
 
I'll try correcting the diode first.

Would that greatly increase the collector current if R1 is dropped to 100ohms? I just fear I might cause an accident. This motor with a centrifugal fan is rather fierce.

I suppose it's got a TO220 casing, such as most regulators. I'll need to on this vacuum for as long as my robot is moving around the room. I fear it may not even last as long as 5 minutes with this much of heat.

Yeah, the transistor marking is facing me.

Connect the diode between 1 & 2 to the nodes in the diagram or 1 & 2 of the transistor (1-B, 2-C, 3-E)?
 
BrownOut said:
Just to be clear, I think he means connect the diode between 1 and 2.
Click to expand...

Thanks, BrownOut.


Like BrownOut said, 1 & 2 from your diagram.
 
teehowe said:
Would that greatly increase the collector current if R1 is dropped to 100ohms? I just fear I might cause an accident. This motor with a centrifugal fan is rather fierce.
Click to expand...

It will help increase it to the correct value - as it stands you're wasting energy overheating the transistor.

I suppose it's got a TO220 casing, such as most regulators. I'll need to on this vacuum for as long as my robot is moving around the room. I fear it may not even last as long as 5 minutes with this much of heat.
Click to expand...

Presumably that means you don't have a heatsink then?.

As you're only switching ON and OFF, have you considered a relay? - I would also expect consumption of a 12V vacuum cleaner to be MUCH higher than you're suggesting.
 
I just tried with the diodes. Cathode at node 1 and anode at node 2. The problem persists.

Then I did with a 100 ohms current, overheating still occurs, but the vacuum spins really fast but the issue with the never-ending violent spin is not there for now. Going to try that the following morning when my family is not sleeping. lol.

I will revise all of these once I get the relays and heat-sinks in my hands. Gotta wait for Monday till the shops open up. Thanks a lot, everyone! Every single one of you has been extremely helpful.

p.s: regarding the relay, am I to assume a simple switching transistor would be enough to switch on the Relay at collector?
 
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The TIP120 darlington transistor already has built-in resistors across its base-emitter junctions.
 
Maybe the vacuum motor is a overload.

The TIP120 maximum rating is 5A of collector current and 65W of case dissipation.

I don't know what is the motor ratings.
 
Connect your circuit to the power but do not connect the base to any wires AT ALL. If the motor runs, the transistor is damaged.
 
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