TIP31C NPN Transistor

[FireAce]

Hey guys, need some info on a TIP31C transistor. Now usually in my circuits when I use an NPN transistor like a BC547, I set the gate high, and it turn fully on. Apparently this is not the nature of the TIP31C. It appears the more voltage i send to the gate, the more power it lets through. Is this normal? Would this also work as a voltage reducer, like running a 2v motor on 12v?

Thanks fellows!

 

[ronv]

How do you have it hooked up?

 

[FireAce]

Supply is 12v. Normally I use 10k res to base. Emittor goes to ground, one leg of motor to Pos voltage, and other leg to collector of the transistor.

 

[ronv]

The base resistor is to large. The Tip31 has Hfe (current gain of 10 to 50 maximum) so if you are driving it with 10K to 5 volts you have .43 ma of base drive so your collector current can be only 4.3ma to 21 ma at the most. Your BC547 has much higher gain (100 to 800) but also much lower current. So reduce your base resistor to get enough current. Note: Make sure your driver can support the needed base drive.
If you want lower voltage to the motor you can put it in the emmiter side. Then the voltage across the motor will be the same as the base -.7 volts. Tie the collector to 12 volts.

 

[FireAce]

The motor was to run off a 4069 Inverter running on 12v. According to my sim, I would have to use about a 680ohm res and that would still only give me less then 10v. Plus, i dont want to overload the 4069, as its running other stuff, so I switched to a relay. I was just surprised at the nature of the TIP31, I didnt know they function a bit different then a smaller NPN like the BC457

 

[MikeMl]

Get a Darlington, like a TIP122

 

[crutschow]

I was just surprised at the nature of the TIP31, I didnt know they function a bit different then a smaller NPN like the BC457

It doesn't really function differently, it just has less current gain (Hfe or beta). Thus you need more base current to turn it on. To use a transistor as a switch you typically used a forced beta of 10, that is, the base current should be at least 0.1 of the maximum collector current you want to switch.

If you need more gain then you can add another transistor stage or go to a Darlington transistor such as MikeML suggested.

 

[ben7]

The motor was to run off a 4069 Inverter running on 12v. According to my sim, I would have to use about a 680ohm res and that would still only give me less then 10v. Plus, i dont want to overload the 4069, as its running other stuff, so I switched to a relay. I was just surprised at the nature of the TIP31, I didnt know they function a bit different then a smaller NPN like the BC457

Commonly you need a higher base current with large power transistors. A transistor that I have is capable of passing 15amps C to E, but it needs 4amps of base current to pass the full 15amps C to E! And just like crutschow was saying, the TIP31 has a low Hfe which causes this to happen.

 

[MrAl]

Hi,

Just to note, if you try to run a 2v motor off of 12v you will be wasting power. How much power is wasted depends on the size of the motor. A very very small motor will waste power but it may not be that much anyway, but a large motor will waste wayyy too much power so you wont want to do it that way.

 

[Mosaic]

Why not use a MOSFET..? No current worries there...just clamp the Motor termianls with a rev. biased diode

 

[FireAce]

Hey fellows, thanks for the input. I went ahead and used a relay so all is well, since I can purchase a relay for less money then a transistor.

Why run a 2v motor on 12v? Well I was kinda in a pickle. I'm an N scale train modeler, and I built a train shed with 2 sets of double doors that open and close via a motor gear set from the local hobby store. The motors are loud and obnoxious at anything over 1.5 volts. I tried using PWM via a 555 timer, but PWM is loud, especially at low speed. The fix?
When I click the buttons to run the gates, a 12v motor from a tape deck turn on, with its shaft hooked right to a smaller motor from the door of an old cd-rom drive. This smaller motor generates the EXACT power I need to run those cheesy motors inside the gear box.