TIP120 transistor overheating

Other Hardware General Electronics
1

hello, i'm having trouble making an arduino controlled seedling/germination heatpad.
I took a heat pad from one of those heated jackets you can get, i plug 5v through it and it works fine, nice and toasty (picture 1).

Now i want my arduino to turn it on /off for me (picture 2). I used a TIP120 transistor, and when the arduino gpio pin goes high to turn it on, the TIP120 gets really hot, and the heat pad doesn't get hot.
What am i doing wrong?
(many thanks)

heatpad
heatpad1208×724 12.4 KB

2

Why are you using an emitter follower?

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3

  • Show us a good image of your actual wiring.

  • What is the collector current and Vce voltage when the TIP120 is ON ?

  • What is the Arduino output voltage when the TIP120 is ON ?

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4

You should be switching the ground side instead of the +5V power:

Transistor Driver Circuit. (You don't need the flyback diode with a non-inductive load.)

The TIP120 may still need a heatsink, depending on the current.

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5

I assume you meant to reply to the OP rather than me?

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6

i'm not sure what an emitter follower is, tried to google it but got confused. sorry my electronics knowledge is terrible. I thought this would just be a simple circuit, where the TIP120 would just act like a switch as in picture 1.

7

Known also as "common collector" - Common collector - Wikipedia

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8

I'm not sure what that means to switch the ground side, my electronics knowledge is is terrible. Do you mean put the heat pad between the 5v and collector? i tried that, but the transistor still gets really hot, the heat pad does get a little warm though. Do i need a more efficient transistor?

9

i'm sorry i don't have a multimeter or access to a camera right now

10

thanks you i'll give it a read

11

Slew to 16m24s in this video for a TIP120 example.

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12

Thanks!

13

  • It should be noted the old TIP120 BJT has a Vce saturation voltage of ~ 2V.
    This means the heater will only see 3V maximum.

  • Today we recommend using (in this case) an N channel logic level MOSFET.
    The MOSFET Vds is close to 0V, therefore the heater would see almost the full 5V.

1 Like
14

Thanks! i'll look into it!

15
  • FYI, I have used these in the past.
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