1
\$\begingroup\$

I am trying to use TIP120 for my inverted switch where I wish to have my DC water pump(rated at 5v, 3W, 600mA) running when switch is open (in actual these will probes that will complete path when there is ample water in between them) and when switch is closed the transistor should shut off.

I have calculated the (base current - Ib) for the above load at 0.6mA (600mA/1000) and base resistor Rb at 4.1 kohms.

I am using circuit wizard for simulation, which is showing explosion on the transistor when the state of switch is changed. Can someone tell me what is wrong with the below mentioned circuit

enter image description here

I tried this sample circuit at

https://cdn.sparkfun.com/assets/learn_tutorials/1/9/3/switch-animated-600h.gif

but had similar prediction of transistor exploding. Can I get some information on why the simulation is predicting exploded transistor. I have also tried placing diode after the Emitter but no luck in simulation

thanks Imran

\$\endgroup\$
6
  • 1
    \$\begingroup\$ Does the motor current exceed the transistor rating? \$\endgroup\$
    – Solar Mike
    Commented May 13, 2018 at 15:37
  • 3
    \$\begingroup\$ You need a flyback diode across the inductive load. I don't know if your simulation is picking that up- you should provide simulated base current and collector current values or we are just guessing. A Darlington is really not a great way to switch 5V since it will leave you with only about 3V for the motor. A logic-level MOSFET is preferably. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented May 13, 2018 at 15:52
  • \$\begingroup\$ @SolarMike which part of rating that might be? The motor here is rated at 4.5V only. Well I tried 2N2222A and 2N3904 but had similar result \$\endgroup\$
    – user482963
    Commented May 13, 2018 at 17:07
  • \$\begingroup\$ @SpehroPefhany well that seems right because VCE is 2V..so that leaves 3V but still does not explain the explosion \$\endgroup\$
    – user482963
    Commented May 13, 2018 at 17:09
  • \$\begingroup\$ Did you try the flyback diode? When the transistor turns off (not on) the voltage will likely spike up to cause the transistor to break down, at least in your simulation. Depends on how far from reality your motor model is. If it's just a resistor that won't happen in simulation. If it's a pure inductor + series resistor it will happen. Neither represents a motor load very well. A stalled motor will draw considerably in excess of its running current. I hope you recognize that you have provided woefully inadequate information. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented May 13, 2018 at 17:28

2 Answers 2

Reset to default
2
\$\begingroup\$

You need a flyback diode (a diode across the load).

When the transistor turns off (not on) the (collector) voltage will likely spike up to cause the transistor to break down, at least in your simulation. This is because the motor has some inductance. Inductance opposes a rapid change in current, by changing the voltage, so when the transistor starts to turn off the voltage at the collector rises to maintain the current until something stops it (in reality it might be transistor breakdown, or ringing from capacitance).

Depends on how far from reality your motor model is. If it's just a resistor that won't happen in simulation. If it's a pure inductor + series resistor it will happen. Neither simplified model represents a motor load very well.

A stalled motor will draw considerably in excess of its running current, because of back-EMF, for example.

\$\endgroup\$
-1
\$\begingroup\$

well this made the explosion go away

enter image description here

\$\endgroup\$
2
  • \$\begingroup\$ Could you explain why it makes the explosion go away? \$\endgroup\$
    – marcelm
    Commented May 13, 2018 at 22:00
  • \$\begingroup\$ I will repeat the same information that is in my comment in an answer. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented May 13, 2018 at 22:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.