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I have some of these (RGB LEDs with drivers). They connect to 12V, and they draw a max of 1.8A total (full white):

Load

I wanted to add a low-side switch to turn the load on/off via a PIC I/O pin output. After getting some advice in this threadthis thread, I created the circuit below:

switch expected

The values above are what I expected the switch to behave like. According to the TIP31C transistor datasheet, Ic=1.8A gives a Vbe(sat) of 1.0V, and a Vce(sat) of 280mV, which apparently shouldn't get very hot:

datasheet

Instead, I'm seeing this:

switch actual

After a few seconds of being on, the transistor starts smoking and is way too hot to touch.

Can somebody please explain why there is a difference between what I expect to see and what I am seeing?

Note: my electronics knowledge is pretty basic, so please assume I don't know much, I won't be offended :). If I've left off any necessary information, please comment. Thanks!

I have some of these (RGB LEDs with drivers). They connect to 12V, and they draw a max of 1.8A total (full white):

Load

I wanted to add a low-side switch to turn the load on/off via a PIC I/O pin output. After getting some advice in this thread, I created the circuit below:

switch expected

The values above are what I expected the switch to behave like. According to the TIP31C transistor datasheet, Ic=1.8A gives a Vbe(sat) of 1.0V, and a Vce(sat) of 280mV, which apparently shouldn't get very hot:

datasheet

Instead, I'm seeing this:

switch actual

After a few seconds of being on, the transistor starts smoking and is way too hot to touch.

Can somebody please explain why there is a difference between what I expect to see and what I am seeing?

Note: my electronics knowledge is pretty basic, so please assume I don't know much, I won't be offended :). If I've left off any necessary information, please comment. Thanks!

I have some of these (RGB LEDs with drivers). They connect to 12V, and they draw a max of 1.8A total (full white):

Load

I wanted to add a low-side switch to turn the load on/off via a PIC I/O pin output. After getting some advice in this thread, I created the circuit below:

switch expected

The values above are what I expected the switch to behave like. According to the TIP31C transistor datasheet, Ic=1.8A gives a Vbe(sat) of 1.0V, and a Vce(sat) of 280mV, which apparently shouldn't get very hot:

datasheet

Instead, I'm seeing this:

switch actual

After a few seconds of being on, the transistor starts smoking and is way too hot to touch.

Can somebody please explain why there is a difference between what I expect to see and what I am seeing?

Note: my electronics knowledge is pretty basic, so please assume I don't know much, I won't be offended :). If I've left off any necessary information, please comment. Thanks!

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parrowdice
parrowdice
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Smoking Transistor in Low-Side Switch. What Am I Doing Wrong?

I have some of these (RGB LEDs with drivers). They connect to 12V, and they draw a max of 1.8A total (full white):

Load

I wanted to add a low-side switch to turn the load on/off via a PIC I/O pin output. After getting some advice in this thread, I created the circuit below:

switch expected

The values above are what I expected the switch to behave like. According to the TIP31C transistor datasheet, Ic=1.8A gives a Vbe(sat) of 1.0V, and a Vce(sat) of 280mV, which apparently shouldn't get very hot:

datasheet

Instead, I'm seeing this:

switch actual

After a few seconds of being on, the transistor starts smoking and is way too hot to touch.

Can somebody please explain why there is a difference between what I expect to see and what I am seeing?

Note: my electronics knowledge is pretty basic, so please assume I don't know much, I won't be offended :). If I've left off any necessary information, please comment. Thanks!