Do you have resistors or diodes in this circuit?

Originally Posted by AxelD

At its lowest setting, the voltage across the transistor is about 12v with a current of about 60ma. Ive monitored the circuit for about an hour and the current moves up by about 2-3 ma as the transitor heats up.

The feedback makes sense but i control the current through the transistor with a POT. If i add feedback then wont any changes i make (by changing the pot and therefore the resistance) be cancelled out?

There are resistors but no diodes. A pot controls current to the transistor and i have the fan connected to the emmiter of the transistor.

Am i reading this correctly then - i dont need a heatsink but it is recommended given the circuit will be enclosed?

Thanks

hi Axel,
When you say the circuit will be enclosed, can you explain.

Axel - regarding the enclosure of the circuit, consider this: if you have a transistor that gets warm it will loose heat to the surrounding air at a rate that is dependent on the difference between the transistor (and heat sink if it has one) and the surrounding air - and also depedent on the resistance to the flow of heat (could be called thermal insulation) between the transistor/heat sink and the surrounding air.

If you enclose your transistor/heat sink it will get warmer as the enclosure adds some resistance to the flow of heat - another way to put it is that it serves as thermal insulation. As you insulate the enclosure the flow of heat will be reduced, the transistor/heat sink temperature will continue to rise - taken to the extreme the temperature inside an infinitely insulated enclosure would rise until the transistor fails.

I offer this explanation as a way of emphasizing that the details of the enclosure do matter. I think it is safe to say that heat sinks and enclosures can be carefully designed - with detailed design of every aspect but often the end result is a combination of design estimates, experience and some guesswork combined with testing - basically "cut-and-try" approach.

Originally Posted by AxelD

Thanks to everyone for the replies.

stevez - im a bit more concerned now given the small size of the plastic box i intend to use and the lack of ventilation. I may need to scrap the box idea ...

Why not use a aluminium diecast box and use that as the heatsink.?

Axel - as a general rule, higher temperatures mean reduced life, reduced reliability however many components are designed to provide long life and high reliability at elevated temperatures. Good design, in my opinion, strikes a balance between use of resources (money, time, space, etc) and meeting your needs - reliability being one of those needs. If a transistor can run warm or hot and still get the job done in a sufficiently reliable way then consuming more resources might not be judged as good design by some.

A possible solution could be to provide some means to let air circulate thru the enclosure. Sometimes one side or all of the enclosure can be metal - with the transistor mounted to the metal as a heat sink - the transistor being mounted so it is electrically but not thermally isoloated. Care must be taken at higher voltage levels.

Nothing wrong with your application - most of us here adapt proven designs to suit our needs. Lots of fun to be had and much learning results.

Originally Posted by mneary

A TIP31 can safely run too hot to touch (125c), but it's much more reliable when you keep it cooler.

Just a square inch of aluminum bolted to the back can work miracles.

Don't forget that, that's the rating for the junction temperature, not case temperature.

It is not that I doubt your circuit, but could you post your circuit AxelD. I think it would help everyone here out a lot more if you did. Is your V(ebo) greater than 5vdc? If so transistor is no good for that voltage.

Why not look at the datasheet for the TIP31 transistor?
It shows a max allowed dissipation of 2.0W when the surrounding air is 25 degrees C.
Then its internal chip temperature is at the max allowed temperature of 150 degrees C and it might fail soon.