IEEE Consultants' Network of Long Island

Thermal Time Constant and Specific Heat, A Second Look - John Dunn, Consultant, Ambertec, P.E., P.C.

A while back, we looked at how the specific heat of a heat sink mass can be used to examine the thermal rise time of an eight pound mass. We will now rescale that to a one pound mass, just to make these numbers a little easier to deal with, as we examine one additional issue regarding thermal rise, the time.

Thermal Time Constant and Specific Heat - John Dunn, Consultant, Ambertec P.E., P.C.

We have all kinds of information about finding maximum the temperatures that some particular things will get to, things like power supply heat sinks for example, but it may also be useful to look of how long will it take the temperature to get to that calculated extreme value.

With the specific heat of that particular something, we can make an estimate of the time constant of that something undergoing a thermal excursion:

Note: In this example, we look up the specific heat of aluminum at    http://hyperphysics.phy-astr.gsu.edu/hbase/tables/sphtt.html    where we find the value as 0.900 joules / °C / gram.

We examine in this example, a real life situation I once came across:  

Localized Thermal Testing - John Dunn, Consultant, Ambertec, P.E., P.C.

Imagine that you have this really complex circuit board which is going awry in some way when it gets hot and you can't seem to figure out which component is the culprit.

You might be able to isolate the problem to some one particular part if you could just heat up that one part by itself without also heating up the nearby surrounding parts.

You could try making a small tool and then do this:    

 

If applying this very localized heat to a suspect part brings about the problem, you've found your culprit.

Note that although you  could use the soldering iron itself to heat a suspected part, the soldering iron might dribble some molten solder, an event that would likely be bad news. The hot nail won't do that!

 

Using One 115V Fan On Two Line Voltages - John Dunn, Consultant, Ambertec, P.E., P.C.

A product's cooling fan was rated only for 115 VAC service in a product that needed to switch between 115 VAC and 230 VAC. To accomplish this, I made the product's power transformer serve the fan as an autotransformer when 230V service was called for:
At the lower line voltage, the fan was simply driven right from the line. At the higher line voltage, the fan acted as a load on the transformer's primary coils which served just fine as an autotransformer.

Power MOSFETs and Thermal Runaway - John Dunn, Consultant, Ambertec, P.E., P.C.

Switching power MOSFETs have a thermal runaway mechanism. A MOSFET's on-resistance rises with rising temperature and if the drain to source current doesn't materially change in response to that, a vicious cycle can occur. A rising Rdson leads to rising temperture which leads to rising Rdson which leads to rising temperature which leads to .......

You can test for this without having to burn anything by measuring MOSFET case temperature. Stasrt by measuring temperature at regular intervals as the devices get hot. Usually intervals of ten seconds are a good choice.

Once the MOSFETs have reached some high temperature, you turn off operating power AND you KEEP ON measuring tempertures as the MOSFETs cool down. Keep using the same time interval and keep on making the measurements until the MOSFET temperature has pretty much returned to the starting value.