Heatsink capacity with fan? And peltier question.

I have some heatsinks that are 90x90x15 (mm) finned aluminium and I think they are rated at around 1.5C/W. Can anyone estimate their rating when fitted with a 90mm fan delivering around 10L/S? Can I expect a 2, 5 or 10 fold increase?

Also, when you power a peltier device and then turn it off whilst its still got a large temperature differential it supplies a current back into the supply. Can I treat this backward voltage in a similar way to the back emf in a motor? Does anyone have any information on calculating this "back emf". I'm assuming that as the differential grows the generated emf will subtract from the applied emf and reduce the current accordingly.

I can test this myself next week but thought I'd ask here as it's amazing the knowledge base that this forum has become.

Thanks,

Mike.

Looks like about 2X with lots of air.
**broken link removed**

As I recall they don't put out much voltage but quite a bit of current unless you buy one for generation.

After I posted I found **broken link removed** which suggests about a 3 fold increase.

The question about the generated emf is how to calculate it. I can find lots of sites that state that the current drops as the temperature differential rises but not how to calculate it. I'm assuming the equation will be something like I = (Vapplied - dT * VperdegreeC)/ Rpeltier but can't find any values for V per degree C.

Mike.

It all has to do with BTUs. If your part produces 20 BTUs and the heat sink can handle 10 BTUs with no fan, the heat sink is too small or it needs a cooling fan.

You can calculate this yourself but you need to know some facts. How many BTUs does your part produce? How many BTUs will the heat sink dissipate? How many BTUs will a certain CFM fan dissipate? Math gets complicated.

You can create your own data. It is easy. Weight the part that needs to be cooled in grams. Attach a heat sink several times heaver. Turn on the power then TIME the temperature increase. Temperature will increase at a steady rate. Use a clock to time the temperature increase from room temperature 70 to 150 degrees F. Assume the heat sink reaches 150 degrees in 60 seconds. Double the size of the heat sink time the tempeature increase again. Assume the temperature increases to 150 degrees in 3 minutes in test #2 and 7 minute in test #3. This is basic math you compare the temperature increase from both experements to calculate how much larger the heat sink needs to be to hold a temperature of 150 degrees.

Example.

First test temperature increase 80 degree in 60 seconds with no fan heat sink = 20 grams.

Second test temperature increase 80 degree in 180 seconds with no fan heat sink = 40 grams.

Third test temperature increases 80 degrees in 420 seconds with no fan heat sink = 60 grams.

Forth test tempeature increases 80 degrees in 600 with a fan.

If the heat sink weight doubles in each test so does the heat transfer rate and the heat radiation rate if your using the same heat sink material. If you draw a graph you will see the natural curve. You can also do the math to determine the exact weight of the heat sink required to hold a tempeature of 150 degrees.

You know the mass of the aluminum heat sink in grams so that makes it easy to calculate how much larger the heat sink needs to be. You also have data that shows the fan improved cooling by 600-420=180 seconds.

You get the idea?

It is often easiest to draw a graph with 3 or more known temperature readings then follow the curve of the graph to find how large the heat sink should be to keep the part cooled to 150 degrees.

You can use any unit of measurement you like. I do not have a scale so I built a balance beam scale with copper wire and used BBs at weights. Buy a pack of BBs for a BB gun or you can also use dimes or pennies as weights.

I hope this answered your question.

Here is some info. Don't know if it will help, but you might rummage around their site. Problem is I don't think the best coolers are the best generators so you may need to do your own curves.

https://www.customthermoelectric.com/powergen/pdf/1261G-7L31-05CQ_20140513_spec_sht.pdf