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Old 02-05-2011, 07:21 AM   #6 (permalink)
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4. The build:

I did cut a piece of plexi, rounded the edges, and attached the two heatsinks with M3 bolts, nuts and distance holders, to allow for a nice airflow all around the heatsinks. Mark the holes to be drilled in the heatsink for the resistor(s) now.

Apply a thin and equal layer of thermal compound between the bottom of the resistor and the heatsink.
This is important, don’t forget ! Like here on the right one:

Fix the resistors on the heatsinks firmly with bolts, nuts and spring washers (threadlock doesn’t like heat).

Wait a minute now, stop the build, and let’s test the supposedly kinda limited heatsinks right away: let’s connect the resistors to an adjustable PSU, and measure the temperatures with an infrared thermometer while allowing more and more current to pass through them, up to the expected max current of about 3 amp. We’re starting with a humble current of 1 amp:

Here are all the testresults, with an ambient temp = 23C. I applied different currents during several minutes, up to the moment that the temp of the heatsinks didn’t rise much anymore:

8V/1 amp = 8 Watt: no noticeable rise in temperature, 3 or 4 degrees at most.
12V/1.5 amp = 18 Watt: heatsinks around 38C, resistors around 42C
16V/2 amp = 32 Watt: heatsinks around 48C, resistors around 57C
20V/2.5 amp = 50 Watt: heatsinks around 64C, resistors around 78C
24V/3 amp = 72 Watt: heatsinks around 79C, resistors around 100C (!!!)
BTW, as a reference for the Fahrenheit lovers: pure water boils at 100Celsius (at normal pressure).

Conclusions: The relationship between power and temperature is linear, and the prototype does get very hot. And it gets about as hot as calculated above. Note that it will only get that hot if it would have been discharging a long time without a break, which is unlikely in real conditions. The whole test took way over an hour.

There are several ways to deal with this heat problem, just to name a few:

1) Change nothing, and take care not to get burned. Bad idea for a final version, IMHO. The circuit wouldn’t suffer from it though, as these resistors support way over 200C in fact.
2) Use a protective, heat resistant cover over the heatsinks, but allowing for a very good airflow.
3) Use active cooling, like two silent 12V computerfans in series, or one 24V fan, and connect these to the 24 Volt circuit like the resistors are connected, avoiding the need for a separate power supply. This is definitely the way to go to build a very powerful discharger. If you use two 12V fans, take care that the power consumption and thus the internal resistance is equal, it’s best to take two identical ones.
4) Use larger heatsinks, about the double in size to get temperature down to a reasonable level.
5) Use resistors with a higher value, which will make a less powerful but cooler discharger. For example, two resistors of 6.8 ohm in series will make for 13.6 ohm. That means 1.77 amps at 24 Volt, or 42 Watt of dissipation power.
6) Use cutting wire as a discharging resistor, to get rid of the resistors and heatsinks. You need to find a heat resistant isolating object to act as a support though. Independent of the type and the resistance/meter of the wire, you want to aim for 8 ohm total resistance, that will determine the length of the wire you need. And you’ll need to check that it doesn’t get too hot at the calculated length.
7) A combination of the above.
Because this is only a first and temporary prototype I choose to do nothing for now, which allowed me to finish the device quickly without any changes. It’s a test, making a cooler or even larger one will be easy once we know exactly what we’re dealing with, and this experience helps. Note: I intend to use larger heatsinks in a second and more final version.

Now after this delay, let’s continue the build: put the relay, diode and light bulb on the circuit board, fix the Cell-Log with some double sided adhesive tape or so, and solder all the connections, wiring and connector per the diagram above. Mount the circuit board on the support plate. You’re basically done, unless you want a nice case around it. Do allow for enough airflow though in that case.

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