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Old 02-05-2011, 07:14 AM   #4 (permalink)
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Join Date: Mar 2009

3. The components, aka shopping list:

- A Cell-Log 8M, you don’t need the advanced 28$ version with USB logging, the 14$ version also has the alarm output connector and the wire, and will do fine:

- A high power resistor, look at the calculations above. I took two 3.9 ohm / 50 Watt pieces in series, to obtain 7.8 ohms / 100 Watts. These have a fixed heatsink around the resistor, but these are not enough to work at the max specs ! They mainly serve to guide the heat to a mandatory external heatsink. Price around 5$ each.

- A male deans connector, or any other connector that fits your packs.

- A single pole relay, with a 24 Volt coil, and contacts capable of switching over 3 amps, I took a 30VDC/16Amp version to be on the (very) safe side. 6$ in my local shop. This one uses 21 mA when it’s working on 24 Volt, not that this matters much.

- A general purpose diode: a simple 1N4148 type (75V/0.2A) might do, but I took a 1N4007 (1000V/1A) out of my spare parts box. You won’t have change small enough in your pocket to pay for one of these.

- Some large alu heatsink(s) and thermal compound. These are 100 x 75 x 65 mm each, with a thermal resistance of 1.6 Kelvin/Watt, together that makes 0.8 K/W. This isn’t really enough in fact, theoretically you need twice that amount of heatsink, see below for the calculations. I expect temperatures to rise rather high. Anyway, one large heatsink is also OK. Think big, it is like the heat that a 80 Watt light bulb produces that you need to dissipate. Real heatsinks like these are rather expensive, 25$ a piece at my local shop, but you might find some in a broken amplifier or large PSU, or even use some thick scrap metal. Painting it black with heat resistant paint also helps. Metal fins pointing upwards allow for a smaller heatsink.

Note 1:
You could use several automotive 24V light bulbs in parallel instead of the resistors, like three 25 Watt types, eliminating the need for these heat sinks, but I don’t like all the intense light they produce, and they will get hot also.

Note 2:
Heatsinks are (roughly) calculated as follows, neglecting thermal resistance between the resistor and heatsink, and neglecting the fact that heatsinks are usually rated for vertical mounting:
Pick the highest ambient temperature at which you would ever use the device, let’s say 30 degrees Celsius, that’s a hot summer day for us down here. Now pick the highest temperature that the heatsink should ever reach when in use, let’s say 60 degrees Celsius for the sake of the example. We also know that we need to dissipate 80 Watts. Thus, the thermal resistance should be (60-30)/80 = 0.375 C(elsius)/W or K(elvin)/W. Any heatsink spec will list this thermal resistance number, lower values are OK, not higher ! Google “heatsinks”, and you’ll see what I mean. That will also give you an idea of the size you’ll need.

My combined heatsinks only have a thermal resistance = 0.8 K/W, so I expect temperatures to rise up to around 100C, if the ambient temperature would be 30C and with 25 Volt applied with the given resistor value. We’ll see what happens later in this post.

Rather optional, depending on how you assemble the project:

- A support plate, I used some 5 mm plexi.

- A piece of circuit board.

- A holder to plug in the relay, but you might solder straight to the relay pins if you wish. If it ever fails, it will be harder to replace though, involving some soldering.

- Some small 24V light bulb, or a LED with a suitable resistor in series (anywhere in between 2000 to 3000 ohm will do fine, at least 0.5 Watt, but take a little larger like 1-3 Watts, to avoid heat here).

- Some wiring, we’re only talking about 3 amps now, it doesn’t need to be so thick.

Not in the pics: some small hardware of course, like bolts and nuts, you’ll see those during the build stage.
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