Custom Heat Sinks

I bought the HobbyCNC kit that included a driver board, stepper motors, power supply and assembly instructions because I knew I didn't have the knowledge to select the components and assemble it all myself. I had considered a cheaper driver board based on a TB6560 that was readily available on eBay but I just didn't know enough about the power requirements nor did I have any idea how I'd put it all together in an appropriate case. But the big problem with the TB6560 was that there seemed to be little documentation and zero support. The HobbyCNC company has a Yahoo group to support their product and that was enough for me to choose it. The fact that they had nearly all the components I would need in one package was icing on the cake.

I knew up front I would be deviating from the plans. I would not be using a computer with a parallel port to communicate with the driver. Instead, I'd be using an Arduino microcontroller. In my last post, I talked about the opto-isolator circuit I am building. Normally, the signals to the driver board get their power from the computer, not the driver. It's the computer, or in my case, the Arduino, where the signal originates that provides the power for the signal. The electricity flows across to the driver, runs around the circuit, then comes back through the ground wire. This ground is common to both the computer and the driver, hence it is the electrical connection through which Bad Things can happen to my computer if the driver and it's 32 volts of power should go south. So the opto-isolator provides the protection I need.

This means the power (electricity) from the computer does not go to the driver and back, just to the opto-isolator and back. The side of the isolator circuit where the driver is needs to have a power source to "reconstitute" the signal that comes across the void in the form of light. It's not like a solar cell that generates electricity when the light hits it. So I needed a 5V power source. Since the driver board logic runs at 5V, it seems a no-brainer to tap off this to power the isolator. So I posed the question to the support group, where is the best place to get 5V from? To my surprise, the response was generally, "You don't want to do that". It was suggested that I use some other source, like a "wall wart" which I really didn't want to do. It's bad enough my transformer doesn't fit in the case. The "help" I got was pretty terse - no real explanation for why so I had to do some work to understand things.

Ultimately I learned that the limiting factor was the 5V regulator on the EZ Driver. This is in the form of a TO220 package that has a small, thin metal "tab" that provides for minimal heat dissipation. According to some calculations I don't understand, this regulator is near the edge of its ability to dissipate heat. I couldn't get anyone to tell me how I could remedy this, I asked if putting heat sinks on it would take care of it, but apparently no one wanted to be responsible for what happens. I finally got some help from someone who contacted me outside the group and said the worst that can happen is the thermal protection circuit would shut it down.

So I figger, if heat's the problem I'll just put heat sinks on the 24V and 5V regulators. I looked them up online. Radio Shack sells 2 varieties for $1.50 or $2.00 each. No sweat, I'll buy a couple and slap 'em on. I went to "The Shack" by my house and found what I needed. I was surprised to find them to be much bigger than I thought they would be. I was advised that since the 2 regulators were at different potentials, the heat sinks should not touch each other, nor anything else. These Radio Shack monsters were pretty close to touching each other, but the real problem was installing them without obstructing access to other components, particularly one of the variable resistors for adjusting the stepper motor current. So clearly not going to work.

Well a working manual milling machine and a box of 1/2 inch aluminum stock was just asking to be made into heat sinks, so who am I to say no. 2 hours later I had 2 custom heat sinks. Here they are next to a TO220 package and the Radio Shack heat sink:

And here they are installed:

So I'll return the Radio Shack heat sinks I paid $3 for. I'm not a great machinist. At a modest $20 an hour, I figure this is a $40 pair of heat sinks. But they fit just right.