Sunday, July 27, 2008

Power supply (now 0.75-30V / 2.7A), part 4.

Mounting the transistors on the heat sink is a yucky job. You have to put paste in between the transistors and the heat sink to help conducting the heat. I used one with silver in it, it conducts heat better. Unfortunately , because of the silver it also conducts electricity better.

I first put a thin layer of paste on the heat sink. And thin means THIN. It's only purpose is to fill up microscopic holes to flatten the surface more for better contact.

Then I put the mica insulating washers in place. I actually made the holes for the transistor pins a lot bigger because I don't want the pins to come in contact with the paste due to the paste's electrical conductivity.

After this I put another very thin layer of paste on the bottom of the transistor and placed them on top of the mica washer.

After this the transistors have to be screwed on to the heat sink. The casing of the transistor will actually carry 34 Volts but I didn't want to put 34 volts on the whole heat sink, hence the mica washers. One of the screws was isolated from the transistor and is simple to use. The other one was in contact with the transistor because it will serve as the +34V contact of the transistor. It therefore is screwed directly on the transistor and the screw is shielded from the heat sink by little plastic cylinders inside the hole in the heat sink. Put a solder lug on it and fix it.

The end result of today's DIY.

PS: A note on the mica washers. The ones you buy can be pretty thick, some times more than 0.25 mm. This influences the thermal conductivity, you want them to be thinner. With a sharp knife and some patience you can "slice" them in thinner ones. The one I bought were 0.1 mm and I sliced them in half.

Tuesday, July 22, 2008

Power supply (now 0.75-30V / 2.7A), part 3.

The power supply does need a fair bit of heat sink. Possibly later on a fan will need to be mounted but I'm not doing that yet.

To work with chunky metal it's handy to be able to use a drill press. It's very stable and it's also safer then drilling from the hand.

I had the vice screwed to the table. This was to prevent it from being lifted while drilling which could cause danger. The holes for the transistors are 4 mm wide. The four outer holes were drilled with a 4.5 mm drill to make M5 screw thread in them. I think actually you need to drill an even smaller hole for M5...

After the holes are drilled, the four outer ones needed to get screw thread in them.

Et voilá, the heat sink with beautiful holes in it, ready to be used.

Sunday, July 20, 2008

Power supply (now 0.75-30V / 2.7A), part 2.

The past few weeks I have redesigned the circuitry a few times and this is the "latest and greatest". Also I have bought a new transformer with 2x12V coils. Because of this I can use less power when using a lower voltage. This is good because it makes the transistors less hot. The power supply will have two voltage ranges: 0.75 - 7V and 5.5 - 30V. The maximum current will be around 2.7A

The new schematics. Click picture for a larger version.

The board with it's components. A heat sink may be added here and there in a later stage.

This is the power supply close to it's current phase. At the moment the board is connected to all front-side controls. The transformer is wired too. Now the board needs to be hooked up to the transformer and the transistors need to be mounted on the heat sink and wired. That will be done soon.