What do I have to know to install a TO-3 regulator?

[Bob M.]

I'm building an amp with DC regulated heaters that uses a LM338K, a three terminal 5 amp positive adjustable voltage regulator in a TO-3 case. I have very limited experience with solid state electronics and have never installed such a device before. It has been recommended to mount this to the chassis but the case itself is the output of the device and I guess I'm missing something (I don't really want to put 6.30 Vdc on my chassis, do I?). I have a insulator kit with a clear wafer and two nylon grommets. Are the nylon grommets for the two pins coming out of the device or are they for the case mounting at either end of the device? Could anyone walk me thru this? Any advice about mounting this regulator would be a great help.

Thanks,
Bob M.

[Enzo]

The TO3 hole pattern is standard. Two legs in the center get holes, and a mounting hole at each end. The mica insulator keeps the body of the part from contacting the heat sink or chassis. I call the grommets washers. The washers are T-washers. You drill a hole to fit the smaller diameter part of the washer. The hole will be oversized for the mounting screw. The washer keeps the screw centered in the hole so it doesn't touch the sides. Put the washer into the hole from the underside of the chassis.

Now the mounting screw goes through the part from the top, passing through the mica wafer, then through the chassis hole with the white washer to keep it centered, and then a nut on the bottom side. I'd add a flat washer and a lock washer under there as well.

With the mica insulating the part on the top, adn the washers under the nuts underneath also insulating, the part can sit ahainst the chassis without electrical contact to it.

[Steve Conner]

Also, if your washer is clear, it's probably made of mica, so don't forget to apply heat transfer compound.

[Bob M.]

Thanks, Enzo, for the thorough explanation.
The only thing I don't understand is where to use the T washers. Do they go on the two protruding middle pins or do they go on the ends where the case mounts?

Yes, I plan to use thermal heatsink compound (white goopy stuff) when mounting the regulator. I'm also using a good-sized heatsink as I'll assume it's gonna heat up pretty much.

Thanks,
Bob M.

[Enzo]

The washers are T-washers. You drill a hole to fit the smaller diameter part of the washer. The hole will be oversized for the mounting screw. The washer keeps the screw centered in the hole so it doesn't touch the sides. Put the washer into the hole from the underside of the chassis.

What is unclear? they cannot go over the middle pins if the mounting screws are going to go through them.

I will say it a different way.

The white washers have shoulders to keep them centered in the chassis hole. The mounting screw fits through them so it does not touch the chassis on the way through. It is OK for the screws to touch the component casing as long as they do not also touch the chassis. In fact, under the chassis it is common to add a solder lug under the mounting screw as a convenient place to connect to the component casing - which as you know is electrically live.

[Bob M.]

Thanks, Enzo,

I just haven't done this before and I'm a 'measure twice and cut once' kind of guy. Thanks for the re-explanation. I got it now.

Bob

[Bob M.]

One related question: I'm using an aux heater transformer for my DC heaters, as I have as I have a bigger current draw with my tube compliment than the power transformer was designed to provide. The aux transformer is mounted a ways from the on/off switch. Is it better to run the primary on the outside or inside the chassis to the on/off switch?

Thanks,

Bob M.