I'm in the process of putting together an Elenco XP-720K bench power supply kit. The kit has 3 ICs and 1 transistor in T0-220 package format. The instructions have the builder bending the legs 90 degrees and then mounting the devices on the heat sink. This seems normal enough. However, to hook wires up to the devices, the instructions instruct the builder to tin the wire and solder it to the correct leg. The instructions are here:

Elenco XP-720K manual

To me, this approach seems likely to result in a fragile device. Is this a normal thing to do? I assume that there are connectors I could use. Would it be better to use connectors. Or can you suggest yet another approach? All suggestions are appreciated.

you mean like this...

...and then like this...

? Not fragile, so long as you don't drop it off the top of a building or something.

Have you already soldered the wires to the board?

Me, I would mount the 220's to the heatsink first, then solder the wires to them after tinning the pins and wares(the heatsink will hold them still), then solder them to the board starting in the middle of the line, or from one end to the other. Whichever is more comfortable for you.

It prolly wouldn't make any difference much if you did all the soldering first, and then bolt them to the heatsink.

Edited September 25, 201015 yr by digger945

It's not fragile at all and you can cover the pins with heatshrink so if the joint would break the two parts won't seperate.

@Scott - Thanks, I hadn't seen that web page. The pictures are useful. I haven't soldered the wires to the board yet, but probably will ahead of time per the instructions. The board is mounted into the chassis by the time you get to this step, so it shouldn't be very hard to solder the wires to the mounted TO-220 packages.

@Birgir - Thanks for the tip about putting shrink wrap in them. I'll to that upgrade.

mikhail and rudi mount to220's like this, with varying results.

mikhail would usually bend the pin into a loop and then solder to what

used to be the back of the pin. A lot of his broke off after a while because

the overstressed transformer generated enough vibration to crack the pins off.

The use of a little piece of perf board might make things more reliable.

I'm on to the next set of kit questions. It involves the 3 wire power cord. In the instructions, it refers to a "smooth side" and a "ribbed side". The "smooth side" is connected to the fuse and the "ribbed side" is connected to the switch. Since the instructions are specific about this, I'm assuming that that the polarity is important. The problem is that in the included cord, both sides are smooth. So I have the following questions:

1. Is the polarity that important? It is AC coming in. So only the phase would be different. Does it really matter how I hook up the non-ground wires?

2. In the schematic diagram on page 18, what is the perspective of the AC male plug? Are we looking from the front of the plug (i.e. at the pins) or are we looking at the wires coming out the back of the plug?

Elenco XP-720K manual

Nevermind. I looked again at the cord in better light and, indeed, one side has 1 rib on it. The rib side is neutral, the middle is ground and the smooth side is hot (looking this up confirmed that this is the normal convention). So I'll just follow the instructions.

this is another one of those UL requirements. Neutral needs to stay hooked up at all times.

For safety reasons in case someone uses a cheater plug. The hot is switched.

this is another one of those UL requirements. Neutral needs to stay hooked up at all times.

For safety reasons in case someone uses a cheater plug. The hot is switched.

That makes sense.

doesnt hooking devices up like this result in some instability due to inductance and lead length?