What to do with extra NC legs? - Page 1
Forum section: electronics / beginners

author: Dan123456
author_rank: Regular Contributor
timestamp: October 28, 2023, 06:00:33 am
content: Hey guysI managed to pick up a LM3875 56W audio amp IC for cheap (data sheet below)https://www.farnell.com/datasheets/2026598.pdfIssue is, it is in a 11 leg TO-220 package which I have never used before and no matter what I do, I can’t seem to make all the legs line up with the holes of a standard piece of perfboard (not unless I were to bend some of them really quite significantly).My questions are:A. Am I just being special and I should just bend the legs more aggressively to make it fit? Or;B. If i am not just a complete dumbass, could / should I just snip some of the NC legs (6 of the 11 legs are NC) without hurting anything? Or;C. Should I really be using a piece of board with the spacings that it comes with (i.e. 1.7mm holes as per the last page in the data sheet) as those NC legs are actually important for something?My understanding is that the extra NC legs on a lot of IC’s are just for support (i.e. to make sure the IC doesn’t physically move / so it can support it’s own weight etc.) so I am guessing it should be ok to snip them, but figured I’d check in case there are often other reasons for them being there (such as maybe being important to assist with heat dissipation? etc.)Thanks in advance!

author: Nusa
author_rank: Super Contributor
timestamp: October 28, 2023, 06:35:00 am
content: The legs are mechanical support when mounted to a PCB layout. You can do what you want when you're making it up as you go.One approach would be to use the heat sink tab for the mechanical mount, then solder wire leads (with heat shrink if it's more than testing) to the 5 pins you actually need to wherever you want them on your perf board. The NC pins could then be ignored or clipped.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 28, 2023, 06:50:41 am
content: No IC was meant for protoboards, so no surprise when some ICs won't fit the 0.1 inch grid.I wouldn't snip any pins away. At 50W it is expected the IC will heat a lot, and thermal dilation might build a lot of mechanical tension. The extra terminals are good for mechanical strength, might help a little with power dissipation, too.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: October 28, 2023, 07:54:09 am
content: Perf board is not suitable for high power audio amps, you really need a proper PC board layout to avoid instability.There are a few boards/kits around. While I haven’t looked in depth, this project placed in the public domain may provide you with a suitable print board:https://www.pcbway.com/project/shareproject/LM3875_Amplifier_58edd73e.htmlSJ

author: wasedadoc
author_rank: Super Contributor
timestamp: October 28, 2023, 06:14:09 pm
content: The TI datasheet has the Mechanical Data on the penultimate page. It shows that the spacing between the two rows of pins is 0.2 inches but the spacing between pins on a row is 0.134 inches. You cannot make that fit into a board that has its holes on a 0.1 inch grid.

author: Ian.M
author_rank: Super Contributor
timestamp: October 28, 2023, 08:02:21 pm
content: Prototype bodgery: 3x 0.134" is near enough 0.4" so it *can* be made to fit 0.1" pitch matrix or veroboard with moderate pin bending, skipping one hole in four. I'd strongly recommend sleeving all the pins in the short pin row with heatshrink (but hold off shrinking it) if you do that to reduce the risk of the pins touching. The long pin row is offset by 0.067" so get the short pins bent and fitted first, working near the board edge so the long pins don't have to go in holes yet, then bend the long pins to suit and finally place it in the desired position on the board.

author: macboy
author_rank: Super Contributor
timestamp: October 30, 2023, 02:54:53 am
content: You will find that if you mount this device at a 45 degree angle, the pins line up quite well. Not quite perfect but very close. It will look odd, but it works.

author: ArdWar
author_rank: Frequent Contributor
timestamp: October 30, 2023, 04:25:59 am
content: Quote from: Dan123456 on October 28, 2023, 06:00:33 amNC pins on this particular IC are there because LM3875 is a subset of LM3886 (or rather 3886 is a superset). In fact, if you design a new PCB using LM3875 for some reason (EOL and all) you might as well use the full LM3886 layout and then DNP-ing the unused features.I was wondering if the layout was cross compatible with ST's amps, apparently not.

author: alligatorblues
author_rank: Regular Contributor
timestamp: October 30, 2023, 04:35:44 am
content: A lot of times there are sockets for ICs like that. But it takes 1//2 a day to find it.

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 30, 2023, 01:16:43 pm
content: I’ll join the chorus of people advising you to design a proper PCBA for this device. Both EasyEda and PCBway allow you to design and order a proper, high quality board whose cost will be very reasonable.A friend of mine attempted to build an audio amp on a perf-board employing a similar device from STMicro, and the thing, under the wrong conditions, would spontaneously burst into violent oscillation.