Attach TO-220 package flat to PCB securely - Page 1
Forum section: electronics / projects

author: Phoxtane
author_rank: Regular Contributor
timestamp: March 21, 2017, 05:46:19 pm
content: My current project that I'm working on requires that I power a microcontroller on a board that will be running off of a 12V supply; as such, I've turned to the 7805 regulator in order to generate the 5V supply.The only 5V device on the board itself will be the the micro. I'm not worried at all about drawing enough current to warrant using a heatsink - assuming 10mA of current draw, I'm calculating a power dissipation of 70mW and a rise over ambient temperature of ~3.5C in my worst-case scenario. I'll be using one that comes in a TO-220 package; however, I don't want to have it just standing straight up from the board, or just bent over haphazardly. Is there a good way to have the package securely attached to the PCB so it doesn't flap around in breeze while avoiding the use of screws? Do I need to keep it electrically isolated from the PCB? Do I solder it in place onto a nice big pad?Here's the datasheet for the specific 7805 I'm using:http://www.mouser.com/ds/2/389/l78-974043.pdf

author: ivan747
author_rank: Super Contributor
timestamp: March 21, 2017, 05:49:55 pm
content: Can't you use a DPAK or D2PAK version? Perhaps another regulator?One way is making a copper pad and bending it in 90 degrees, then soldering the tab to the pad. It would look very similar to the D2PAK version.Sent from my iPhone using Tapatalk

author: JPortici
author_rank: Super Contributor
timestamp: March 21, 2017, 05:52:01 pm
content: -why do you think that it will bend?-why are you not using other through hole or even smd packages if you fear that the part will bend?unless you are getting aliexpress grade parts for cost reasons there are countless alternatives

author: dimkasta
author_rank: Regular Contributor
timestamp: March 21, 2017, 06:10:40 pm
content: If you worry that it might touch something it should not, then just use a TO-92 part.10mA is a joke anyway. barely enough to let the part maintain voltage regulation.If do not want to change the part, you can either solder the tab as ivan suggested (a wimpy soldering iron/station won't do this properly), or you can screw it on the pcb (on a pad or on plain solder mask) with a properly sized screw and nut. All you have to do is provision the space.Rather a waste of space and effort though

author: Phoxtane
author_rank: Regular Contributor
timestamp: March 21, 2017, 06:25:10 pm
content: Quote from: dimkasta on March 21, 2017, 06:10:40 pmI think you're right - however, the big downside to using a TO-92 (this one, specificallyhttp://www.mouser.com/ds/2/149/MC78L05A-889580.pdf) is that its thermal resistance: junction-to-air is three times greater than the TO-220 part I had in mind originally - which means now that the temperature rise over ambient is 10.5C, to a total of 35.5C with an ambient temperature of 25C.I'm not sure if this is a bad thing or not.EDIT: Yes, this is well below the junction temperature, but I'm not entirely sure if it's safe for usage inside the plastic scale model building it'll be used in. I suppose it won't really matter too much if I just keep it from touching anything.As for maintaining voltage regulation, what part of the datasheet do I look at to determine if my load is sufficient to maintain the regulation?

author: Benta
author_rank: Super Contributor
timestamp: March 21, 2017, 06:33:07 pm
content: Use a 78L05 in SO-8.

author: dimkasta
author_rank: Regular Contributor
timestamp: March 21, 2017, 07:29:01 pm
content: Quote from: Phoxtane on March 21, 2017, 06:25:10 pmI remember seeing it in some datasheets as a footnote. Can't see it on this one. It was 5mA for the TO220 if I remember correctly.Now for the plastics, it really depends. Only you can check this. But even if you do not see an immediate problem, increased heat might increase the polymerization rate.If it is a static model, just put the pcb on the base. If it is an RC model, then I would not worry that much. The plastics on those things are practically consumables.

author: Benta
author_rank: Super Contributor
timestamp: March 21, 2017, 07:35:30 pm
content: QuoteThis is irrelevant for the 78xx, 78Lxx series of regulators, they will keep regulation down to zero output current.You've probably seen it mentioned for LM317 regulators, but that's a different story.

author: Avacee
author_rank: Unknown
timestamp: March 21, 2017, 07:36:47 pm
content: Quote from: Phoxtane on March 21, 2017, 06:25:10 pmIt's a footnote in Section 5: Electrical Characteristics of the datasheet in the first post.Same value for the 7805A and 7805CQuote

author: Benta
author_rank: Super Contributor
timestamp: March 21, 2017, 07:49:29 pm
content: QuoteWell, it seems ST is not very good at making 78-series regulators.Here is a link to the MC78xx series from ON:https://www.onsemi.com/pub/Collateral/MC7800-D.PDFNote Figure 14, which even specifies dropout voltage at 0 mA. And no strange footnotes.