Just one data point: Last night, I taped a thermocouple to the top case of a unit and read 50C.
Joe Gray W5JG On Wed, Jan 11, 2012 at 7:12 AM, Robert Benward <[email protected]> wrote: > Arthur, > 800mW will give you a 52C rise in junction temperature over ambient when not > mounted to anything. The 5680s get pretty hot, probably 50C internally (has > anyone measured it?). That will put you at 102C. Make sure the flavor of > 7805 you selected is rated for that (most are rated for 125C). Not all are, > and not all have thermal shutdown. As for reliability, without any > derating, this won't give you much margin. > > Don't forget the cap on the output. > > Bob > > > > ----- Original Message ----- From: "Arthur Dent" <[email protected]> > To: <[email protected]> > Sent: Tuesday, January 10, 2012 10:50 PM > Subject: [time-nuts] Adding adjustment pot to 5680 > > > >> You are driving the 7805 near its limit. As a rule of thumb, >> a TO-220 case can disipate about 1W of power, if it's >> free-standing. You have max 100mA with a voltage difference of >> 10V, resulting in 1W max. Ie the 7805 will be running at a >> considerable high temperature. As you have there a good heat sink >> already (the case), i'd mount the 7805 against the case and use >> wires to connect it. >> Attila Kinali > > ++++++++++++++++++++++++++ > I had considered mounting the 7805 on the vertical heatsink you see > to the left of the 7805 in my photo, using a short screws in either side > of the the tapped hole that is already there and use leads to connect > the 7805 to the connector pins, but felt it wasn't necessary. Directly > connecting the regulator to the DB-9 pins was much simpler. > > FYI, the load is not 100Ma as you assumed, that is the max rating of > the 78L05 that I didn't use. The actual load the 7805 sees in the 5680A > is about 80Ma so even at the higher temperatures inside the case I > don't see a problem. You are correct that it might be a better practice > to mount the regulator on the heat sink, especially if this were a > production item, but in this case I felt it was overkill. > http://farm8.staticflickr.com/7013/6668466093_90782cf7e9_b.jpg > > > -Arthur > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
