[time-nuts] Adding adjustment pot to 5680

Joseph Gray jgray at zianet.com
Wed Jan 11 14:41:53 UTC 2012


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 <rbenward at verizon.net> 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" <golgarfrincham at yahoo.com>
> To: <time-nuts at febo.com>
> 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
>
>
>
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