Power resistors, heat on PCBs - was Re: [TekScopes] 2236 Focus range.


toby@...
 

On 2021-08-26 10:33 a.m., toby@telegraphics.com.au wrote:
On 2021-08-26 10:19 a.m., n4buq wrote:
I recently had a 1/2W film resistor flame-out on the LV Supply board in my 2445 and it burned a significant divot in the board. I mounted the replacement about 1/8" or more off the board to give it some breathing room. Probably unnecessary as the cap that caused the resistor to burn out is now replaced but I did it just for good measure.

Thanks,
Barry - N4BUQ
In the Hewlett-Packard 'scope I have been working on (1340A), they have
spaced every power resistor _at least_ 5-6mm from the board. This seems
like a very good idea.

There is also a nice ventilation measure using 1/2" circular holes where
the components are on the bottom of the board (so the hole provides an
upward exit for warm air right next to the hot compoents). They have
done this in the X/Y/Z amplifiers where there are 2-4 transistors and
power resistors clustered around the hole. Photos on request!
Fwiw, I had a photo lying around. This PCB is on the bottom of the
instrument, facing down.

https://imgur.com/viKpRhb

--Toby



--Toby