Re: Can a 1W resistor handle 50W for 7msec?

In article <[EMAIL PROTECTED]>,
 Bill Turner <[EMAIL PROTECTED]> wrote:

> On 24 Nov 2003 22:50:40 -0800, [EMAIL PROTECTED] (Jason Hsu) wrote:
> 
> >Can the 1W-51 ohm resistors handle this 50 RF volts 0-peak (about 50W
> >PEP) for .007 sec?  50W over .007 seconds is .35 Joules.  .35W for 1
> >second is also .35 Joules, which a 1W resistor should have no trouble
> >handling.  Can the resistors be damaged during that .007 seconds?
> 
> _________________________________________________________
> 
> You really need to ask the manufacturer of the resistor.  They are well
> aware of the problem - if you get to the right person.
> 
> Having said that, here is a generalization:  If the resistor's element
> is a solid block of material, such as in a carbon composition type, it
> will have very good pulse power ratings.  On the other hand, if the
> element is a film, it may develop tiny hot spots during pulsing and
> eventually fail.
> 

Many years ago I analyzed a problem with resistors which were 
discoloring in the field. This involved two identical circuits with 
identical singal inputs. In one circuit, a resistor kept turning brown 
over time, in the other it did not. We were getting field returns as a 
result of troubleshooting that focused on the brown resistors.

What was the difference? The resistor which did not discolor was resting 
on two metal traces which ran under it, the one which discolored, was 
resting on the PCB. That tiny amount of heatsinking provided by the 
traces was the difference. Obviously, the circuit needed to be 
redesigned, but it worked great on the breadboard when it went into 
production.

Al

-- 
There's never enough time to do it right the first time.......