Output resistors for parellell 4780

[jaste]

Is there any problem using 0.1 ohm 2w dale resistors as output resistors instead of 0.1 ohm 3w when driving the llm4780 in parallell config?

 

[Leolabs]

It depends on much output power is needed.

 

[jaste]

so how restricted am I if I use the 2w instead of the 3w?

 

[tlmadsen]

You should be fine under normal use.

Remember: P(W)=I*I*R

So in your case you can pull around 4,5 amp before it should get to hot.
H
Have fun

Thomas

 

[sixtek]

No-no, Tlmadsen you aren't right.
A 2W rated resistor will get to hot even at 0,5-0,7W. At 1-1,5W it will be very, very hot.

 

[AndrewT]

Output Emitter Resistors

Hi,
how long will it take for them to get hot?
and very very hot?

How long will the current be at or exceed the value required to get the emitter resistors to either of these temperatures?

If the time constant of the signal is shorter than the time constant of the thermal capacity of the resistor then provided your are not running with maximum continuous ouput power nor a severely clipped output then the resistor will never get up to these temperatures.

What is needed is surviveability at the peak current output into low loads and reasonable running temperatures at normal to loud music level.

Keep in mind that each emitter resistor carries halfwave rms current when sending unclipped sinewave to the output. Power dissipation will be related to the half wave Isquared R loss.

 

[jaste]

alright, sp if I drive a pair of 4 ohms speakers and dont go too loud for too long then I should be ok with my 2w resistors?

 

[AndrewT]

Hi,
2w will easily cope with normal music reproducing operation.

But will they survive power testing or accidental shorting of output?