Hi,

Its a pretty straight forward question. My project for my HNC is work based and it involves Changing our PCB's from through hole to Surface Mount. My question is about resistor Power ratings and temperature coefficient.

For example the current through hole resistor, 10K ohms, 200V, 1% tolerance has a power rating of 125mW and temp coefficient of 50ppm/C. I have found an equivelant value resistor that is SM type but its power rating is 250mW and temp coefficient is 100ppm/C.

Will there be any noticable preformance issues if I choose to go with the SM replacement regarding their differences in power rating and temp coefficients? Are there other paramaters to consider when converting like components?

Thanks in advance.

 

There is no way to tell without a schematic. Very few circuits are so critical about temperature change that dR/dT is rarely even calculated. Why would you think twice as much dR/dT would be a problem when dT/W is 1/2 what it used to be? Is this board in a place where the power applied to the resistor is insignificant compared to environmental temperature considerations? Is your output so critical and accurate that 25 ohms difference in a 10k resistor (1/4th of 1%) would throw it out of calibration? Is the design of this hyper-critical circuit lacking resistors which are placed to compensate by changing resistance in a way that would cancel out temperature drift?

See? No way to know without seeing the circuit and knowing the external temperature conditions it has to work with and how accurate it is required to be.

 

Thanks for replying.

Its for a control board in a generator case. The generator supplies voltage to a transducer to provide ultrasound. I have the circuit diagram and board files but i am unable to view the gerber files.

 

As #12 said, there is no way of really knowing. Here is a 0.5%, 25 ppm, 150 V resistor. You may be able to use two resistors of 25 ppm to get the voltage rating and tolerance you need.

Say you got two 5K resistors at 0.5% ; the equivalent would be 10K, +-1%, +-50 ppm, 300 V. 5K may not exist.

 

5K may not exist.

Actually, it's becoming amazingly easy to get nearly any resistance you want. Here's a page with (29) surface mount resistors of exactly 5K rating and various tempcos, watt ratings, accuracy specs, and voltage ratings.
http://www.mouser.com/Passive-Components/Resistors/SMD-Resistors-Chip-Resistors/_/N-7h7yu?P=1yzekim

 

I just didn't look. The TS is probably happy for your work.