Question about resistor power ratings (general design rules)

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cradleinflames
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Posted by cradleinflames: Thu. Oct 1, 2015 - 04:23 PM
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Hello everyone!

 

I'm going to be dropping a max 8.65V across a resistor and want to select the smallest resistor possible to minimize the effects of current draw under nominal conditions (much less than 8.65V).  Looking at the max resistor size I can use for the application (an 0603 thin film resistor, 0.1% tolerance, 25 ppm), it looks like the max wattage I can get is just under 200 ohms (technically, 199.527 ohms).  

 

Because of how specific the resistor specifications are, my options appear to be 200 or 301 ohms.  My question is is there a general rule for selecting power of a resistor (ie select a power that is 1.5 times more than what you would use, you can use the max power rating assuming you don't exceed it, etc).  For this application, it would be a one time PULSE of 8.65V so I'm assuming I might be able to safely choose a 200 ohm resistor without any problems but I figured I'd ask if anyone knows of a general guideline for selecting resistor values based on power ratings.  Let me know when you get the chance.  Thanks!

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Bronze Markbobgardner
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Posted by bobgardner: Thu. Oct 1, 2015 - 04:25 PM
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I've always heard factor of 2 in resistor power rating. Got an EE deg in '73, so this rule of thumb has stood the test of time.

 

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cradleinflames
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Posted by cradleinflames: Thu. Oct 1, 2015 - 04:35 PM (Reply to #2)
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Makes sense.  If my typical power rating under normal operating conditions is about 0.5V across the resistor or less (at 200 ohms, that means I'm using about 1.25 mW max) and the only time I'll see anywhere near the 8.65V is a one time pulse, do you think it would probably be fine to use a 200 ohm resistor or would you play it safe and go with a 300 ohm resistor?

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ka7ehk
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Posted by ka7ehk: Thu. Oct 1, 2015 - 05:34 PM
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Places  I have worked at also used the 50% guideline for resistors.

 

It is much less clear about pulse conditions because it depends, very strongly, on the duration of the pulse. Some resistor family spec sheets will include a graph of  allowable pulse power as a function of pulse duration, and whether repetitive or isolated pulses. 

 

There is often a maximum voltage rating, which can come into play with large value resistors.

 

Capacitors are FAR more difficult!

 

Jim

 

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Bronze Markjs
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Posted by js: Thu. Oct 1, 2015 - 09:03 PM
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it looks like the max wattage I can get is just under 200 ohms (technically, 199.527 ohms).  

WATT? You mean resistance?

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

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Torby
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Posted by Torby: Thu. Oct 1, 2015 - 10:27 PM
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He believes resistance is futile, so he measures watts in ohms.

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jgmdesign
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Posted by jgmdesign: Fri. Oct 2, 2015 - 01:53 AM (Reply to #6)
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Torby wrote:

He believes resistance is futile, so he measures watts in ohms.

Resistance is not futile....It's voltage divided by current.

Jim

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Who-me
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Posted by Who-me: Fri. Oct 2, 2015 - 02:12 AM
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cradleinflames wrote:

0.1% tolerance, 25 ppm

... it would be a one time PULSE of 8.65V

Pulse means very little - what duration ?

That 200 ohms gives 0.374W, which seems a lot to be pushing into a 0.1% 0603 for other than a very short time (way less than the thermal time constant)

 

If you want the resistor to still be 0.1% after the pulse, you will need to be careful.

I can find just one brand that goes to 0.375W in 0603, and it also says

"Dependent on component mounting by user"

Their test says "testing was conducted mounting
one device to 2" x 3" test cards with 2.5 mil copper plating on
both surfaces. Thermal vias on 120 mil centers were utilized for
heat transfer between surfaces of the test card."

 

That's quite a lot of copper cooling ! - have you allowed that much in your design ?

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valusoft
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Posted by valusoft: Fri. Oct 2, 2015 - 03:09 AM
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Candle,

 

I am confused by your confused usage of units for parameters. Watts is the unit of power and is determined by knowing the voltage across a known resistance value [P = (V*V)/R] or the current through the same resistance [P=I*I*R].

 

So what is the current that you expect to be passing through your resistor to cause 8.65 volts to be "dropped" across it?

 

The rest is then maths.

 

Cheers,

 

Ross

 

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Who-me
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Posted by Who-me: Fri. Oct 2, 2015 - 04:11 AM (Reply to #9)
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valusoft wrote:

Candle,

 

I am confused by your confused usage of units for parameters. Watts is the unit of power and is determined by knowing the voltage across a known resistance value [P = (V*V)/R] or the current through the same resistance [P=I*I*R].

 

So what is the current that you expect to be passing through your resistor to cause 8.65 volts to be "dropped" across it?

 

The rest is then maths.

 

I think the OP knows that, just used a somewhat terse wording - that 200 ohms maps to 374mW and I can find a vendor claiming 375mW, making 200 Ohms the resistance to give operation at maximum rated power.

375mW is still a lot to push into a 0603, but if it is ms in duration, genuine one off, that may be ok.

 

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cradleinflames
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Posted by cradleinflames: Fri. Oct 2, 2015 - 05:51 AM (Reply to #10)
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Who-me wrote:

 

valusoft wrote:

 

Candle,

 

I am confused by your confused usage of units for parameters. Watts is the unit of power and is determined by knowing the voltage across a known resistance value [P = (V*V)/R] or the current through the same resistance [P=I*I*R].

 

So what is the current that you expect to be passing through your resistor to cause 8.65 volts to be "dropped" across it?

 

The rest is then maths.

 

 

 

I think the OP knows that, just used a somewhat terse wording - that 200 ohms maps to 374mW and I can find a vendor claiming 375mW, making 200 Ohms the resistance to give operation at maximum rated power.

375mW is still a lot to push into a 0603, but if it is ms in duration, genuine one off, that may be ok.

 

 

Heh, that's a very diplomatic way of saying what everyone else was thinking . . . my wording is terrible!  But you hit what I was trying to say right on the head.  At 200 ohms, the measured wattage is 0.374W so, it's at the limit even before the 0.1% tolerance (which would obviously put it outside of the limits of the power rating).  

 

Who-Me, that is a lot of very valuable information that I clearly didn't read (and should have).  I'm 100% sure we haven't taken all of that into consideration so the case is likely worse than the test conditions.  

 

I think at the end of the day, even though the pulse is short, it probably isn't worth risking it.  Truthfully, this is all a result of another issue having to do with the protection circuit method we're using being very current dependent (resulting in needing lower value, higher power resistors.  I'm actually moving away from zener diodes (which are going to cause non linearity issues as a result of the current spikes with higher voltages).  In the end, I'm moving towards schottky diodes for protection, which don't have the current draw issues of zener diodes and can afford to use higher value resistors.  

 

Thanks for all the feedback, I was definitely wanting to know how other people decide what to use based on power ratings and such.  I appreciate the help!

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gchapman
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Posted by gchapman: Fri. Oct 2, 2015 - 05:53 AM
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cradleinflames wrote:
... (an 0603 thin film resistor, 0.1% tolerance, 25 ppm) ...
An ESD event will slightly change the resistance; might consider a surge capable resistor.

"Dare to be naïve." - Buckminster Fuller

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meslomp
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Posted by meslomp: Fri. Oct 2, 2015 - 07:10 AM
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Always make sure that you can handle the pulse current constantly.

If things crash or not react at the speed you expect you do not want smoke, unless you added that resistor to give smoke and make sure the rest of the design is safe.

 

a 0603 resistor typically does 64mW to 125mW there are vendors as you found out that have a higher current rating, but then you will be depenadnt on the availability through that one vendor.

 

what is the normal current consumption by the system? 200Ohm x 10mA is a 2V drop.

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DocJC
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Posted by DocJC: Fri. Oct 2, 2015 - 07:45 PM
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And, obviously, two 400 ohm resistors in parallel will still give you a 200 ohm resistance, but twice the power capacity.

It also doesn't take much more space than one such resistor.

 

JC

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Torby
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Posted by Torby: Sun. Oct 4, 2015 - 01:39 AM (Reply to #7)
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jgmdesign wrote:

 

Torby wrote:

He believes resistance is futile, so he measures watts in ohms.

 

 

Resistance is not futile....It's voltage divided by current.

Jim

 

You'll have to explain that to him.

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