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High peak power pulse in resistor will eventually kill it?

Flyback

Well-Known Member
Hi,

We are using the RSF300JB-91-100R resistor (100R,3W) in an inrush overvoltage damping circuit as attached. (LTspice also attached)
Will this resistor be able to handle these power pulses several times per day for 20 years?
As shown, the power pulse is 1700W peak and lasts for some 35us.

RSF300JB-91-100R resistor datasheet
https://www.yageo.com/upload/media/product/products/datasheet/lr/Yageo_LR_RSF_1.pdf
 

Attachments

gophert

Well-Known Member
Most Helpful Member
Power pulses? There is a 2M ohm resistor in series. How are you measuring / calculating 1700 watts?

The thermal time constant for a 3 watt resistor is likely in the range of a second. So, at 1/16th watt averaged over 1 second per event, assuming you calculated correcty, I am sure the resistor can handle it as long as you don't exceed the resistor's max working voltage.
 

Flyback

Well-Known Member
Power pulses? There is a 2M ohm resistor in series. How are you measuring / calculating 1700 watts?
Thanks, i am speaking of the 100R resistor thats ringed...at switch on, it gets a 1700w peak power pulse....the LTspice sim attached to top post shows it.
 

simonbramble

Active Member
Have you tried contacting the resistor manufacturer or one of their distributors? They might have conducted some tests already. Resistors die through heat dissipation, so if the device does not get hot, it should be OK. I agree with you about the thermal time constant, so that is the sort of time frame you should be integrating over, but I cannot give you a precise answer (and I would be surprised if they give you one either). I would be interested in knowing what they come back with, if anything. You could run some tests yourself
 

JimW

Member
Wow, this goes back some 30 years. Yes, you can destroy a resistor with a high energy pulse. It is not the wattage and heat, it is the energy in joules. When we did this in a lightning strike simulator in a telephony company, the paralleled 2 watt carbon comp resistors exploded. Lots and lots of very small pieces. I remember the lead engineer getting energy ratings for different types of resistors and we ended up with a fair number in parallel to shape our lighting strike pulses. Sorry I can't help you more than that, it is just s good memory.
 

ronsimpson

Well-Known Member
Most Helpful Member
Hi FlyBack,
I am playing with this right now. I am using "pulse" resistors. Many resistors will not tell you what happens when you pound them for a uS. Using Digikey.com I am looking at only resistors that have a pulse rating. Look for 'shunt' or 'pulse' resistors. If they don't give you a graph like this move on. I had some resistors open up under large pulses.
1595982005756.png
For inrush I would change R1 to a posistor. 100 ohm cold and 1 ohm hot.
 

gophert

Well-Known Member
Most Helpful Member
I'm still having trouble seeing how 1700 W is generated on that resistor, the one in series with a 2.2M resistor. So only the low-side discharge of a nanofarad cap Is generating this "huge" spike? Are you sure your simulator is correct?
1700W across that 100R resistor means 410 volts at 4+ amps. To achieve that when a 2.2M resistor is in series with it means the supply rail has to reach 10M VOLTS. Unless you're building a Mr Fusion, I'd check the model again.
 
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Diver300

Well-Known Member
Most Helpful Member
1700 W is more than you are going to get, but the worst case is when the system is turned on at the peak of the mains voltage, so the 2.2 M discharge resistor isn't the limiting factor. The current in the 100 Ohm resistor limits the charge rate of the 470 nF capacitor. There will be some voltage drop in the diodes etc. but that won't be much compared to the peak supply voltage of 373 V, so there will be most of that across the 100 Ohm resistor.

The time constant of 100 Ohms and 470 nF is 47 microseconds, so the pulse will last about that long. I calculate that the peak power will be around 1.4 kW and the total energy in the resistor will be around 33 mJ.

Obviously the resistor doesn't need to be rated at 1.4 kW, but it does need to be able to absorb 33 mJ very quickly without getting too hot or having too much thermally induced stress. The resistor also has to handle the 373 V.
 

picbits

Well-Known Member
We have a control board made by a very well known manufacturer fail every couple of years or so - they use a 10R SMD resistor in series with a diode feeding a large cap so the board can run on either 24v AC or DC. The 10R resistors eventually fail open circuit at which point the unit is brought up to me for another fix. I've done some mods with two 20R SMD resistors in parallel and not had one back yet.
 

gophert

Well-Known Member
Most Helpful Member
We have a control board made by a very well known manufacturer fail every couple of years or so - they use a 10R SMD resistor in series with a diode feeding a large cap so the board can run on either 24v AC or DC. The 10R resistors eventually fail open circuit at which point the unit is brought up to me for another fix. I've done some mods with two 20R SMD resistors in parallel and not had one back yet.
well, they may not have come back to YOU, but certainly your solution couldn't hav worked. Nigel said you have to replace it with a single bigger wattage resistor.

:p:D:):happy:
 

picbits

Well-Known Member
well, they may not have come back to YOU, but certainly your solution couldn't hav worked. Nigel said you have to replace it with a single bigger wattage resistor.

:p:D:):happy:
By putting two 20R resistors of the same wattage in parallel, each is now dissipating half of what it was before so fingers crossed .......

They will only come back to me as I'm the only one with a stash of 1210 10R and 20R resistors ;)
 

gophert

Well-Known Member
Most Helpful Member
By putting two 20R resistors of the same wattage in parallel, each is now dissipating half of what it was before so fingers crossed .......

They will only come back to me as I'm the only one with a stash of 1210 10R and 20R resistors ;)
I know, my post was sarcasm. I'll never understand Nigel's repeated claim of "a very common design flaw" in the link above.
 

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